26. Transaction Support

26.1 Understanding Transactions in Message flows

Spring Integration exposes several hooks to address transactional needs of you message flows. But to better understand these hooks and how you can benefit from them we must first revisit the 6 mechanisms that could be used to initiate Message flows and see how transactional needs of these flows could be addressed within each of these mechanisms.

Here are the 6 mechanisms to initiate a Message flow and their short summary (details for each are provided throughout this manual):

  • Gateway Proxy - Your basic Messaging Gateway

  • MessageChannel - Direct interactions with MessageChannel methods (e.g., channel.send(message))

  • Message Publisher - the way to initiate message flow as a bi-product of method invocations on Spring beans

  • Inbound Channel Adapters/Gateways - the way to initiate message flow based on connecting third-party system with Spring Integration messaging system(e.g., [JmsMessage] -> Jms Inbound Adapter[SI Message] -> SI Channel)

  • Scheduler - the way to initiate message flow based on scheduling events distributed by a pre-configured Scheduler

  • Poller - similar to the Scheduler and is the way to initiate message flow based on scheduling or interval-based events distributed by a pre-configured Poller

These 6 cold be split in 2 general categories:

  • Message flows initiated by a USER process - Example scenarios in this category would be invoking a Gateway method or explicitly sending a Message to a MessageChannel. In other words these message flows depend on third party process (e.g., some code that we wrote) to be initiated

  • Message flows initiated by the DAEMON process - Example scenarios in this category would be a Poller polling for a Message queue to initiate a new Message flow with the polled Message or a Scheduler scheduling the process, by creating a new Message and initiating a message flow at a predefined time

Clearly the Gateway Proxy, MessageChannel.send(..) and MessagePublisher are all belong to the 1st category and Inbound Adapters/Gateways, Scheduler and Poller belong to the 2nd.

So, how do we address transactional needs in various scenarios within each category and is there a need for Spring Integration to provide something explicitly with regard to transaction for a particular scenario or Spring's Transaction Support could be leveraged instead?.

First of all, the first and obvious goal is NOT to re-invent something that has already been invented unless you can provide a beter solution. In our case Spring itself provides a first class support for transaction management. So our goal here is not to provide something new but rather delegate/use Spring to benefit from the existing support for transactions. In other words as a framework we must expose hooks to the Transaction management functionality provided by Spring. But since Spring Integration configuration is based on Spring Configuration it is not always neccessery to expose these hooks as they already expposed via Spring natively. Remeber every Spring Integration component is a Spring Bean after all.

With this goal in mind let's look at the two scenarios. 

If you think about it, Message flows that are initiated by the USER process (Category 1) and obviously configured in Spring Application Context, are subject to transactional configuration of such process and therefore don't need to be explicitly configured by Spring Integration to support transactions. The transaction could and should be initiated by such process through standard Transaction support provided by Spring and Spring Integration message flow will honor transactional semantics of the components naturally because it is Spring configured. For example; A Gateway or ServiceActivator methods could be annotated with @Transactional or TransactionInterceptor could be configured in XML configuration with point-cut expression pointing to specific methods that should be transactional. The bottom line you have full control over transaction configuration and boundaries in these scenarios.

However, things are a bit different when it comes to Message flows initiated by the DAEMON process (Category 2). Although configured by the developer these flows do not directly involve human or some other process to be initiated. These are trigger-based flows that are initiated by a trigger process (DAEMON process) based on the configuration of such process. For example, we could have a Scheduler initiating a message flow every Friday night of every week. We can also configure a trigger that initiates a Message flow every second, etc. So, we obviously need the same way to let these trigger-based processes know of our intention to make these Message flows transactional so Transaction context could be created whenever a new Message flow is initiated. In other words we need to expose some Transaction configuration, but ONLY enough to delegate to Transaction support already provided by Spring (as we do in other scenarios).

Spring Integration provides transactional support for Pollers. Pollers are a special case comoponents becouse we can call receive() within that poller task against a resource that is itself transactional thus including receive() call in the the boundaries of the Transaction allowing it to be rolled back in case of a task failure. If we were to add the same support for channels, the added transactions would affect all downstream components starting with that send() call. That is providing a rather wide scope for transaction demarcation without any strong reason especially when Spring already provides several way to address transactional needs of any component downstream. However the receive() method being included in a transaction boundary is the "strong reason" for pollers. 

26.1.1 Poller Transaction Support

Any time you configure a Poller you can provide transactional configuration via transactional element and its attributes:

<poller max-messages-per-poll="1" fixed-rate="1000">
    <transactional transaction-manager="txManager" 

As you can see this configuration looks evry similar to native Spring transaction configuration. You must still provide reference to Transaction manager and specify transaction attributes or rely on defauls (e.g., if 'transaction-manager'' attribute is not specified then it will default to the bean with the name 'transactionManager'). Internally the process would be wrapped in the Spring's native Transaction where TransactionInterceptor is responsible to handle transactions. For more information on how to configure Transaction Manager, the types of Transaction Managers (e.g., JTA, Datasource etc.) and other details related to transaction configuration please refer to Spring's Reference manual (Chapter 10 - Transaction Management).

With the above configuration all Message flows initiated by this poller will be transactional. For more information and details on Poller's transactional configuration please refer to section - 21.1.1. Polling and Transactions.

There times when besides transaction several more cross cutting concerns needs to be addressed when running Poller. To help with that, Poller element defines <advice-chain> sub-element which allows you to define a custom chain of Advices to be applied on the Poller. (see section 4.4 for more details) In Spring Integration 2.0 Poller went through the major  refactoring effort and is now using proxy mechanism to address transactional concerns as well as other cross cutting concerns, one of the significant changes evolving from this effort is that we made <transactional> and <advice-chain> elements mutually exclusive. The rational behind this is; If you need more then one advice, and one of them is Transaction advice, then you can simply include it in the <advice-chain> with the same convenience as before but with much more control since you now have an option to position any advice in the desired order. 

<poller max-messages-per-poll="1" fixed-rate="10000">
    <ref bean="txAdvice"/>
    <ref bean="someAotherAdviceBean" />
    <beans:bean class="foo.bar.SampleAdvice"/>

<tx:advice id="txAdvice" transaction-manager="txManager">
    <tx:method name="get*" read-only="true"/>
    <tx:method name="*"/>

As yo can see from the example above, we have provided a very basic XML-based configuration of Spring Transaction advice  - "txAdvice" and included it within the <advice-chain> defined by the Poller. And if you only need to address transactional concerns of the Poller, then you can still use <transactional> element as a convinience.

26.2 Transaction Boundaries

Another important factor that needs to be understood is the boundaries of the Transactions within the Message flow. When transaction is started, transaction context is bound to the current thread. So regardless of how many endpoints and channels you have in your Message flow you transaction context will be preserved as long as you are ensuring that the flow continues on the same thread. As soon as you break it by introducing a Pollable Channel or Executor Channel or initiate a new thread manually in some service, the Transactional boundary will be broken as well. Essentially the Transaction will END right there and if successfull hand of happened between the threads, the flow would be considered a success and COMMIT signal would be sent even though the flow might still result in the exception somewhere downstream. If such flow was synchronous the exception would be thrown back to the initiator of the Message flow who is also the initiator of the transactional context and transaction would result in a ROLLBACK.