To simplify configuration, SHDP provides a dedicated namespace for most of its components. However, one can opt to configure the beans directly through the usual <bean> definition. For more information about XML Schema-based configuration in Spring, see this appendix in the Spring Framework reference documentation.

To use the SHDP namespace, one just needs to import it inside the configuration:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:hdp="http://www.springframework.org/schema/hadoop"
   xsi:schemaLocation="
    http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/hadoop
    http://www.springframework.org/schema/hadoop/spring-hadoop.xsd">

   <bean/>

   <hdp:configuration/>
</beans>

Once imported, the namespace elements can be declared simply by using the aforementioned prefix. Note that is possible to change the default namespace, for example from <beans> to <hdp>. This is useful for configuration composed mainly of Hadoop components as it avoids declaring the prefix. To achieve this, simply swap the namespace prefix declarations above:

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans
xmlns="http://www.springframework.org/schema/hadoop"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:beans="http://www.springframework.org/schema/beans"
   xsi:schemaLocation="
    http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/hadoop http://www.springframework.org/schema/hadoop/spring-hadoop.xsd">

    <beans:bean id ... >

    <configuration ...>

</beans:beans>

For the remainder of this doc, to improve readability, the XML examples may simply refer to the <hdp> namespace without the namespace declaration, where possible.

Annotation based configuration is designed to work via a SpringHadoopConfigurerAdapter which is loosely trying to use same type of dsl language familiar from xml. Within the adapter you need to override configure method which is exposing HadoopConfigConfigurer containing familiar attributes to work with a Hadoop configuration.

import org.springframework.context.annotation.Configuration;
import org.springframework.data.hadoop.config.annotation.EnableHadoop
import org.springframework.data.hadoop.config.annotation.SpringHadoopConfigurerAdapter
import org.springframework.data.hadoop.config.annotation.builders.HadoopConfigConfigurer;

@Configuration
@EnableHadoop
static class Config extends SpringHadoopConfigurerAdapter {

  @Override
  public void configure(HadoopConfigConfigurer config) throws Exception {
    config
      .fileSystemUri("hdfs://localhost:8021");
  }

}

	Note
	@EnableHadoop annotation is required to mark Spring @Configuration class to be a candidate for Spring Hadoop configuration.

In order to use Hadoop, one needs to first configure it namely by creating a Configuration object. The configuration holds information about the job tracker, the input, output format and the various other parameters of the map reduce job.

In its simplest form, the configuration definition is a one liner:

<hdp:configuration />

The declaration above defines a Configuration bean (to be precise a factory bean of type ConfigurationFactoryBean) named, by default, hadoopConfiguration. The default name is used, by conventions, by the other elements that require a configuration - this leads to simple and very concise configurations as the main components can automatically wire themselves up without requiring any specific configuration.

For scenarios where the defaults need to be tweaked, one can pass in additional configuration files:

<hdp:configuration resources="classpath:/custom-site.xml, classpath:/hq-site.xml">

In this example, two additional Hadoop configuration resources are added to the configuration.

	Note
	Note that the configuration makes use of Spring’s Resource abstraction to locate the file. This allows various search patterns to be used, depending on the running environment or the prefix specified (if any) by the value - in this example the classpath is used.

In addition to referencing configuration resources, one can tweak Hadoop settings directly through Java Properties. This can be quite handy when just a few options need to be changed:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:hdp="http://www.springframework.org/schema/hadoop"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/hadoop http://www.springframework.org/schema/hadoop/spring-hadoop.xsd">

     <hdp:configuration>
        fs.defaultFS=hdfs://localhost:8020
        hadoop.tmp.dir=/tmp/hadoop
        electric=sea
     </hdp:configuration>
</beans>

One can further customize the settings by avoiding the so called hard-coded values by externalizing them so they can be replaced at runtime, based on the existing environment without touching the configuration:

	Note
	Usual configuration parameters for fs.defaultFS, mapred.job.tracker and yarn.resourcemanager.address can be configured using tag attributes file-system-uri, job-tracker-uri and rm-manager-uri respectively.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:hdp="http://www.springframework.org/schema/hadoop"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/hadoop http://www.springframework.org/schema/hadoop/spring-hadoop.xsd">

     <hdp:configuration>
        fs.defaultFS=${hd.fs}
        hadoop.tmp.dir=file://${java.io.tmpdir}
        hangar=${number:18}
     </hdp:configuration>

     <context:property-placeholder location="classpath:hadoop.properties" />
</beans>

Through Spring’s property placeholder support, SpEL and the environment abstraction. one can externalize environment specific properties from the main code base easing the deployment across multiple machines. In the example above, the default file system is replaced based on the properties available in hadoop.properties while the temp dir is determined dynamically through SpEL. Both approaches offer a lot of flexibility in adapting to the running environment - in fact we use this approach extensivly in the Spring for Apache Hadoop test suite to cope with the differences between the different development boxes and the CI server.

Additionally, external Properties files can be loaded, Properties beans (typically declared through Spring’s util namespace). Along with the nested properties declaration, this allows customized configurations to be easily declared:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:hdp="http://www.springframework.org/schema/hadoop"
    xmlns:context="http://www.springframework.org/schema/context"
    xmlns:util="http://www.springframework.org/schema/util"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd
        http://www.springframework.org/schema/hadoop http://www.springframework.org/schema/hadoop/spring-hadoop.xsd">

   <!-- merge the local properties, the props bean and the two properties files -->
   <hdp:configuration properties-ref="props" properties-location="cfg-1.properties, cfg-2.properties">
      star=chasing
      captain=eo
   </hdp:configuration>

   <util:properties id="props" location="props.properties"/>
</beans>

When merging several properties, ones defined locally win. In the example above the configuration properties are the primary source, followed by the props bean followed by the external properties file based on their defined order. While it’s not typical for a configuration to refer to so many properties, the example showcases the various options available.

	Note
	For more properties utilities, including using the System as a source or fallback, or control over the merging order, consider using Spring’s PropertiesFactoryBean (which is what Spring for Apache Hadoop and util:properties use underneath).

It is possible to create configurations based on existing ones - this allows one to create dedicated configurations, slightly different from the main ones, usable for certain jobs (such as streaming - more on that #hadoop:job:streaming[below]). Simply use the configuration-ref attribute to refer to the parent configuration - all its properties will be inherited and overridden as specified by the child:

<!-- default name is 'hadoopConfiguration' -->
<hdp:configuration>
    fs.defaultFS=${hd.fs}
    hadoop.tmp.dir=file://${java.io.tmpdir}
</hdp:configuration>

<hdp:configuration id="custom" configuration-ref="hadoopConfiguration">
    fs.defaultFS=${custom.hd.fs}
</hdp:configuration>

...

Make sure though that you specify a different name since otherwise, because both definitions will have the same name, the Spring container will interpret this as being the same definition (and will usually consider the last one found).

Another option worth mentioning is register-url-handler which, as the name implies, automatically registers an URL handler in the running VM. This allows urls referrencing hdfs resource (by using the hdfs prefix) to be properly resolved - if the handler is not registered, such an URL will throw an exception since the VM does not know what hdfs means.

	Note
	Since only one URL handler can be registered per VM, at most once, this option is turned off by default. Due to the reasons mentioned before, once enabled if it fails, it will log the error but will not throw an exception. If your hdfs URLs stop working, make sure to investigate this aspect.

Last but not least a reminder that one can mix and match all these options to her preference. In general, consider externalizing Hadoop configuration since it allows easier updates without interfering with the application configuration. When dealing with multiple, similar configurations use configuration composition as it tends to keep the definitions concise, in sync and easy to update.

	Note
	Configuring security and kerberos refer to chapter Chapter 11, Security Support.

Spring Boot support is enabled automatically if spring-data-hadoop-boot-2.3.0.M1-phd30.jar is found from a classpath. Currently Boot auto-configuration is a little limited and only supports configuring of hadoopConfiguration and fsShell beans.

Configuration properties can be defined using various methods. See a Spring Boot dodumentation for details.

@Grab('org.springframework.data:spring-data-hadoop-boot:2.3.0.M1-phd30')

import org.springframework.data.hadoop.fs.FsShell

public class App implements CommandLineRunner {

  @Autowired FsShell shell

  void run(String... args) {
    shell.lsr("/tmp").each() {println "> ${it.path}"}
  }

}

Above example which can be run using Spring Boot CLI shows how auto-configuration ease use of Spring Hadoop. In this example Hadoop configuration and FsShell are configured automatically.

Once the Hadoop configuration is taken care of, one needs to actually submit some work to it. SHDP makes it easy to configure and run Hadoop jobs whether they are vanilla map-reduce type or streaming. Let us start with an example:

<hdp:job id="mr-job"
  input-path="/input/" output-path="/ouput/"
  mapper="org.apache.hadoop.examples.WordCount.TokenizerMapper"
  reducer="org.apache.hadoop.examples.WordCount.IntSumReducer"/>

The declaration above creates a typical Hadoop Job: specifies its input and output, the mapper and the reducer classes. Notice that there is no reference to the Hadoop configuration above - that’s because, if not specified, the default naming convention (hadoopConfiguration) will be used instead. Neither is there to the key or value types - these two are automatically determined through a best-effort attempt by analyzing the class information of the mapper and the reducer. Of course, these settings can be overridden: the former through the configuration-ref element, the latter through key and value attributes. There are plenty of options available not shown in the example (for simplicity) such as the jar (specified directly or by class), sort or group comparator, the combiner, the partitioner, the codecs to use or the input/output format just to name a few - they are supported, just take a look at the SHDP schema (?) or simply trigger auto-completion (usually CTRL+SPACE) in your IDE; if it supports XML namespaces and is properly configured it will display the available elements. Additionally one can extend the default Hadoop configuration object and add any special properties not available in the namespace or its backing bean (JobFactoryBean).

It is worth pointing out that per-job specific configurations are supported by specifying the custom properties directly or referring to them (more information on the pattern is available #hadoop:config:properties[here]):

<hdp:job id="mr-job"
  input-path="/input/" output-path="/ouput/"
  mapper="mapper class" reducer="reducer class"
  jar-by-class="class used for jar detection"
  properties-location="classpath:special-job.properties">
    electric=sea
</hdp:job>

<hdp:job> provides additional properties, such as the #hadoop:generic-options[generic options], however one that is worth mentioning is jar which allows a job (and its dependencies) to be loaded entirely from a specified jar. This is useful for isolating jobs and avoiding classpath and versioning collisions. Note that provisioning of the jar into the cluster still depends on the target environment - see the aforementioned #hadoop:generic-options[section] for more info (such as libs).

<hdp:streaming id="streaming"
  input-path="/input/" output-path="/ouput/"
  mapper="${path.cat}" reducer="${path.wc}"/>

<hdp:streaming id="streaming-env"
  input-path="/input/" output-path="/ouput/"
  mapper="${path.cat}" reducer="${path.wc}">
  <hdp:cmd-env>
     EXAMPLE_DIR=/home/example/dictionaries/
     ...
  </hdp:cmd-env>
</hdp:streaming>

The jobs, after being created and configured, need to be submitted for execution to a Hadoop cluster. For non-trivial cases, a coordinating, workflow solution such as Spring Batch is recommended . However for basic job submission SHDP provides the job-runner element (backed by JobRunner class) which submits several jobs sequentially (and waits by default for their completion):

<hdp:job-runner id="myjob-runner" pre-action="cleanup-script" post-action="export-results" job-ref="myjob" run-at-startup="true"/>

<hdp:job id="myjob"  input-path="/input/" output-path="/output/"
    mapper="org.apache.hadoop.examples.WordCount.TokenizerMapper"
    reducer="org.apache.hadoop.examples.WordCount.IntSumReducer" />

Multiple jobs can be specified and even nested if they are not used outside the runner:

<hdp:job-runner id="myjobs-runner" pre-action="cleanup-script" job-ref="myjob1, myjob2" run-at-startup="true"/>

<hdp:job id="myjob1"  ... />
<hdp:streaming id="myjob2"  ... />

One or multiple Map-Reduce jobs can be specified through the job attribute in the order of the execution. The runner will trigger the execution during the application start-up (notice the run-at-startup flag which is by default false). Do note that the runner will not run unless triggered manually or if run-at-startup is set to true. Additionally the runner (as in fact do all runners in SHDP) allows one or multiple pre and post actions to be specified to be executed before and after each run. Typically other runners (such as other jobs or scripts) can be specified but any JDK Callable can be passed in. For more information on runners, see the dedicated chapter.

Note

As the Hadoop job submission and execution (when wait-for-completion is true) is blocking, JobRunner uses a JDK Executor to start (or stop) a job. The default implementation, SyncTaskExecutor uses the calling thread to execute the job, mimicking the hadoop command line behaviour. However, as the hadoop jobs are time-consuming, in some cases this can lead to application freeze, preventing normal operations or even application shutdown from occuring properly. Before going into production, it is recommended to double-check whether this strategy is suitable or whether a throttled or pooled implementation is better. One can customize the behaviour through the executor-ref parameter.

The job runner also allows running jobs to be cancelled (or killed) at shutdown. This applies only to jobs that the runner waits for (wait-for-completion is true) using a different executor then the default - that is, using a different thread then the calling one (since otherwise the calling thread has to wait for the job to finish first before executing the next task). To customize this behaviour, one should set the kill-job-at-shutdown attribute to false and/or change the executor-ref implementation.

<hdp:job-tasklet id="hadoop-tasklet" job-ref="mr-job" wait-for-completion="true" />

It is common for Hadoop utilities and libraries to be started from the command-line (ex: hadoop jar some.jar). SHDP offers generic support for such cases provided that the packages in question are built on top of Hadoop standard infrastructure, namely Tool and ToolRunner classes. As opposed to the command-line usage, Tool instances benefit from Spring’s IoC features; they can be parameterized, created and destroyed on demand and have their properties (such as the Hadoop configuration) injected.

Consider the typical jar example - invoking a class with some (two in this case) arguments (notice that the Hadoop configuration properties are passed as well):

bin/hadoop jar -conf hadoop-site.xml -jt darwin:50020 -Dproperty=value someJar.jar

Since SHDP has first-class support for #hadoop:config[configuring] Hadoop, the so called generic options aren’t needed any more, even more so since typically there is only one Hadoop configuration per application. Through tool-runner element (and its backing ToolRunner class) one typically just needs to specify the Tool implementation and its arguments:

<hdp:tool-runner id="someTool" tool-class="org.foo.SomeTool" run-at-startup="true">
   <hdp:arg value="data/in.txt"/>
   <hdp:arg value="data/out.txt"/>

   property=value
</hdp:tool-runner>

Additionally the runner (just like the job runner) allows one or multiple pre and post actions to be specified to be executed before and after each run. Typically other runners (such as other jobs or scripts) can be specified but any JDK Callable can be passed in. Do note that the runner will not run unless triggered manually or if run-at-startup is set to true. For more information on runners, see the dedicated chapter.

The previous example assumes the Tool dependencies (such as its class) are available in the classpath. If that is not the case, tool-runner allows a jar to be specified:

<hdp:tool-runner ... jar="myTool.jar">
    ...
</hdp:tool-runner>

The jar is used to instantiate and start the tool - in fact all its dependencies are loaded from the jar meaning they no longer need to be part of the classpath. This mechanism provides proper isolation between tools as each of them might depend on certain libraries with different versions; rather then adding them all into the same app (which might be impossible due to versioning conflicts), one can simply point to the different jars and be on her way. Note that when using a jar, if the main class (as specified by the Main-Class entry) is the target Tool, one can skip specifying the tool as it will picked up automatically.

Like the rest of the SHDP elements, tool-runner allows the passed Hadoop configuration (by default hadoopConfiguration but specified in the example for clarity) to be #hadoop:config:properties[customized] accordingly; the snippet only highlights the property initialization for simplicity but more options are available. Since usually the Tool implementation has a default argument, one can use the tool-class attribute. However it is possible to refer to another Tool instance or declare a nested one:

<hdp:tool-runner id="someTool" run-at-startup="true">
   <hdp:tool>
      <bean class="org.foo.AnotherTool" p:input="data/in.txt" p:output="data/out.txt"/>
   </hdp:tool>
</hdp:tool-runner>

This is quite convenient if the Tool class provides setters or richer constructors. Note that by default the tool-runner does not execute the Tool until its definition is actually called - this behavior can be changed through the run-at-startup attribute above.

hadoop jar job1.jar -files fullpath:props.properties -Dconfig=config.properties ...
hadoop jar job2.jar arg1 arg2...
...
hadoop jar job10.jar ...

<hdp:tool-runner id="job1" tool-class="job1.Tool" jar="job1.jar" files="fullpath:props.properties" properties-location="config.properties"/>
<hdp:tool-runner id="job2" jar="job2.jar">
   <hdp:arg value="arg1"/>
   <hdp:arg value="arg2"/>
</hdp:tool-runner>
<hdp:tool-runner id="job3" jar="job3.jar"/>
...

<hdp:tool-tasklet id="tool-tasklet" tool-ref="some-tool" />

SHDP also provides support for executing vanilla Hadoop jars. Thus the famous WordCount example:

bin/hadoop jar hadoop-examples.jar wordcount /wordcount/input /wordcount/output

becomes

<hdp:jar-runner id="wordcount" jar="hadoop-examples.jar" run-at-startup="true">
    <hdp:arg value="wordcount"/>
    <hdp:arg value="/wordcount/input"/>
    <hdp:arg value="/wordcount/output"/>
</hdp:jar-runner>

	Note
	Just like the hadoop jar command, by default the jar support reads the jar’s Main-Class if none is specified. This can be customized through the main-class attribute.

Additionally the runner (just like the job runner) allows one or multiple pre and post actions to be specified to be executed before and after each run. Typically other runners (such as other jobs or scripts) can be specified but any JDK Callable can be passed in. Do note that the runner will not run unless triggered manually or if run-at-startup is set to true. For more information on runners, see the dedicated chapter.

The jar support provides a nice and easy migration path from jar invocations from the command-line to SHDP (note that Hadoop #hadoop:generic-options[generic options] are also supported). Especially since SHDP enables Hadoop Configuration objects, created during the jar execution, to automatically inherit the context Hadoop configuration. In fact, just like other SHDP elements, the jar element allows #hadoop:config:properties[configurations properties] to be declared locally, just for the jar run. So for example, if one would use the following declaration:

<hdp:jar-runner id="wordcount" jar="hadoop-examples.jar" run-at-startup="true">
    <hdp:arg value="wordcount"/>
    ...
    speed=fast
</hdp:jar-runner>

inside the jar code, one could do the following:

assert "fast".equals(new Configuration().get("speed"));

This enabled basic Hadoop jars to use, without changes, the enclosing application Hadoop configuration.

And while we think it is a useful feature (that is why we added it in the first place), we strongly recommend using the tool support instead or migrate to it; there are several reasons for this mainly because there are no contracts to use, leading to very poor embeddability caused by:

SHDP tries to use sensible defaults to provide the best integration experience possible but at the end of the day, without any contract in place, there are no guarantees. Hence using the Tool interface is a much better alternative.

<hdp:jar-tasklet id="jar-tasklet" jar="some-jar.jar" />

DistributedCache is a Hadoop facility for distributing application-specific, large, read-only files (text, archives, jars and so on) efficiently. Applications specify the files to be cached via urls (hdfs://) using DistributedCache and the framework will copy the necessary files to the slave nodes before any tasks for the job are executed on that node. Its efficiency stems from the fact that the files are only copied once per job and the ability to cache archives which are un-archived on the slaves. Note that DistributedCache assumes that the files to be cached (and specified via hdfs:// urls) are already present on the Hadoop FileSystem.

SHDP provides first-class configuration for the distributed cache through its cache element (backed by DistributedCacheFactoryBean class), allowing files and archives to be easily distributed across nodes:

<hdp:cache create-symlink="true">
   <hdp:classpath value="/cp/some-library.jar#library.jar" />
   <hdp:cache value="/cache/some-archive.tgz#main-archive" />
   <hdp:cache value="/cache/some-resource.res" />
   <hdp:local value="some-file.txt" />
</hdp:cache>

The definition above registers several resources with the cache (adding them to the job cache or classpath) and creates symlinks for them. As described in the DistributedCache documentation the declaration format is (absolute-path#link-name). The link name is determined by the URI fragment (the text following the # such as #library.jar or #main-archive above) - if no name is specified, the cache bean will infer one based on the resource file name. Note that one does not have to specify the hdfs://node:port prefix as these are automatically determined based on the configuration wired into the bean; this prevents environment settings from being hard-coded into the configuration which becomes portable. Additionally based on the resource extension, the definition differentiates between archives (.tgz, .tar.gz, .zip and .tar) which will be uncompressed, and regular files that are copied as-is. As with the rest of the namespace declarations, the definition above relies on defaults - since it requires a Hadoop Configuration and FileSystem objects and none are specified (through configuration-ref and file-system-ref) it falls back to the default naming and is wired with the bean named hadoopConfiguration, creating the FileSystem automatically.

Warning

Clients setting up a classpath in the DistributedCache, running on Windows platforms should set the System path.separator property to :. Otherwise the classpath will be set incorrectly and will be ignored; see HADOOP-9123 bug report for more information. There are multiple ways to change the path.separator System property - a quick one being a simple script in Javascript (that uses the Rhino package bundled with the JDK) that runs at start-up: <hdp:script language="javascript" run-at-startup="true"> // set System 'path.separator' to ':' - see HADOOP-9123 java.lang.System.setProperty("path.separator", ":") </hdp:script>

The job, streaming and tool all support a subset of generic options, specifically archives, files and libs. libs is probably the most useful as it enriches a job classpath (typically with some jars) - however the other two allow resources or archives to be copied throughout the cluster for the job to consume. Whenver faced with provisioning issues, revisit these options as they can help up significantly. Note that the fs, jt or conf options are not supported - these are designed for command-line usage, for bootstrapping the application. This is no longer needed, as the SHDP offers first-class support for defining and customizing Hadoop configurations.

The Hadoop file-system, HDFS, can be accessed in various ways - this section will cover the most popular protocols for interacting with HDFS and their pros and cons. SHDP does not enforce any specific protocol to be used - in fact, as described in this section any FileSystem implementation can be used, allowing even other implementations than HDFS to be used.

The table below describes the common HDFS APIs in use:

This chapter focuses on the core file-system protocols supported by Hadoop. S3, FTP and the rest of the other FileSystem implementations are supported as well - Spring for Apache Hadoop has no dependency on the underlying system rather just on the public Hadoop API.

hdfs:// protocol should be familiar to most readers - most docs (and in fact the previous chapter as well) mention it. It works out of the box and it’s fairly efficient. However because it is RPC based, it requires both the client and the Hadoop cluster to share the same version. Upgrading one without the other causes serialization errors meaning the client cannot interact with the cluster. As an alternative one can use hftp:// which is HTTP-based or its more secure brother hsftp:// (based on SSL) which gives you a version independent protocol meaning you can use it to interact with clusters with an unknown or different version than that of the client. hftp is read only (write operations will fail right away) and it is typically used with disctp for reading data. webhdfs:// is one of the additions in Hadoop 1.0 and is a mixture between hdfs and hftp protocol - it provides a version-independent, read-write, REST-based protocol which means that you can read and write to/from Hadoop clusters no matter their version. Furthermore, since webhdfs:// is backed by a REST API, clients in other languages can use it with minimal effort.

	Note
	Not all file systems work out of the box. For example WebHDFS needs to be enabled first in the cluster (through dfs.webhdfs.enabled property, see this document for more information) while the secure hftp, hsftp requires the SSL configuration (such as certificates) to be specified. More about this (and how to use hftp/hsftp for proxying) in this page.

Once the scheme has been decided upon, one can specify it through the standard Hadoop configuration, either through the Hadoop configuration files or its properties:

<hdp:configuration>
  fs.defaultFS=webhdfs://localhost
  ...
</hdp:configuration>

This instructs Hadoop (and automatically SHDP) what the default, implied file-system is. In SHDP, one can create additional file-systems (potentially to connect to other clusters) and specify a different scheme:

<!-- manually creates the default SHDP file-system named 'hadoopFs' -->
<hdp:file-system uri="webhdfs://localhost"/>

<!-- creates a different FileSystem instance -->
<hdp:file-system id="old-cluster" uri="hftp://old-cluster/"/>

As with the rest of the components, the file systems can be injected where needed - such as file shell or inside scripts (see the next section).

In Spring the ResourceLoader interface is meant to be implemented by objects that can return (i.e. load) Resource instances.

public interface ResourceLoader {
  Resource getResource(String location);
}

All application contexts implement the ResourceLoader interface, and therefore all application contexts may be used to obtain Resource instances.

When you call getResource() on a specific application context, and the location path specified doesn’t have a specific prefix, you will get back a Resource type that is appropriate to that particular application context. For example, assume the following snippet of code was executed against a ClassPathXmlApplicationContext instance:

Resource template = ctx.getResource("some/resource/path/myTemplate.txt");

What would be returned would be a ClassPathResource; if the same method was executed against a FileSystemXmlApplicationContext instance, you’d get back a FileSystemResource. For a WebApplicationContext, you’d get back a ServletContextResource, and so on.

As such, you can load resources in a fashion appropriate to the particular application context.

On the other hand, you may also force ClassPathResource to be used, regardless of the application context type, by specifying the special classpath: prefix:

Resource template = ctx.getResource("classpath:some/resource/path/myTemplate.txt");

	Note
	More information about the generic usage of resource loading, check the Spring Framework Documentation.

Spring Hadoop is adding its own functionality into generic concept of resource loading. Resource abstraction in Spring has always been a way to ease resource access in terms of not having a need to know where there resource is and how it’s accessed. This abstraction also goes beyond a single resource by allowing to use patterns to access multiple resources.

Lets first see how HdfsResourceLoader is used manually.

<hdp:file-system />
<hdp:resource-loader id="loader" file-system-ref="hadoopFs" />
<hdp:resource-loader id="loaderWithUser" user="myuser" uri="hdfs://localhost:8020" />

In above configuration we created two beans, one with reference to existing Hadoop FileSystem bean and one with impersonated user.

// get path '/tmp/file.txt'
Resource resource = loader.getResource("/tmp/file.txt");
// get path '/tmp/file.txt' with user impersonation
Resource resource = loaderWithUser.getResource("/tmp/file.txt");

// get path '/user/<current user>/file.txt'
Resource resource = loader.getResource("file.txt");
// get path '/user/myuser/file.txt'
Resource resource = loaderWithUser.getResource("file.txt");

// get all paths under '/tmp/'
Resource[] resources = loader.getResources("/tmp/*");
// get all paths under '/tmp/' recursively
Resource[] resources = loader.getResources("/tmp/**/*");
// get all paths under '/tmp/' using more complex ant path matching
Resource[] resources = loader.getResources("/tmp/?ile?.txt");

What would be returned in above examples would be instances of HdfsResources.

If there is a need for Spring Application Context to be aware of HdfsResourceLoader it needs to be registered using hdp:resource-loader-registrar namespace tag.

<hdp:file-system />
<hdp:resource-loader file-system-ref="hadoopFs" handle-noprefix="false" />
<hdp:resource-loader-registrar />

	Note
	On default the HdfsResourceLoader will handle all resource paths without prefix. Attribute handle-noprefix can be used to control this behaviour. If this attribute is set to false, non-prefixed resource uris will be handled by Spring Application Context.

// get 'default.txt' from current user's home directory
Resource[] resources = context.getResources("hdfs:default.txt");
// get all files from hdfs root
Resource[] resources = context.getResources("hdfs:/*");
// let context handle classpath prefix
Resource[] resources = context.getResources("classpath:cfg*properties");

What would be returned in above examples would be instances of HdfsResources and ClassPathResource for the last one. If requesting resource paths without existing prefix, this example would fall back into Spring Application Context. It may be advisable to let HdfsResourceLoader to handle paths without prefix if your application doesn’t rely on loading resources from underlying context without prefixes.

SHDP scripting supports any JSR-223 (also known as javax.scripting) compliant scripting engine. Simply add the engine jar to the classpath and the application should be able to find it. Most languages (such as Groovy or JRuby) provide JSR-233 support out of the box; for those that do not see the scripting project that provides various adapters.

Since Hadoop is written in Java, accessing its APIs in a native way provides maximum control and flexibility over the interaction with Hadoop. This holds true for working with its file systems; in fact all the other tools that one might use are built upon these. The main entry point is the org.apache.hadoop.fs.FileSystem abstract class which provides the foundation of most (if not all) of the actual file system implementations out there. Whether one is using a local, remote or distributed store through the FileSystem API she can query and manipulate the available resources or create new ones. To do so however, one needs to write Java code, compile the classes and configure them which is somewhat cumbersome especially when performing simple, straightforward operations (like copy a file or delete a directory).

JVM scripting languages (such as Groovy, JRuby, Jython or Rhino to name just a few) provide a nice solution to the Java language; they run on the JVM, can interact with the Java code with no or few changes or restrictions and have a nicer, simpler, less ceremonial syntax; that is, there is no need to define a class or a method - simply write the code that you want to execute and you are done. SHDP combines the two, taking care of the configuration and the infrastructure so one can interact with the Hadoop environment from her language of choice.

Let us take a look at a JavaScript example using Rhino (which is part of JDK 6 or higher, meaning one does not need any extra libraries):

<beans xmlns="http://www.springframework.org/schema/beans" ...>
  <hdp:configuration .../>

  <hdp:script id="inlined-js" language="javascript" run-at-startup="true">
    try {load("nashorn:mozilla_compat.js");} catch (e) {} // for Java 8
    importPackage(java.util);

    name = UUID.randomUUID().toString()
    scriptName = "src/test/resources/test.properties"
    //  - FileSystem instance based on 'hadoopConfiguration' bean
    // call FileSystem#copyFromLocal(Path, Path)
    .copyFromLocalFile(scriptName, name)
    // return the file length
    .getLength(name)
  </hdp:script>

</beans>

The script element, part of the SHDP namespace, builds on top of the scripting support in Spring permitting script declarations to be evaluated and declared as normal bean definitions. Furthermore it automatically exposes Hadoop-specific objects, based on the existing configuration, to the script such as the FileSystem (more on that in the next section). As one can see, the script is fairly obvious: it generates a random name (using the UUID class from java.util package) and then copies a local file into HDFS under the random name. The last line returns the length of the copied file which becomes the value of the declaring bean (in this case inlined-js) - note that this might vary based on the scripting engine used.

	Note
	The attentive reader might have noticed that the arguments passed to the FileSystem object are not of type Path but rather String. To avoid the creation of Path object, SHDP uses a wrapper class SimplerFileSystem which automatically does the conversion so you don’t have to. For more information see the implicit variables section.

Note that for inlined scripts, one can use Spring’s property placeholder configurer to automatically expand variables at runtime. Using one of the examples seen before:

<beans ... >
  <context:property-placeholder location="classpath:hadoop.properties" />

  <hdp:script language="javascript" run-at-startup="true">
    ...
    tracker=
    ...
  </hdp:script>
</beans>

Notice how the script above relies on the property placeholder to expand ${hd.fs} with the values from hadoop.properties file available in the classpath.

As you might have noticed, the script element defines a runner for JVM scripts. And just like the rest of the SHDP runners, it allows one or multiple pre and post actions to be specified to be executed before and after each run. Typically other runners (such as other jobs or scripts) can be specified but any JDK Callable can be passed in. Do note that the runner will not run unless triggered manually or if run-at-startup is set to true. For more information on runners, see the dedicated chapter.

<hdp:script location="org/company/basic-script.py" run-at-startup="true"/>

from java.util import UUID
from org.apache.hadoop.fs import Path

print "Home dir is " + str(fs.homeDirectory)
print "Work dir is " + str(fs.workingDirectory)
print "/user exists " + str(fs.exists("/user"))

name = UUID.randomUUID().toString()
scriptName = "src/test/resources/test.properties"
fs.copyFromLocalFile(scriptName, name)
print Path(name).makeQualified(fs)

To ease the interaction of the script with its enclosing context, SHDP binds by default the so-called implicit variables. These are:

	Note
	If no Hadoop Configuration can be detected (either by name hadoopConfiguration or by type), several log warnings will be made and none of the Hadoop-based variables (namely cfg , distcp , fs , fsh , distcp or hdfsRL) will be bound.

As mentioned in the Description column, the variables are first looked (either by name or by type) in the application context and, in case they are missing, created on the spot based on the existing configuration. Note that it is possible to override or add new variables to the scripts through the property sub-element that can set values or references to other beans:

<hdp:script location="org/company/basic-script.js" run-at-startup="true">
   <hdp:property name="foo" value="bar"/>
   <hdp:property name="ref" ref="some-bean"/>
</hdp:script>

<script-tasklet id="script-tasklet">
  <script language="groovy">
    inputPath = "/user/gutenberg/input/word/"
    outputPath = "/user/gutenberg/output/word/"
    if (fsh.test(inputPath)) {
      fsh.rmr(inputPath)
    }
    if (fsh.test(outputPath)) {
      fsh.rmr(outputPath)
    }
    inputFile = "src/main/resources/data/nietzsche-chapter-1.txt"
    fsh.put(inputFile, inputPath)
  </script>
</script-tasklet>

<script-tasklet id="script-tasklet" script-ref="clean-up"/>
    <hdp:script id="clean-up" location="org/company/myapp/clean-up-wordcount.groovy"/>

A handy utility provided by the Hadoop distribution is the file system shell which allows UNIX-like commands to be executed against HDFS. One can check for the existence of files, delete, move, copy directories or files or set up permissions. However the utility is only available from the command-line which makes it hard to use from/inside a Java application. To address this problem, SHDP provides a lightweight, fully embeddable shell, called FsShell which mimics most of the commands available from the command line: rather than dealing with System.in or System.out, one deals with objects.

Let us take a look at using FsShell by building on the previous scripting examples:

<hdp:script location="org/company/basic-script.groovy" run-at-startup="true"/>

name = UUID.randomUUID().toString()
scriptName = "src/test/resources/test.properties"
fs.copyFromLocalFile(scriptName, name)

// use the shell made available under variable
dir = "script-dir"
if (!fsh.test(dir)) {
   fsh.mkdir(dir); fsh.cp(name, dir); fsh.chmodr(700, dir)
   println "File content is " + fsh.cat(dir + name).toString()
}
println fsh.ls(dir).toString()
fsh.rmr(dir)

As mentioned in the previous section, a FsShell instance is automatically created and configured for scripts, under the name fsh. Notice how the entire block relies on the usual commands: test, mkdir, cp and so on. Their semantics are exactly the same as in the command-line version however one has access to a native Java API that returns actual objects (rather than String`s) making it easy to use them programmatically whether in Java or another language. Furthermore, the class offers enhanced methods (such as `chmodr which stands for recursive chmod) and multiple overloaded methods taking advantage of varargs so that multiple parameters can be specified. Consult the API for more information.

To be as close as possible to the command-line shell, FsShell mimics even the messages being displayed. Take a look at line 9 which prints the result of fsh.cat(). The method returns a Collection of Hadoop Path objects (which one can use programatically). However when invoking toString on the collection, the same printout as from the command-line shell is being displayed:

File content is

The same goes for the rest of the methods, such as ls. The same script in JRuby would look something like this:

require 'java'
name = java.util.UUID.randomUUID().to_s
scriptName = "src/test/resources/test.properties"
$fs.copyFromLocalFile(scriptName, name)

# use the shell
dir = "script-dir/"
...
print $fsh.ls(dir).to_s

which prints out something like this:

drwx------   - user     supergroup          0 2012-01-26 14:08 /user/user/script-dir
-rw-r--r--   3 user     supergroup        344 2012-01-26 14:08 /user/user/script-dir/520cf2f6-a0b6-427e-a232-2d5426c2bc4e

As you can see, not only can you reuse the existing tools and commands with Hadoop inside SHDP, but you can also code against them in various scripting languages. And as you might have noticed, there is no special configuration required - this is automatically inferred from the enclosing application context.

Note

The careful reader might have noticed that besides the syntax, there are some minor differences in how the various languages interact with the java objects. For example the automatic toString call called in Java for doing automatic String conversion is not necessarily supported (hence the to_s in Ruby or str in Python). This is to be expected as each language has its own semantics - for the most part these are easy to pick up but do pay attention to details.

<hdp:script language="groovy">distcp.copy("${distcp.src}", "${distcp.dst}")</hdp:script>

Native store abstractions provide various writer and reader interfaces so that the end user don’t have to worry about the underlying implementation actually doing the work on files in HDFS. Implementations are usually strongly typed and provides constructors and setters for additional setup to work with naming, compression codecs and everything else defining the behaviour. Interfaces are meant to be used from integration components which don’t need to know the internal workings of writers and readers.

6.1.1 Writing Data

Main interface writing into a store is a DataWriter which have one method write which simply writes an entity and the backing implementation will handle the rest.

public interface DataWriter<T> {
  void write(T entity) throws IOException;
}

The DataStoreWriter interface adds methods to close and flush a writer. Some of the writers have a property to close a stream after an idle time or a close time has been reached but generally this interface is meant for programmatic control of these operations.

public interface DataStoreWriter<T> extends DataWriter<T>, Flushable, Closeable {
}

File Naming

Different file naming strategies are used to automatically determine the name of a file to be used. Writers without additional naming configuration will usually use a given base path as is. As soon as any type of a strategy is configured, given base path is considered to be a base directory and the name of the file is resolved by file naming strategies.

For example, if defined base path is “/tmp/path” and the StaticFileNamingStrategy with “data” parameter is used then the actual file path resolved would be “/tmp/path/data”.

Path path = new Path("/tmp/path");
Configuration config = new Configuration();
TextFileWriter writer = new TextFileWriter(config, path, null);
StaticFileNamingStrategy fileNamingStrategy = new StaticFileNamingStrategy("data")
writer.setFileNamingStrategy(fileNamingStrategy);

At first look this may feel a little complicated, but it will make sense after more file naming strategies are added. These will also provide facilities for using writers in parallel, or for a re-launched writer to be able to create a new file based on already existing files in the directry. For example, RollingFileNamingStrategy will add a simple increasing value to a file name and will try to initialize itself with the correct position.

Built-in strategies currently supported are StaticFileNamingStrategy, RollingFileNamingStrategy, UuidFileNamingStrategy and CodecFileNamingStrategy. ChainedFileNamingStrategy can be used to chain multiple strategies together where each individual strategy will provide its own part.

File Rollover

File rolling strategy is used to determine a condition in a writer when a current stream should be automatically closed and the next file should be opened. This is usually done together with RollingFileNamingStrategy to rollover when a certain file size limit has been reached.

Currently, only one strategy SizeRolloverStrategy is supported.

Partitioning

Partitioning is a concept of choosing a target file on demand either based on content to be written or any other information available to a writer at the time of the write operation. While it would be perfectly alright to use multiple writers manually, the framework already does all the heavy lifting around partitioning. We work through interfaces and provide a generic default implementation still allowing to plug a customized version if there’s a need for it.

PartitionStrategy is a strategy interface defining PartitionResolver and PartitionKeyResolver.

public interface PartitionStrategy<T,K> {
  PartitionResolver<K> getPartitionResolver();
  PartitionKeyResolver<T, K> getPartitionKeyResolver();
}

PartitionResolver is an interface used to resolve arbitrary partition keys into a path. We don’t force any specific partition key type in the interface level itself but usually the implementation needs to be aware of its type.

public interface PartitionResolver<K> {
  Path resolvePath(K partitionKey);
}

PartitionKeyResolver is an interface which is responsible for creating a partition key from an entity. This is needed because writer interfaces allow us to write entities without an explicit partition key.

public interface PartitionKeyResolver<T, K> {
  K resolvePartitionKey(T entity);
}

PartitionDataStoreWriter is an extension of DataStoreWriter adding a method to write an entity with a partition key. In this context the partition key is something what the partition strategy is able to use.

public interface PartitionDataStoreWriter<T,K> extends DataStoreWriter<T> {
  void write(T entity, K partitionKey) throws IOException;
}

DefaultPartitionStrategy

DefaultPartitionStrategy is a generic default implementation meant to be used together with an expression using Spring’s SpEL expression language. PartitionResolver used in DefaultPartitionStrategy expects partition key to be a type of Map<String,Object> and partition key created by PartitionKeyResolver is a DefaultPartitionKey which itself is a Map<String,Object>.

In order to make it easy to work with SpEL and partitioning, map values can be directly accessed with keys and additional partitioning methods has been registered.

Partition Path Expression

SpEL expression is evaluated against a partition key passed into a HDFS writer.

Accessing Properties

If partition key is a type of Map any property given to a SpEL expression is automatically resolved from a map.

Custom Methods

In addition to normal SpEL functionality, a few custom methods have been added to make it easier to build partition paths. These custom methods can be used to work with normal partition concepts like date formatting, lists, ranges and hashes.

path

path(String... paths)

You can concatenate paths together with a / delimiter. This method can be used to make the expression less verbose than using a native SpEL functionality to combine path parts together. To create a path part1/part2, expression 'part1' + '/' + 'part2' is equivalent to path('part1','part2').

Parameters

paths

Any number of path parts

Return Value

Concatenated value of paths delimited with /.

dateFormat

dateFormat(String pattern)
dateFormat(String pattern, Long epoch)
dateFormat(String pattern, Date date)
dateFormat(String pattern, String datestring)
dateFormat(String pattern, String datestring, String dateformat)

Creates a path using date formatting. Internally this method delegates to SimpleDateFormat and needs a Date and a pattern.

Method signature with three parameters can be used to create a custom Date object which is then passed to SimpleDateFormat conversion using a dateformat pattern. This is useful in use cases where partition should be based on a date or time string found from a payload content itself. Default dateformat pattern if omitted is yyyy-MM-dd.

Parameters

pattern

Pattern compatible with SimpleDateFormat to produce a final output.

epoch

Timestamp as Long which is converted into a Date.

date

A Date to be formatted.

dateformat

Secondary pattern to convert datestring into a Date.

datestring

Date as a String

Return Value

A path part representation which can be a simple file or directory name or a directory structure.

list

list(Object source, List<List<Object>> lists)

Creates a partition path part by matching a source against a lists denoted by lists.

Lets assume that data is being written and it’s possible to extract an appid from the content. We can automatically do a list based partition by using a partition method list(appid,\{\{'1TO3','APP1','APP2','APP3'},\{'4TO6','APP4','APP5','APP6'}}). This method would create three partitions, 1TO3_list, 4TO6_list and list. The latter is used if no match is found from partition lists passed to lists.

Parameters

source

An Object to be matched against lists.

lists

A definition of list of lists.

Return Value

A path part prefixed with a matched key i.e. XXX_list or list if no match.

range

range(Object source, List<Object> list)

Creates a partition path part by matching a source against a list denoted by list using a simple binary search.

The partition method takes source as first argument and a list as the second argument. Behind the scenes this is using the JVM’s binarySearch which works on an Object level so we can pass in anything. Remember that meaningful range match only works if passed in Object and types in list are of same type like Integer. Range is defined by a binarySearch itself so mostly it is to match against an upper bound except the last range in a list. Having a list of \{1000,3000,5000} means that everything above 3000 will be matched with 5000. If that is an issue then simply adding Integer.MAX_VALUE as last range would overflow everything above 5000 into a new partition. Created partitions would then be 1000_range, 3000_range and 5000_range.

Parameters

source

An Object to be matched against list.

list

A definition of list.

Return Value

A path part prefixed with a matched key i.e. XXX_range.

hash

hash(Object source, int bucketcount)

Creates a partition path part by calculating hashkey using source`s hashCode and bucketcount. Using a partition method hash(timestamp,2) would then create partitions named 0_hash, 1_hash and 2_hash. Number suffixed with hash is simply calculated using _Object.hashCode() % bucketcount.

Parameters

source

An Object which hashCode will be used.

bucketcount

A number of buckets

Return Value

A path part prefixed with a hash key i.e. XXX_hash.

Creating a Custom Partition Strategy

Creating a custom partition strategy is as easy as just implementing needed interfaces. Custom strategy may be needed in use cases where it is just not feasible to use SpEL expressions. This will then give total flexibility to implement partitioning as needed.

Below sample demonstrates how a simple customer id could be used as a base for partitioning.

public class CustomerPartitionStrategy implements PartitionStrategy<String, String> {

  CustomerPartitionResolver partitionResolver = new CustomerPartitionResolver();
  CustomerPartitionKeyResolver keyResolver = new CustomerPartitionKeyResolver();

  @Override
  public PartitionResolver<String> getPartitionResolver() {
    return partitionResolver;
  }

  @Override
  public PartitionKeyResolver<String, String> getPartitionKeyResolver() {
    return keyResolver;
  }
}

public class CustomerPartitionResolver implements PartitionResolver<String> {

  @Override
  public Path resolvePath(String partitionKey) {
    return new Path(partitionKey);
  }
}

public class CustomerPartitionKeyResolver implements PartitionKeyResolver<String, String> {

  @Override
  public String resolvePartitionKey(String entity) {
    if (entity.startsWith("customer1")) {
      return "customer1";
    } else if (entity.startsWith("customer2")) {
      return "customer2";
    } else if (entity.startsWith("customer3")) {
      return "customer3";
    }
    return null;
  }
}

Writer Implementations

We provide a number of writer implementations to be used based on the type of file to write.

• TextFileWriter. an implementation meant to write a simple text data where entities are separated by a delimiter. Simple example for this is a text file with line terminations.
• DelimitedTextFileWriter. an extension atop of TextFileWriter where written entity itself is also delimited. Simple example for this is a csv file.
• TextSequenceFileWriter. a similar implementation to TextFileWriter except that backing file is a Hadoop’s SequenceFile.
• PartitionTextFileWriter. wraps multiple TextFileWriters providing automatic partitioning functionality.

Append and Sync Data

HDFS client library which is usually referred as a DFS Client is using a rather complex set of buffers to make writes fast. Using a compression codec adds yet another internal buffer. One big problem with these buffers is that if a jvm suddenly dies bufferred data is naturally lost.

With TextFileWriter and TextSequenceFileWriter it is possible to enable either append or syncable mode which effectively is causing our store libraries to call sync method which will flush buffers from a client side into a currently active datanodes.

	Note
	Appending or synching data will be considerably slower than a normal write. It is always a trade-off between fast write and data integrity. Using append or sync with a compression is also problematic because it’s up to a codec implementation when it can actually flush its own data to a datanode.

public interface DataReader<T> {
  T read() throws IOException;
}

public interface DataStoreReader<T> extends DataReader<T>, Closeable {
}

public interface Split {
  long getStart();
  long getLength();
  long getEnd();
}

public interface Splitter {
  List<Split> getSplits(Path path) throws IOException;
}

6.1.3 Using Codecs

Supported compression codecs are denoted via an interface CodecInfo which simply defines if codec supports splitting, what is it’s fully qualified java class and what is its default file suffix.

public interface CodecInfo {
  boolean isSplittable();
  String getCodecClass();
  String getDefaultSuffix();
}

Codecs provides an enum for easy access to supported codecs.

• GZIP - org.apache.hadoop.io.compress.GzipCodec
• SNAPPY - org.apache.hadoop.io.compress.SnappyCodec
• BZIP2 - org.apache.hadoop.io.compress.BZip2Codec
• LZO - com.hadoop.compression.lzo.LzoCodec
• LZOP - com.hadoop.compression.lzo.LzopCodec

	Note
	Lzo based compression codecs doesn’t exist in maven dependencies due to licensing restrictions and need for native libraries. Order to use it add codec classes to classpath and its native libs using java.library.path.

One common requirement is to persist a large number of POJOs in serialized form using HDFS. The Kite SDK project provides a Kite Data Module that provides an API for working with datasets stored in HDFS. We are using this functionality and provide a some simple helper classes to aid in configuration and use in a Spring environment.

6.2.1 Data Formats

The Kite SDK project provides support for writing data using both the Avro and Parquet data formats. The data format you choose to use influences the data types you can use in your POJO classes. We’ll discuss the basics of the Java type mapping for the two data formats but we recommend that you consult each project’s documentation for additional details.

	Note
	Currently, you can’t provide your own schema. This is something that we are considering changing in upcomming releases. We are also planning to provide better mapping support in line with the support we currently provide for NoSQL stores like MongoDB.

Using Avro

When using Avro as the data format the schema generation is based on reflection of thet POJO class used. Primitive data types and their corresponding wrapper classes are mapped to the corresponding Avro data type. More complex types, as well as the POJO itself, are mapped to a record type consisting of one or more fields.

The table below shows the mapping from some common types:

Table 6.1. Some common Java to Avro data types mapping

Java type	Avro type	Comment
String	string	[multiblock cell omitted]
int / Integer	int	32-bit signed integer
long / Long	long	64-bit signed integer
float / Float	float	32-bit floating point
double / Double	double	64-bit floating point
boolean / Boolean	boolean	[multiblock cell omitted]
byte[]	bytes	byte array
java.util.Date	record	[multiblock cell omitted]

Using Parquet

When using Parquet as the data format the schema generation is based on reflection of thet POJO class used. The POJO class must be a proper JavaBean and not have any nested types. We only support primitive data types and their corresponding wrapper classes plus byte arrays. We do rely on the Avro-to-Parquet mapping support that the Kite SDK uses, so the schema will be generated by Avro.

	Note
	The Parquet support we currently povide is considered experimental. We are planning to relax a lot of the restrictions on the POJO class in upcoming releases.

The table below shows the mapping from some common types:

Table 6.2. Some common Java to Parquet data types mapping

Java type	Parquet type	Comment
String	BINARY/UTF8	[multiblock cell omitted]
int / Integer	INT32	32-bit signed integer
long / Long	INT64	64-bit signed integer
float / Float	FLOAT	32-bit floating point
double / Double	DOUBLE	64-bit floating point
boolean / Boolean	BOOLEAN	[multiblock cell omitted]
byte[]	BINARY/BYTE_ARRAY	byte array

@Configuration
@ImportResource("hadoop-context.xml")
public class DatasetConfig {

  private @Autowired org.apache.hadoop.conf.Configuration hadoopConfiguration;

  @Bean
  public DatasetRepositoryFactory datasetRepositoryFactory() {
    DatasetRepositoryFactory datasetRepositoryFactory = new DatasetRepositoryFactory();
    datasetRepositoryFactory.setConf(hadoopConfiguration);
    datasetRepositoryFactory.setBasePath("/user/spring");
    return datasetRepositoryFactory;
  }

  @Bean
  public DatasetDefinition fileInfoDatasetDefinition() {
    DatasetDefinition definition = new DatasetDefinition();
    definition.setFormat(Formats.AVRO.getName());
    definition.setTargetClass(FileInfo.class);
    definition.setAllowNullValues(false);
    return definition;
  }
}

6.2.3 Writing datasets

To write datasets to Hadoop you should use either the AvroPojoDatasetStoreWriter or the ParquetDatasetStoreWriter depending on the data format you want to use.

	Tip
	To mark your fields as nullable use the @Nullable annotation (org.apache.avro.reflect.Nullable). This will result in the schema defining your field as a union of null and your datatype.

We are using a FileInfo POJO that we have defined to hold some information based on the files we read from our local file system. The dataset will be stored in a directory that is the name of the class using lowercase, so in this case it would be fileinfo. This directory is placed inside the basePath specified in the configuration of the DatasetRepositoryFactory.:

package org.springframework.samples.hadoop.dataset;

import org.apache.avro.reflect.Nullable;

public class FileInfo {
  private String name;
  private @Nullable String path;
  private long size;
  private long modified;

  public FileInfo(String name, String path, long size, long modified) {
    this.name = name;
    this.path = path;
    this.size = size;
    this.modified = modified;
  }

  public FileInfo() {
  }

  public String getName() {
      return name;
  }

  public String getPath() {
      return path;
  }

  public long getSize() {
      return size;
  }

  public long getModified() {
      return modified;
  }
}

To create a writer add the following bean definition to your configuration class:

  @Bean
  public DataStoreWriter<FileInfo> dataStoreWriter() {
    return new AvroPojoDatasetStoreWriter<FileInfo>(FileInfo.class,
        datasetRepositoryFactory(), fileInfoDatasetDefinition());
  }

Next, have your class use the writer bean:

    private DataStoreWriter<FileInfo> writer;

    @Autowired
    public void setDataStoreWriter(DataStoreWriter dataStoreWriter) {
        this.writer = dataStoreWriter;
    }

Now we can use the writer, it will be opened automatically once we start writing to it:

      FileInfo fileInfo = new FileInfo(file.getName(),
          file.getParent(), (int)file.length(), file.lastModified());
      writer.write(fileInfo);

Once we are done writing we should close the writer:

    try {
      writer.close();
    } catch (IOException e) {
      throw new StoreException("Error closing FileInfo", e);
    }

We should now have dataset containing all the FileInfo entries in a /user/spring/demo/fileinfo directory:

$ hdfs dfs -ls /user/spring/*
Found 2 items
drwxr-xr-x   - spring supergroup          0 2014-06-09 17:09 /user/spring/fileinfo/.metadata
-rw-r--r--   3 spring supergroup   13824695 2014-06-09 17:10 /user/spring/fileinfo/6876f250-010a-404a-b8c8-0ce1ee759206.avro

The .metadata directory contains dataset information including the Avro schema:

$ hdfs dfs -cat /user/spring/fileinfo/.metadata/schema.avsc
{
  "type" : "record",
  "name" : "FileInfo",
  "namespace" : "org.springframework.samples.hadoop.dataset",
  "fields" : [ {
    "name" : "name",
    "type" : "string"
  }, {
    "name" : "path",
    "type" : [ "null", "string" ],
    "default" : null
  }, {
    "name" : "size",
    "type" : "long"
  }, {
    "name" : "modified",
    "type" : "long"
  } ]
}

 @Bean
  public DatasetOperations datasetOperations() {
    DatasetTemplate datasetOperations = new DatasetTemplate();
    datasetOperations.setDatasetRepositoryFactory(datasetRepositoryFactory());
    return datasetOperations;
  }

  private DatasetOperations datasetOperations;

  @Autowired
  public void setDatasetOperations(DatasetOperations datasetOperations) {
      this.datasetOperations = datasetOperations;
  }

        final AtomicLong count = new AtomicLong();
        datasetOperations.read(FileInfo.class, new RecordCallback<FileInfo>() {
            @Override
            public void doInRecord(FileInfo record) {
                count.getAndIncrement();
            }
        });
        System.out.println("File count: " + count.get());

DatasetDefinition definition = new DatasetDefinition();
definition.setPartitionStrategy(new PartitionStrategy.Builder().year("modified").build());

identity("region", "R", String.class, 10).year("timestamp").month("timestamp")

Spring Hadoop doesn’t have support for configuring store components using xml but have a support using JavaConfig for writer configuration.

JavaConfig is using same concepts found from other parts of a Spring Hadoop where whole configuration logic works around use of an adapter.

@Configuration
@EnableDataStoreTextWriter
static class Config
    extends SpringDataStoreTextWriterConfigurerAdapter {

  @Override
  public void configure(DataStoreTextWriterConfigurer config)
      throws Exception {
    config
      .basePath("/tmp/foo");
  }
}

What happened in above example:

We can also do configuration for other usual properties like, idleTimeout, closeTimeout, partitioning strategy, naming strategy and rollover strategy.

@Configuration
@EnableDataStoreTextWriter
static class Config
    extends SpringDataStoreTextWriterConfigurerAdapter {

  @Override
  public void configure(DataStoreTextWriterConfigurer config)
      throws Exception {
    config
      .basePath("/tmp/store")
      .idleTimeout(60000)
      .closeTimeout(120000)
      .inWritingSuffix(".tmp")
      .withPartitionStrategy()
        .map("dateFormat('yyyy/MM/dd/HH/mm', timestamp)")
        .and()
      .withNamingStrategy()
        .name("data")
        .uuid()
        .rolling()
        .name("txt", ".")
        .and()
      .withRolloverStrategy()
        .size("1M");
  }
}

What happened in above example:

Writer can be auto-wired using DataStoreWriter.

	Important
	Autowiring by type PartitionDataStoreWriter only works if adapter is used with annotation @EnableDataStorePartitionTextWriter which will introduce a correct bean type.

static class MyBean {

  @Autowired
  DataStoreWriter<String> writer;

  @Autowired
  PartitionDataStoreWriter<String, Map<String, Object>> writer;
}

In some cases it is more convenient to name the bean instead letting Spring to create that name automatically. @EnableDataStoreTextWriter and @EnableDataStorePartitionTextWriter both have a name field which works in a same way than normal Spring @Bean annotation. You’d use this custom naming in cases where multiple writers are created and auto-wiring by type would no longer work.

@Configuration
@EnableDataStoreTextWriter(name={"mywriter", "myalias"})
static class Config
    extends SpringDataStoreTextWriterConfigurerAdapter {
}

In above example bean was created with a name mywriter having an alias named myalias.

One of the most popular and powerful feature in Spring Framework is the Data Access Object (or DAO) support. It makes dealing with data access technologies easy and consistent allowing easy switch or interconnection of the aforementioned persistent stores with minimal friction (no worrying about catching exceptions, writing boiler-plate code or handling resource acquisition and disposal). Rather than reiterating here the value proposal of the DAO support, we recommend the JDBC section in the Spring Framework reference documentation

SHDP provides the same functionality for Apache HBase through its org.springframework.data.hadoop.hbase package: an HbaseTemplate along with several callbacks such as TableCallback, RowMapper and ResultsExtractor that remove the low-level, tedious details for finding the HBase table, run the query, prepare the scanner, analyze the results then clean everything up, letting the developer focus on her actual job (users familiar with Spring should find the class/method names quite familiar).

At the core of the DAO support lies HbaseTemplate - a high-level abstraction for interacting with HBase. The template requires an HBase configuration, once it’s set, the template is thread-safe and can be reused across multiple instances at the same time:

// default HBase configuration
<hdp:hbase-configuration/>

// wire hbase configuration (using default name 'hbaseConfiguration') into the template
<bean id="htemplate" class="org.springframework.data.hadoop.hbase.HbaseTemplate" p:configuration-ref="hbaseConfiguration"/>

The template provides generic callbacks, for executing logic against the tables or doing result or row extraction, but also utility methods (the so-called _one-liner_s) for common operations. Below are some examples of how the template usage looks like:

// writing to 'MyTable'
template.execute("MyTable", new TableCallback<Object>() {
  @Override
  public Object doInTable(HTable table) throws Throwable {
    Put p = new Put(Bytes.toBytes("SomeRow"));
    p.add(Bytes.toBytes("SomeColumn"), Bytes.toBytes("SomeQualifier"), Bytes.toBytes("AValue"));
    table.put(p);
    return null;
  }
});

// read each row from 'MyTable'
List<String> rows = template.find("MyTable", "SomeColumn", new RowMapper<String>() {
  @Override
  public String mapRow(Result result, int rowNum) throws Exception {
    return result.toString();
  }
}));

The first snippet showcases the generic TableCallback - the most generic of the callbacks, it does the table lookup and resource cleanup so that the user code does not have to. Notice the callback signature - any exception thrown by the HBase API is automatically caught, converted to Spring’s DAO exceptions and resource clean-up applied transparently. The second example, displays the dedicated lookup methods - in this case find which, as the name implies, finds all the rows matching the given criteria and allows user code to be executed against each of them (typically for doing some sort of type conversion or mapping). If the entire result is required, then one can use ResultsExtractor instead of RowMapper.

Besides the template, the package offers support for automatically binding HBase table to the current thread through HbaseInterceptor and HbaseSynchronizationManager. That is, each class that performs DAO operations on HBase can be wrapped by HbaseInterceptor so that each table in use, once found, is bound to the thread so any subsequent call to it avoids the lookup. Once the call ends, the table is automatically closed so there is no leakage between requests. Please refer to the Javadocs for more information.

The new HiveServer2 now supports multi-user access and is typically run in the Hadoop cluster. See the Hive Project for details.

We provide a dedicated namespace element for configuring a Hive client (that is Hive accessing a server node through JDBC). You also need a hiveDataSource using the JDBC driver for HiveServer2:

<!-- by default, the definition name is 'hiveClientFactory' -->
<hive-client-factory id="hiveClientFactory" hive-data-source-ref="hiveDataSource"/>
<beans:bean id="hiveDriver" class="org.apache.hive.jdbc.HiveDriver"/>
<beans:bean id="hiveDataSource" class="org.springframework.jdbc.datasource.SimpleDriverDataSource">
  <beans:constructor-arg name="driver" ref="hiveDriver"/>
  <beans:constructor-arg name="url" value="jdbc:hive2://localhost:1000"/>
</beans:bean>

Like the rest of the Spring Hadoop components, a runner is provided out of the box for executing Hive scripts, either inlined or from various locations through hive-runner element:

<hdp:hive-runner id="hiveRunner" run-at-startup="true">
   <hdp:script>
     DROP TABLE IF EXITS testHiveBatchTable;
     CREATE TABLE testHiveBatchTable (key int, value string);
   </hdp:script>
   <hdp:script location="hive-scripts/script.q"/>
</hdp:hive-runner>

The runner will trigger the execution during the application start-up (notice the run-at-startup flag which is by default false). Do note that the runner will not run unless triggered manually or if run-at-startup is set to true. Additionally the runner (as in fact do all runners in SHDP) allows one or multiple pre and post actions to be specified to be executed before and after each run. Typically other runners (such as other jobs or scripts) can be specified but any JDK Callable can be passed in. For more information on runners, see the dedicated chapter.

<hdp:hive-tasklet id="hive-script">
   <hdp:script>
     DROP TABLE IF EXITS testHiveBatchTable;
     CREATE TABLE testHiveBatchTable (key int, value string);
   </hdp:script>
   <hdp:script location="classpath:org/company/hive/script.q" />
</hdp:hive-tasklet>

For those that need to programmatically interact with the Hive API, Spring for Apache Hadoop provides a dedicated template, similar to the aforementioned JdbcTemplate. The template handles the redundant, boiler-plate code, required for interacting with Hive such as creating a new HiveClient, executing the queries, catching any exceptions and performing clean-up. One can programmatically execute queries (and get the raw results or convert them to longs or ints) or scripts but also interact with the Hive API through the HiveClientCallback. For example:

<hdp:hive-client-factory ... />
<!-- Hive template wires automatically to 'hiveClientFactory'-->
<hdp:hive-template />

<!-- wire hive template into a bean -->
<bean id="someBean" class="org.SomeClass" p:hive-template-ref="hiveTemplate"/>

public class SomeClass {

  private HiveTemplate template;

  public void setHiveTemplate(HiveTemplate template) { this.template = template; }

  public List<String> getDbs() {
      return hiveTemplate.execute(new HiveClientCallback<List<String>>() {
         @Override
         public List<String> doInHive(HiveClient hiveClient) throws Exception {
            return hiveClient.get_all_databases();
         }
      }));
  }
}

The example above shows a basic container configuration wiring a HiveTemplate into a user class which uses it to interact with the HiveClient Thrift API. Notice that the user does not have to handle the lifecycle of the HiveClient instance or catch any exception (out of the many thrown by Hive itself and the Thrift fabric) - these are handled automatically by the template which converts them, like the rest of the Spring templates, into `DataAccessException`s. Thus the application only has to track only one exception hierarchy across all data technologies instead of one per technology.

Like the rest of the Spring Hadoop components, a runner is provided out of the box for executing Pig scripts, either inlined or from various locations through pig-runner element:

<hdp:pig-runner id="pigRunner" run-at-startup="true">
   <hdp:script>
        A = LOAD 'src/test/resources/logs/apache_access.log' USING PigStorage() AS (name:chararray, age:int);
        ...
   </hdp:script>
   <hdp:script location="pig-scripts/script.pig"/>
</hdp:pig-runner>

The runner will trigger the execution during the application start-up (notice the run-at-startup flag which is by default false). Do note that the runner will not run unless triggered manually or if run-at-startup is set to true. Additionally the runner (as in fact do all runners in SHDP) allows one or multiple pre and post actions to be specified to be executed before and after each run. Typically other runners (such as other jobs or scripts) can be specified but any JDK Callable can be passed in. For more information on runners, see the dedicated chapter.

<hdp:pig-tasklet id="pig-script">
   <hdp:script location="org/company/pig/handsome.pig" />
</hdp:pig-tasklet>

For those that need to programmatically interact directly with Pig , Spring for Apache Hadoop provides a dedicated template, similar to the aforementioned HiveTemplate. The template handles the redundant, boiler-plate code, required for interacting with Pig such as creating a new PigServer, executing the scripts, catching any exceptions and performing clean-up. One can programmatically execute scripts but also interact with the Hive API through the PigServerCallback. For example:

<hdp:pig-factory ... />
<!-- Pig template wires automatically to 'pigFactory'-->
<hdp:pig-template />

<!-- use component scanning-->
<context:component-scan base-package="some.pkg" />

public class SomeClass {
  @Inject
  private PigTemplate template;

  public Set<String> getDbs() {
      return pigTemplate.execute(new PigCallback<Set<String>() {
         @Override
         public Set<String> doInPig(PigServer pig) throws ExecException, IOException {
            return pig.getAliasKeySet();
         }
      });
  }
}

The example above shows a basic container configuration wiring a PigTemplate into a user class which uses it to interact with the PigServer API. Notice that the user does not have to handle the lifecycle of the PigServer instance or catch any exception - these are handled automatically by the template which converts them, like the rest of the Spring templates, into `DataAccessException`s. Thus the application only has to track only one exception hierarchy across all data technologies instead of one per technology.

HDFS layer provides file permissions designed to be similar to those present in *nix OS. The official guide explains the major components but in short, the access for each file (whether it’s for reading, writing or in case of directories accessing) can be restricted to certain users or groups. Depending on the user identity (which is typically based on the host operating system), code executing against the Hadoop cluster can see or/and interact with the file-system based on these permissions. Do note that each HDFS or FileSystem implementation can have slightly different semantics or implementation.

SHDP obeys the HDFS permissions, using the identity of the current user (by default) for interacting with the file system. In particular, the HdfsResourceLoader considers when doing pattern matching, only the files that it’s supposed to see and does not perform any privileged action. It is possible however to specify a different user, meaning the ResourceLoader interacts with HDFS using that user’s rights - however this obeys the #security:kerberos[user impersonation] rules. When using different users, it is recommended to create separate ResourceLoader instances (one per user) instead of assigning additional permissions or groups to one user - this makes it easier to manage and wire the different HDFS views without having to modify the ACLs. Note however that when using impersonation, the ResourceLoader might (and will typically) return restricted files that might not be consumed or seen by the callee.

Securing a Hadoop cluster can be a difficult task - each machine can have a different set of users and groups, each with different passwords. Hadoop relies on Kerberos, a ticket-based protocol for allowing nodes to communicate over a non-secure network to prove their identity to one another in a secure manner. Unfortunately there is not a lot of documentation on this topic out there. However there are some resources to get you started.

SHDP does not require any extra configuration - it simply obeys the security system in place. By default, when running inside a secure Hadoop, SHDP uses the current user (as expected). It also supports user impersonation, that is, interacting with the Hadoop cluster with a different identity (this allows a superuser to submit job or access hdfs on behalf of another user in a secure way, without leaking permissions). The major MapReduce components, such as job, streaming and tool as well as pig support user impersonation through the user attribute. By default, this property is empty, meaning the current user is used - however one can specify the different identity (also known as ugi) to be used by the target component:

<hdp:job id="jobFromJoe" user="joe" .../>

Note that the user running the application (or the current user) must have the proper kerberos credentials to be able to impersonate the target user (in this case joe).

To simplify configuration, SHDP provides a dedicated namespace for Yarn components. However, one can opt to configure the beans directly through the usual <bean> definition. For more information about XML Schema-based configuration in Spring, see this appendix in the Spring Framework reference documentation.

To use the SHDP namespace, one just needs to import it inside the configuration:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:yarn="http://www.springframework.org/schema/yarn"
  xmlns:yarn-int="http://www.springframework.org/schema/yarn/integration"
  xmlns:yarn-batch="http://www.springframework.org/schema/yarn/batch"
  xsi:schemaLocation="
    http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/yarn
    http://www.springframework.org/schema/yarn/spring-yarn.xsd
    http://www.springframework.org/schema/yarn/integration
    http://www.springframework.org/schema/yarn/integration/spring-yarn-integration.xsd
    http://www.springframework.org/schema/yarn/batch
    http://www.springframework.org/schema/yarn/batch/spring-yarn-batch.xsd">

  <bean id ... >

  <yarn:configuration ...>
</beans>

Once declared, the namespace elements can be declared simply by appending the aforementioned prefix. Note that is possible to change the default namespace, for example from <beans> to <yarn>. This is useful for configuration composed mainly of Hadoop components as it avoids declaring the prefix. To achieve this, simply swap the namespace prefix declaration above:

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans
xmlns="http://www.springframework.org/schema/yarn"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:beans="http://www.springframework.org/schema/beans"
  xsi:schemaLocation="
    http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/yarn
    http://www.springframework.org/schema/yarn/spring-yarn.xsd">

    <beans:bean id ... >

    <configuration ...>

</beans:beans>

It is also possible to work without XML configuration and rely on Annotation based configuration model. XML and JavaConfig for Spring YARN are not full replacement for each others but we try to mimic the behaviour as much as we can.

We basically rely on two concepts when working with JavaConfig. Firstly an annotation @EnableYarn is used to activate different parts of a Spring Configuration depending on enable attribute. We can enable configuration for CONTAINER, APPMASTER or CLIENT. Secondly when configuration is enabled one can use SpringYarnConfigurerAdapter whose callback methods can be used to do further configuration for components familiar from XML.

@Configuration
@EnableYarn(enable=Enable.CONTAINER)
public class ContainerConfiguration extends SpringYarnConfigurerAdapter {

  @Override
  public void configure(YarnContainerConfigurer container) throws Exception {
    container
      .containerClass(MultiContextContainer.class);
  }

}

In above example we enabled configuration for CONTAINER and used SpringYarnConfigurerAdapter and its configure callback method for YarnContainerConfigurer. In this method we instructed container class to be a MultiContextContainer.

@Configuration
@EnableYarn(enable=Enable.APPMASTER)
public class AppmasterConfiguration extends SpringYarnConfigurerAdapter {

  @Override
  public void configure(YarnAppmasterConfigurer master) throws Exception {
    master
      .withContainerRunner();
  }

}

In above example we enabled configuration for APPMASTER and because of this a callback method for YarnAppmasterConfigurer is called automatically.

@Configuration
@EnableYarn(enable=Enable.CLIENT)
@PropertySource("classpath:hadoop.properties")
public class ClientConfiguration extends SpringYarnConfigurerAdapter {

  @Autowired
  private Environment env;

  @Override
  public void configure(YarnConfigConfigurer config) throws Exception {
    config
      .fileSystemUri(env.getProperty("hd.fs"))
      .resourceManagerAddress(env.getProperty("hd.rm"));
  }

  @Override
  public void configure(YarnClientConfigurer client) throws Exception {
    Properties arguments = new Properties();
    arguments.put("container-count", "4");
    client
      .appName("multi-context-jc")
      .withMasterRunner()
        .contextClass(AppmasterConfiguration.class)
        .arguments(arguments);
}

In above example we enabled configuration for CLIENT. Here one will get yet another callback for YarnClientConfigurer. Additionally this shows how a Hadoop configuration can be customized using a callback for YarnConfigConfigurer.

In order to use Hadoop and Yarn, one needs to first configure it namely by creating a YarnConfiguration object. The configuration holds information about the various parameters of the Yarn system.

	Note
	Configuration for <yarn:configuration> looks very similar than <hdp:configuration>. Reason for this is a simple separation for Hadoop’s YarnConfiguration and JobConf classes.

In its simplest form, the configuration definition is a one liner:

<yarn:configuration />

The declaration above defines a YarnConfiguration bean (to be precise a factory bean of type ConfigurationFactoryBean) named, by default, yarnConfiguration. The default name is used, by conventions, by the other elements that require a configuration - this leads to simple and very concise configurations as the main components can automatically wire themselves up without requiring any specific configuration.

For scenarios where the defaults need to be tweaked, one can pass in additional configuration files:

<yarn:configuration resources="classpath:/custom-site.xml, classpath:/hq-site.xml">

In this example, two additional Hadoop configuration resources are added to the configuration.

	Note
	Note that the configuration makes use of Spring’s Resource abstraction to locate the file. This allows various search patterns to be used, depending on the running environment or the prefix specified(if any) by the value - in this example the classpath is used.

In addition to referencing configuration resources, one can tweak Hadoop settings directly through Java Properties. This can be quite handy when just a few options need to be changed:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:yarn="http://www.springframework.org/schema/yarn"
  xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/yarn http://www.springframework.org/schema/yarn/spring-yarn.xsd">

  <yarn:configuration>
    fs.defaultFS=hdfs://localhost:9000
    hadoop.tmp.dir=/tmp/hadoop
    electric=sea
  </yarn:configuration>
</beans>

One can further customize the settings by avoiding the so called hard-coded values by externalizing them so they can be replaced at runtime, based on the existing environment without touching the configuration:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:yarn="http://www.springframework.org/schema/yarn"
  xmlns:context="http://www.springframework.org/schema/context"
  xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
    http://www.springframework.org/schema/yarn http://www.springframework.org/schema/yarn/spring-yarn.xsd">

  <yarn:configuration>
    fs.defaultFS=${hd.fs}
    hadoop.tmp.dir=file://${java.io.tmpdir}
    hangar=${number:18}
  </yarn:configuration>

  <context:property-placeholder location="classpath:hadoop.properties" />
</beans>

Through Spring’s property placeholder support, SpEL and the environment abstraction. one can externalize environment specific properties from the main code base easing the deployment across multiple machines. In the example above, the default file system is replaced based on the properties available in hadoop.properties while the temp dir is determined dynamically through SpEL. Both approaches offer a lot of flexbility in adapting to the running environment - in fact we use this approach extensivly in the Spring for Apache Hadoop test suite to cope with the differences between the different development boxes and the CI server.

Additionally, external Properties files can be loaded, Properties beans (typically declared through Spring’s ` ` namespace). Along with the nested properties declaration, this allows customized configurations to be easily declared:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:yarn="http://www.springframework.org/schema/yarn"
  xmlns:context="http://www.springframework.org/schema/context"
  xmlns:util="http://www.springframework.org/schema/util"
  xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
    http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd
    http://www.springframework.org/schema/yarn http://www.springframework.org/schema/yarn/spring-yarn.xsd">

  <!-- merge the local properties, the props bean and the two properties files -->
  <yarn:configuration properties-ref="props" properties-location="cfg-1.properties, cfg-2.properties">
    star=chasing
    captain=eo
  </yarn:configuration>

  <util:properties id="props" location="props.properties"/>
</beans>

When merging several properties, ones defined locally win. In the example above the configuration properties are the primary source, followed by the props bean followed by the external properties file based on their defined order. While it’s not typical for a configuration to refer to use so many properties, the example showcases the various options available.

	Note
	For more properties utilities, including using the System as a source or fallback, or control over the merging order, consider using Spring’s PropertiesFactoryBean (which is what Spring for Apache Hadoop Yarn and util:properties use underneath).

It is possible to create configuration based on existing ones - this allows one to create dedicated configurations, slightly different from the main ones, usable for certain jobs (such as streaming - more on that #yarn:job:streaming[below]). Simply use the configuration-ref attribute to refer to the parent configuration - all its properties will be inherited and overridden as specified by the child:

<!-- default name is 'yarnConfiguration' -->
<yarn:configuration>
  fs.defaultFS=${hd.fs}
  hadoop.tmp.dir=file://${java.io.tmpdir}
</yarn:configuration>

<yarn:configuration id="custom" configuration-ref="yarnConfiguration">
  fs.defaultFS=${custom.hd.fs}
</yarn:configuration>

...

Make sure though you specify a different name since otherwise, since both definitions will have the same name, the Spring container will interpret this as being the same definition (and will usually consider the last one found).

Last but not least a reminder that one can mix and match all these options to her preference. In general, consider externalizing configuration since it allows easier updates without interfering with the application configuration. When dealing with multiple, similar configuration use configuration composition as it tends to keep the definitions concise, in sync and easy to update.

When Application Master or any other Container is run in a hadoop cluster, there are usually dependencies to various application and configuration files. These files needs to be localized into a running Container by making a physical copy. Localization is a process where dependent files are copied into node’s directory structure and thus can be used within the Container itself. Yarn itself tries to provide isolation in a way that multiple containers and applications would not clash.

In order to use local resources, one needs to create an implementation of ResourceLocalizer interface. In its simplest form, resource localizer can be defined as:

<yarn:localresources>
  <yarn:hdfs path="/path/in/hdfs/my.jar"/>
</yarn:localresources>

The declaration above defines a ResourceLocalizer bean (to be precise a factory bean of type LocalResourcesFactoryBean) named, by default, yarnLocalresources. The default name is used, by conventions, by the other elements that require a reference to a resource localizer. It’s explained later how this reference is used when container launch context is defined.

It is also possible to define path as pattern. This makes it easier to pick up all or subset of files from a directory.

<yarn:localresources>
  <yarn:hdfs path="/path/in/hdfs/*.jar"/>
</yarn:localresources>

Behind the scenes it’s not enough to simple have a reference to file in a hdfs file system. Yarn itself when localizing resources into container needs to do a consistency check for copied files. This is done by checking file size and timestamp. This information needs to passed to yarn together with a file path. Order to do this the one who defines these beans needs to ask this information from hdfs prior to sending out resouce localizer request. This kind of behaviour exists to make sure that once localization is defined, Container will fail fast if dependant files were replaced during the process.

On default the hdfs base address is coming from a Yarn configuration and ResourceLocalizer bean will use configuration named yarnLocalresources. If there is a need to use something else than the default bean, configuration parameter can be used to make a reference to other defined configurations.

<yarn:localresources configuration="yarnConfiguration">
  <yarn:hdfs path="/path/in/hdfs/my.jar"/>
</yarn:localresources>

For example, client defining a launch context for Application Master needs to access dependent hdfs entries. Effectively hdfs entry given to resource localizer needs to be accessed from a Node Manager.

Yarn resource localizer is using additional parameters to define entry type and visibility. Usage is described below:

<yarn:localresources>
  <yarn:hdfs path="/path/in/hdfs/my.jar" type="FILE" visibility="APPLICATION"/>
</yarn:localresources>

For convenience it is possible to copy files into hdfs during the localization process using a yarn:copy tag. Currently base staging directory is /syarn/staging/xx where xx is a unique identifier per application instance.

<yarn:localresources>
  <yarn:copy src="file:/local/path/to/files/*jar" staging="true"/>
  <yarn:hdfs path="/*" staging="true"/>
</yarn:localresources>

One central concept in Yarn is to use environment variables which then can be read from a container. While it’s possible to read those variable at any time it is considered bad design if one chooce to do so. Spring Yarn will pass variable into application before any business methods are executed, which makes things more clearly and testing becomes much more easier.

<yarn:environment/>

The declaration above defines a Map bean (to be precise a factory bean of type EnvironmentFactoryBean) named, by default, yarnEnvironment. The default name is used, by conventions, by the other elements that require a reference to a environment variables.

For conveniance it is possible to define a classpath entry directly into an environment. Most likely one is about to run some java code with libraries so classpath needs to be defined anyway.

<yarn:environment include-local-system-env="false">
  <yarn:classpath use-yarn-app-classpath="true" delimiter=":">
    ./*
  </yarn:classpath>
</yarn:environment>

If use-yarn-app-classpath parameter is set to true(default value) a default yarn entries will be added to classpath automatically. These entries are on default resolved from a normal Hadoop Yarn Configuration using its yarn.application.classpath property or if site-yarn-app-classpath has a any content entries are resolved from there.

	Note
	Be carefull if passing environment variables between different systems. For example if running a client on Windows and passing variables to Application Master running on Linux, execution wrapper in Yarn may silently fail.

Client is always your entry point when interacting with a Yarn system whether one is about to submit a new application instance or just querying Resource Manager for running application(s) status. Currently support for client is very limited and a simple command to start Application Master can be defined. If there is just a need to query Resource Manager, command definition is not needed.

<yarn:client app-name="customAppName">
  <yarn:master-command>
    <![CDATA[
      /usr/local/java/bin/java
      org.springframework.yarn.am.CommandLineAppmasterRunner
      appmaster-context.xml
      yarnAppmaster
      container-count=2
      1><LOG_DIR>/AppMaster.stdout
      2><LOG_DIR>/AppMaster.stderr
    ]]>
  </yarn:master-command>
</yarn:client>

The declaration above defines a YarnClient bean (to be precise a factory bean of type YarnClientFactoryBean) named, by default, yarnClient. It also defines a command launching an Application Master using <master-command> entry which is also a way to define the raw commands. If this yarnClient instance is used to submit an application, its name would come from a app-name attribute.

<yarn:client app-name="customAppName">
  <yarn:master-runner/>
</yarn:client>

For a convinience entry <master-runner> can be used to define same command entries.

<yarn:client app-name="customAppName">
  <util:properties id="customArguments">
    container-count=2
  </util:properties>
  <yarn:master-runner
    command="java"
    context-file="appmaster-context.xml"
    bean-name="yarnAppmaster"
    arguments="customArguments"
    stdout="<LOG_DIR>/AppMaster.stdout"
    stderr="<LOG_DIR>/AppMaster.stderr" />
</yarn:client>

All previous three examples are effectively identical from Spring Yarn point of view.

	Note
	The <LOG_DIR> refers to Hadoop’s dedicated log directory for the running container.

<yarn:client app-name="customAppName"
  configuration="customConfiguration"
  resource-localizer="customResources"
  environment="customEnv"
  priority="1"
  virtualcores="2"
  memory="11"
  queue="customqueue">
  <yarn:master-runner/>
</yarn:client>

If there is a need to change some of the parameters for the Application Master submission, memory and virtualcores defines the container settings. For submission, queue and priority defines how submission is actually done.

Application master is responsible for container allocation, launching and monitoring.

<yarn:master>
  <yarn:container-allocator virtualcores="1" memory="64" priority="0"/>
  <yarn:container-launcher username="whoami"/>
  <yarn:container-command>
    <![CDATA[
      /usr/local/java/bin/java
      org.springframework.yarn.container.CommandLineContainerRunner
      container-context.xml
      1><LOG_DIR>/Container.stdout
      2><LOG_DIR>/Container.stderr
    ]]>
  </yarn:container-command>
</yarn:master>

The declaration above defines a YarnAppmaster bean (to be precise a bean of type StaticAppmaster) named, by default, yarnAppmaster. It also defines a command launching a Container(s) using <container-command> entry, parameters for allocation using <container-allocator> entry and finally a launcher parameter using <container-launcher> entry.

Currently there is a simple implementation of StaticAppmaster which is able to allocate and launch a number of containers. These containers are monitored by querying resource manager for container execution completion.

<yarn:master>
  <yarn:container-runner/>
</yarn:master>

For a convinience entry <container-runner> can be used to define same command entries.

<yarn:master>
  <util:properties id="customArguments">
    some-argument=myvalue
  </util:properties>
  <yarn:container-runner
    command="java"
    context-file="container-context.xml"
    bean-name="yarnContainer"
    arguments="customArguments"
    stdout="<LOG_DIR>/Container.stdout"
    stderr="<LOG_DIR>/Container.stderr" />
</yarn:master>

There is very little what Spring Yarn needs to know about the Container in terms of its configuration. There is a simple contract between org.springframework.yarn.container.CommandLineContainerRunner and a bean it’s trying to run on default. Default bean name is yarnContainer.

There is a simple interface org.springframework.yarn.container.YarnContainer which container needs to implement.

public interface YarnContainer {
  void run();
  void setEnvironment(Map<String, String> environment);
  void setParameters(Properties parameters);
}

There are few different ways how Container can be defined in Spring xml configuration. Natively without using namespaces bean can be defined with a correct name:

<bean id="yarnContainer" class="org.springframework.yarn.container.TestContainer">

Spring Yarn namespace will make it even more simpler. Below example just defines class which implements needed interface.

<yarn:container container-class="org.springframework.yarn.container.TestContainer"/>

It’s possible to make a reference to existing bean. This is usefull if bean cannot be instantiated with default constructor.

<bean id="testContainer" class="org.springframework.yarn.container.TestContainer"/>
<yarn:container container-ref="testContainer"/>

It’s also possible to inline the bean definition.

<yarn:container>
  <bean class="org.springframework.yarn.container.TestContainer"/>
</yarn:container>

It is fairly easy to create an application which launches a few containers and then leave those to do their tasks. This is pretty much what Distributed Shell example application in Yarn is doing. In that example a container is configured to run a simple shell command and Application Master only tracks when containers have finished. If only need from a framework is to be able to fire and forget then that’s all you need, but most likely a real-world Yarn application will need some sort of collaboration with Application Master. This communication is initiated either from Application Client or Application Container.

Yarn framework itself doesn’t define any kind of general communication API for Application Master. There are APIs for communicating with Container Manager and Resource Manager which are used on within a layer not necessarily exposed to a user. Spring Yarn defines a general framework to talk to Application Master through an abstraction and currently a JSON based rpc system exists.

This chapter concentrates on developer concepts to create a custom services for Application Master, configuration options for built-in services can be found from sections below - #yarn:masterservice[Appmaster Service] and #yarn:masterserviceclient[Appmaster Service Client].

public interface AppmasterService {
  int getPort();
  boolean hasPort();
  String getHost();
}

<bean id="yarnAmservice" class="AppmasterServiceImpl" />
<bean id="yarnClientAmservice" class="AppmasterClientServiceImpl" />

public interface AppmasterServiceClient {
}

<yarn-int:amservice
  service-impl="org.springframework.yarn.integration.ip.mind.TestService"
  default-port="1234"/>
<yarn-int:amservice-client
  service-impl="org.springframework.yarn.integration.ip.mind.DefaultMindAppmasterServiceClient"
  host="localhost"
  port="1234"/>

@Autowired
AppmasterServiceClient appmasterServiceClient;

@Test
public void testServiceInterfaces() throws Exception {
  SimpleTestRequest request = new SimpleTestRequest();
  SimpleTestResponse response =
  (SimpleTestResponse) ((MindAppmasterServiceClient)appmasterServiceClient).
    doMindRequest(request);
  assertThat(response.stringField, is("echo:stringFieldValue"));
}

<bean id="mapper"
  class="org.springframework.yarn.integration.support.Jackson2ObjectMapperFactoryBean" />

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindObjectToHolderConverter">
    <constructor-arg ref="mapper"/>
  </bean>
</yarn-int:converter>

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindHolderToObjectConverter">
    <constructor-arg ref="mapper"/>
    <constructor-arg value="org.springframework.yarn.batch.repository.bindings"/>
  </bean>
</yarn-int:converter>

This section of this document is about configuration, more about general concepts for see a ?.

Currently Spring Yarn have support for services using Spring Integration tcp channels as a transport.

<bean id="mapper"
  class="org.springframework.yarn.integration.support.Jackson2ObjectMapperFactoryBean" />

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindObjectToHolderConverter">
    <constructor-arg ref="mapper"/>
  </bean>
</yarn-int:converter>

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindHolderToObjectConverter">
    <constructor-arg ref="mapper"/>
    <constructor-arg value="org.springframework.yarn.integration.ip.mind"/>
  </bean>
</yarn-int:converter>

<yarn-int:amservice
  service-impl="org.springframework.yarn.integration.ip.mind.TestService"/>

If there is a need to manually configure the server side dispatch channel, a little bit more configuration is needed.

<bean id="serializer"
  class="org.springframework.yarn.integration.ip.mind.MindRpcSerializer" />
<bean id="deserializer"
  class="org.springframework.yarn.integration.ip.mind.MindRpcSerializer" />
<bean id="socketSupport"
  class="org.springframework.yarn.integration.support.DefaultPortExposingTcpSocketSupport" />

<ip:tcp-connection-factory id="serverConnectionFactory"
  type="server"
  port="0"
  socket-support="socketSupport"
  serializer="serializer"
  deserializer="deserializer"/>

<ip:tcp-inbound-gateway id="inboundGateway"
  connection-factory="serverConnectionFactory"
  request-channel="serverChannel" />

<int:channel id="serverChannel" />

<yarn-int:amservice
  service-impl="org.springframework.yarn.integration.ip.mind.TestService"
  channel="serverChannel"
  socket-support="socketSupport"/>

This section of this document is about configuration, more about general concepts for see a ?.

Currently Spring Yarn have support for services using Spring Integration tcp channels as a transport.

<bean id="mapper"
  class="org.springframework.yarn.integration.support.Jackson2ObjectMapperFactoryBean" />

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindObjectToHolderConverter">
    <constructor-arg ref="mapper"/>
  </bean>
</yarn-int:converter>

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindHolderToObjectConverter">
    <constructor-arg ref="mapper"/>
    <constructor-arg value="org.springframework.yarn.integration.ip.mind"/>
  </bean>
</yarn-int:converter>

<yarn-int:amservice-client
  service-impl="org.springframework.yarn.integration.ip.mind.DefaultMindAppmasterServiceClient"
  host="${SHDP_AMSERVICE_HOST}"
  port="${SHDP_AMSERVICE_PORT}"/>

If there is a need to manually configure the server side dispatch channel, a little bit more configuration is needed.

<bean id="serializer"
  class="org.springframework.yarn.integration.ip.mind.MindRpcSerializer" />
<bean id="deserializer"
  class="org.springframework.yarn.integration.ip.mind.MindRpcSerializer" />

<ip:tcp-connection-factory id="clientConnectionFactory"
  type="client"
  host="localhost"
  port="${SHDP_AMSERVICE_PORT}"
  serializer="serializer"
  deserializer="deserializer"/>

<ip:tcp-outbound-gateway id="outboundGateway"
  connection-factory="clientConnectionFactory"
  request-channel="clientRequestChannel"
  reply-channel="clientResponseChannel" />

<int:channel id="clientRequestChannel" />
<int:channel id="clientResponseChannel" >
  <int:queue />
</int:channel>

<yarn-int:amservice-client
  service-impl="org.springframework.yarn.integration.ip.mind.DefaultMindAppmasterServiceClient"
  request-channel="clientRequestChannel"
  response-channel="clientResponseChannel"/>

In this chapter we assume you are fairly familiar with concepts using Spring Batch. Many batch processing problems can be solved with single threaded, single process jobs, so it is always a good idea to properly check if that meets your needs before thinking about more complex implementations. When you are ready to start implementing a job with some parallel processing, Spring Batch offers a range of options. At a high level there are two modes of parallel processing: single process, multi-threaded; and multi-process.

Spring Hadoop contains a support for running Spring Batch jobs on a Hadoop cluster. For better parallel processing Spring Batch partitioned steps can be executed on a Hadoop cluster as remote steps.

<bean id="transactionManager" class="org.springframework.batch.support.transaction.ResourcelessTransactionManager"/>

<bean id="jobRepository" class="org.springframework.batch.core.repository.support.MapJobRepositoryFactoryBean">
  <property name="transactionManager" ref="transactionManager"/>
</bean>

<bean id="jobLauncher" class="org.springframework.batch.core.launch.support.SimpleJobLauncher">
  <property name="jobRepository" ref="jobRepository"/>
</bean>

<bean id="yarnEventPublisher" class="org.springframework.yarn.event.DefaultYarnEventPublisher"/>

<yarn-batch:master/>

<bean id="hello" class="org.springframework.yarn.examples.PrintTasklet">
  <property name="message" value="Hello"/>
</bean>

<batch:job id="job">
  <batch:step id="master">
    <batch:tasklet transaction-manager="transactionManager" ref="hello"/>
  </batch:step>
</batch:job>

12.12.2 Partitioning

Let’s take a quick look how Spring Batch partitioning is handled. Concept of running a partitioned job involves three things, Remote steps, Partition Handler and a Partitioner. If we do a little bit of oversimplification a remote step is like any other step from a user point of view. Spring Batch itself does not contain implementations for any proprietary grid or remoting fabrics. Spring Batch does however provide a useful implementation of PartitionHandler that executes Steps locally in separate threads of execution, using the TaskExecutor strategy from Spring. Spring Hadoop provides implementation to execute Steps remotely on a Hadoop cluster.

	Note
	For more background information about the Spring Batch Partitioning, read the Spring Batch reference documentation.

Configuring Master

As we previously mentioned a step executed on a remote host also need to access a job repository. If job repository would be based on a database instance, configuration could be similar on a container compared to application master. In our configuration example the job repository is in-memory based and remote steps needs access for it. Spring Yarn Batch contains implementation of a job repository which is able to proxy request via json requests. Order to use that we need to enable application client service which is exposing this service.

<bean id="jobRepositoryRemoteService" class="org.springframework.yarn.batch.repository.JobRepositoryRemoteService" >
  <property name="mapJobRepositoryFactoryBean" ref="&amp;jobRepository"/>
</bean>

<bean id="batchService" class="org.springframework.yarn.batch.repository.BatchAppmasterService" >
  <property name="jobRepositoryRemoteService" ref="jobRepositoryRemoteService"/>
</bean>

<yarn-int:amservice service-ref="batchService"/>

he declaration above defines JobRepositoryRemoteService bean named jobRepositoryRemoteService which is then connected into Application Master Service exposing job repository via Spring Integration Tcp channels.

As job repository communication messages are exchanged via custom json messages, converters needs to be defined.

<bean id="mapper" class="org.springframework.yarn.integration.support.Jackson2ObjectMapperFactoryBean" />

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindObjectToHolderConverter">
    <constructor-arg ref="mapper"/>
  </bean>
</yarn-int:converter>

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindHolderToObjectConverter">
    <constructor-arg ref="mapper"/>
    <constructor-arg value="org.springframework.yarn.batch.repository.bindings"/>
  </bean>
</yarn-int:converter>

Configuring Container

Previously we made a choice to use in-memore job repository running inside the application master. Now we need to talk to this repository via client service. We start by adding same converters as in application master.

<bean id="mapper" class="org.springframework.yarn.integration.support.Jackson2ObjectMapperFactoryBean" />

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindObjectToHolderConverter">
    <constructor-arg ref="mapper"/>
  </bean>
</yarn-int:converter>

<yarn-int:converter>
  <bean class="org.springframework.yarn.integration.convert.MindHolderToObjectConverter">
    <constructor-arg ref="mapper"/>
    <constructor-arg value="org.springframework.yarn.batch.repository.bindings"/>
  </bean>
</yarn-int:converter>

We use general client implementation able to communicate with a service running on Application Master.

<yarn-int:amservice-client
  service-impl="org.springframework.yarn.integration.ip.mind.DefaultMindAppmasterServiceClient"
  host="${SHDP_AMSERVICE_HOST}"
  port="${SHDP_AMSERVICE_PORT}" />

Remote step is just like any other step.

<bean id="hello" class="org.springframework.yarn.examples.PrintTasklet">
  <property name="message" value="Hello"/>
</bean>

<batch:step id="remoteStep">
  <batch:tasklet transaction-manager="transactionManager" start-limit="100" ref="hello"/>
</batch:step>

We need to have a way to locate the step from an application context. For this we can define a step locator which is later configured into running container.

<bean id="stepLocator" class="org.springframework.yarn.batch.partition.BeanFactoryStepLocator"/>

Spring Hadoop contains a custom job repository implementation which is able to talk back to a remote instance via custom json protocol.

<bean id="transactionManager" class="org.springframework.batch.support.transaction.ResourcelessTransactionManager"/>

<bean id="jobRepository" class="org.springframework.yarn.batch.repository.RemoteJobRepositoryFactoryBean">
  <property name="transactionManager" ref="transactionManager"/>
  <property name="appmasterScOperations" ref="yarnAmserviceClient"/>
</bean>

<bean id="jobExplorer" class="org.springframework.yarn.batch.repository.RemoteJobExplorerFactoryBean">
  <property name="repositoryFactory" ref="&amp;jobRepository" />
</bean>

Finally we define a Container understanding how to work with a remote steps.

<bean id="yarnContainer" class="org.springframework.yarn.batch.container.DefaultBatchYarnContainer">
  <property name="stepLocator" ref="stepLocator"/>
  <property name="jobExplorer" ref="jobExplorer"/>
  <property name="integrationServiceClient" ref="yarnAmserviceClient"/>
</bean>

We have additional support for leveraging Spring Boot when creating applications using Spring YARN. All dependencies for this exists in a sub-module named spring-yarn-boot which itself depends on Spring Boot.

Spring Boot extensions in Spring YARN are used to ease following issues:

Before we get into details let’s go through how simple it is to create and deploy a custom application to a Hadoop cluster. Notice that there are no need to use XML.

@Configuration
@EnableAutoConfiguration
public class ContainerApplication {

  public static void main(String[] args) {
    SpringApplication.run(ContainerApplication.class, args);
  }

  @Bean
  public HelloPojo helloPojo() {
    return new HelloPojo();
  }

}

In above ContainerApplication, notice how we added @Configuration in a class level itself and @Bean for a helloPojo() method.

@YarnComponent
public class HelloPojo {

  private static final Log log = LogFactory.getLog(HelloPojo.class);

  @Autowired
  private Configuration configuration;

  @OnContainerStart
  public void publicVoidNoArgsMethod() {
    log.info("Hello from HelloPojo");
    log.info("About to list from hdfs root content");
    FsShell shell = new FsShell(configuration);
    for (FileStatus s : shell.ls(false, "/")) {
      log.info(s);
    }
  }

}

HelloPojo class is a simple POJO in a sense that it doesn’t extend any Spring YARN base classes. What we did in this class:

To demonstrate that we actually have some real functionality in this class, we simply use Spring Hadoop’s FsShell to list entries from a root of a HDFS file system. For this we need to have access to Hadoop’s Configuration which is prepared for you so that you can just autowire it.

@EnableAutoConfiguration
public class ClientApplication {

  public static void main(String[] args) {
    SpringApplication.run(ClientApplication.class, args)
      .getBean(YarnClient.class)
      .submitApplication();
  }

}

The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. From there we simply request a bean of type YarnClient and execute its submitApplication() method. What happens next depends on application configuration, which we go through later in this document.

@EnableAutoConfiguration
public class AppmasterApplication {

  public static void main(String[] args) {
    SpringApplication.run(AppmasterApplication.class, args);
  }

}

Application class for YarnAppmaster looks even simpler than what we just did for ClientApplication. Again the main() method uses Spring Boot’s SpringApplication.run() method to launch an application.

In real life, you most likely need to start adding more custom functionality to your application component and you’d do that by start adding more beans. To do that you need to define a Spring @Configuration or @ComponentScan. AppmasterApplication would then act as your main starting point to define more custom functionality.

spring:
  hadoop:
    fsUri: hdfs://localhost:8020
    resourceManagerHost: localhost
  yarn:
    appName: yarn-boot-simple
    applicationDir: /app/yarn-boot-simple/
    client:
      files:
       - "file:build/libs/yarn-boot-simple-container-0.1.0.jar"
       - "file:build/libs/yarn-boot-simple-appmaster-0.1.0.jar"
      launchcontext:
        archiveFile: yarn-boot-simple-appmaster-0.1.0.jar
    appmaster:
      containerCount: 1
      launchcontext:
        archiveFile: yarn-boot-simple-container-0.1.0.jar

Final part for your application is its runtime configuration which glues all the components together which then can be called as a Spring YARN application. This configuration act as source for Spring Boot’s @ConfigurationProperties and contains relevant configuration properties which cannot be auto-discovered or otherwise needs to have an option to be overwritten by an end user.

You can then write your own defaults for your own environment. Because these @ConfigurationProperties are resolved at runtime by Spring Boot, you even have an easy option to overwrite these properties either by using command-line options or provide additional configuration property files.

spring:
  yarn:
    client:
      launchcontext:
        archiveFile: yarn-boot-appmaster-0.1.0.jar
    appmaster:
      launchcontext:
        archiveFile: yarn-boot-container-0.1.0.jar

spring:
  yarn:
    siteYarnAppClasspath: "/path/to/hadoop/libs/*"
    appmaster:
      launchcontext:
        useYarnAppClasspath: true
        archiveFile: yarn-boot-container-0.1.0.zip

spring:
  yarn:
    appmaster:
      launchcontext:
        archiveFile: yarn-boot-container-0.1.0.zip
        containerAppClasspath:
         - "./yarn-boot-container-0.1.0.zip/config"
         - "./yarn-boot-container-0.1.0.zip/lib"

spring:
  yarn:
    appmaster:
      launchcontext:
        runnerClass: com.example.MyMainClazz

spring:
  yarn:
    client:
      localizer:
        patterns:
         - "*appmaster*jar"
         - "*appmaster*zip"
        zipPattern: "*zip"
        propertiesNames: [application]
        propertiesSuffixes: [properties, yml]
    appmaster:
      localizer:
        patterns:
         - "*container*jar"
         - "*container*zip"
        zipPattern: "*zip"
        propertiesNames: [application]
        propertiesSuffixes: [properties, yml]

public interface LocalResourcesSelector {
  List<Entry> select(String dir);
}

@Configuration
@EnableConfigurationProperties({ SpringYarnAppmasterLocalizerProperties.class })
public static class LocalResourcesSelectorConfig {

  @Autowired
  private SpringYarnAppmasterLocalizerProperties syalp;

  @Bean
  @ConditionalOnMissingBean(LocalResourcesSelector.class)
  public LocalResourcesSelector localResourcesSelector() {
    BootLocalResourcesSelector selector = new BootLocalResourcesSelector(Mode.CONTAINER);
    if (StringUtils.hasText(syalp.getZipPattern())) {
      selector.setZipArchivePattern(syalp.getZipPattern());
    }
    if (syalp.getPropertiesNames() != null) {
      selector.setPropertiesNames(syalp.getPropertiesNames());
    }
    if (syalp.getPropertiesSuffixes() != null) {
      selector.setPropertiesSuffixes(syalp.getPropertiesSuffixes());
    }
    selector.addPatterns(syalp.getPatterns());
    return selector;
  }
}

@EnableAutoConfiguration
public class AppmasterApplication {

  @Bean
  public LocalResourcesSelector localResourcesSelector() {
    return MyLocalResourcesSelector();
  }

  public static void main(String[] args) {
    SpringApplication.run(AppmasterApplication.class, args);
  }

}

12.13.6 Container as POJO

In Boot application model if YarnContainer is not explicitly defined it defaults to DefaultYarnContainer which expects to find a POJO created as a bean having a specific annotations instructing the actual functionality.

@YarnComponent is a stereotype annotation itself having a Spring’s @Component defined in it. This is automatically marking a class to be a candidate having a @YarnComponent functionality.

Within a POJO class we can use @OnContainerStart annotation to mark a public method to act as an activator for a method endpoint.

	Note
	Return values from a @OnContainerStart will participate to a container exit value. If you omit these methods from a @YarnComponent, no return values are present thus making container not to exist automatically. This is useful in cases where you just want to have a mvc endpoints interacting with other containers. Otherwise you need to use dummy thread sleep or return a Future value.

@OnContainerStart
public void publicVoidNoArgsMethod() {
}

Returning type of int participates in a YarnContainer exit value.

@OnContainerStart
public int publicIntNoArgsMethod() {
  return 0;
}

Returning type of boolean participates in a YarnContainer exit value where true would mean complete and false failed container.

@OnContainerStart
public boolean publicBooleanNoArgsMethod() {
  return true;
}

Returning type of String participates in a YarnContainer exit value by matching ExitStatus and getting exit value from ExitCodeMapper.

@OnContainerStart
public String publicStringNoArgsMethod() {
  return "COMPLETE";
}

If method throws any Exception YarnContainer is marked as failed.

@OnContainerStart
public void publicThrowsException() {
  throw new RuntimeExection("My Error");
}

Method parameter can be bound with @YarnEnvironments to get access to current YarnContainer environment variables.

@OnContainerStart
public void publicVoidEnvironmentsArgsMethod(@YarnEnvironments Map<String,String> env) {
}

Method parameter can be bound with @YarnEnvironment to get access to specific YarnContainer environment variable.

@OnContainerStart
public void publicVoidEnvironmentArgsMethod(@YarnEnvironment("key") String value) {
}

Method parameter can be bound with @YarnParameters to get access to current YarnContainer arguments.

@OnContainerStart
public void publicVoidParametersArgsMethod(@YarnParameters Properties properties) {
}

Method parameter can be bound with @YarnParameter to get access to a specific YarnContainer arguments.

@OnContainerStart
public void publicVoidParameterArgsMethod(@YarnParameter("key") String value) {
}

It is possible to use multiple @YarnComponent classes and @OnContainerStart methods but a care must be taken in a way execution happens. In default these methods are executed synchronously and ordering is pretty much random. Few tricks can be used to overcome synchronous execution and ordering.

We support `@Order' annotation both on class and method levels. If `@Order' is defined on both the one from method takes a presense.

@YarnComponent
@Order(1)
static class Bean {

  @OnContainerStart
  @Order(10)
  public void method1() {
  }

  @OnContainerStart
  @Order(11)
  public void method2() {
  }

}

@OnContainerStart also supports return values of Future or ListenableFuture. This is a convenient way to do something asynchronously because future is returned immediately and execution goes to a next method and later waits future values to be set.

@YarnComponent
static class Bean {

  @OnContainerStart
  Future<Integer> void method1() {
    return new AsyncResult<Integer>(1);
  }

  @OnContainerStart
  Future<Integer> void method1() {
    return new AsyncResult<Integer>(2);
  }

}

Below is an example to use more sophisticated functionality with a ListenableFuture and scheduling work within a @OnContainerStart method. In this case YarnContainerSupport class simply provides an easy access to a TaskScheduler.

@YarnComponent
static class Bean extends YarnContainerSupport {

  @OnContainerStart
  public ListenableFuture<?> method() throws Exception {

    final MyFuture future = new MyFuture();

    getTaskScheduler().schedule(new FutureTask<Void>(new Runnable() {

      @Override
      public void run() {
        try {
          while (!future.interrupted) {
            // do something
          }
        } catch (Exception e) {
          // bail out from error
          future.set(false);
        }
      }
    }, null), new Date());

    return future;
  }

  static class MyFuture extends SettableListenableFuture<Boolean> {
    boolean interrupted = false;

    @Override
    protected void interruptTask() {
      interrupted = true;
    }
  }

}