Distributed Configuration with Consul

Consul provides a Key/Value Store for storing configuration and other metadata. Spring Cloud Consul Config is an alternative to the Config Server and Client. Configuration is loaded into the Spring Environment during the special "bootstrap" phase. Configuration is stored in the /config folder by default. Multiple PropertySource instances are created based on the application’s name and the active profiles that mimics the Spring Cloud Config order of resolving properties. For example, an application with the name "testApp" and with the "dev" profile will have the following property sources created:

config/testApp,dev/
config/testApp/
config/application,dev/
config/application/

The most specific property source is at the top, with the least specific at the bottom. Properties in the config/application folder are applicable to all applications using consul for configuration. Properties in the config/testApp folder are only available to the instances of the service named "testApp".

Configuration is currently read on startup of the application. Sending a HTTP POST to /refresh will cause the configuration to be reloaded. Config Watch will also automatically detect changes and reload the application context.

How to activate

To get started with Consul Configuration use the starter with group org.springframework.cloud and artifact id spring-cloud-starter-consul-config. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

Spring Boot Config Data Import

Spring Boot 2.4 introduced a new way to import configuration data via the spring.config.import property. This is now the default way to get configuration from Consul.

To optionally connect to Consul set the following in application.properties:

application.properties

spring.config.import=optional:consul:

This will connect to the Consul Agent at the default location of "http://localhost:8500". Removing the optional: prefix will cause Consul Config to fail if it is unable to connect to Consul. To change the connection properties of Consul Config either set spring.cloud.consul.host and spring.cloud.consul.port or add the host/port pair to the spring.config.import statement such as, spring.config.import=optional:consul:myhost:8500. The location in the import property has precedence over the host and port propertie.

Consul Config will try to load values from four automatic contexts based on spring.cloud.consul.config.name (which defaults to the value of the spring.application.name property) and spring.cloud.consul.config.default-context (which defaults to application). If you want to specify the contexts rather than using the computed ones, you can add that information to the spring.config.import statement.

application.properties

spring.config.import=optional:consul:myhost:8500/contextone;/context/two

This will optionally load configuration only from /contextone and /context/two.

A bootstrap file (properties or yaml) is not needed for the Spring Boot Config Data method of import via spring.config.import.

Customizing

Consul Config may be customized using the following properties:

spring:
  cloud:
    consul:
      config:
        enabled: true
        prefix: configuration
        defaultContext: apps
        profileSeparator: '::'

If you have set spring.cloud.bootstrap.enabled=true or spring.config.use-legacy-processing=true, or included spring-cloud-starter-bootstrap, then the above values will need to be placed in bootstrap.yml instead of application.yml.

enabled setting this value to "false" disables Consul Config
prefix sets the base folder for configuration values
defaultContext sets the folder name used by all applications
profileSeparator sets the value of the separator used to separate the profile name in property sources with profiles

Config Watch

The Consul Config Watch takes advantage of the ability of consul to watch a key prefix. The Config Watch makes a blocking Consul HTTP API call to determine if any relevant configuration data has changed for the current application. If there is new configuration data a Refresh Event is published. This is equivalent to calling the /refresh actuator endpoint.

To change the frequency of when the Config Watch is called change spring.cloud.consul.config.watch.delay. The default value is 1000, which is in milliseconds. The delay is the amount of time after the end of the previous invocation and the start of the next.

To disable the Config Watch set spring.cloud.consul.config.watch.enabled=false.

The watch uses a Spring TaskScheduler to schedule the call to consul. By default it is a ThreadPoolTaskScheduler with a poolSize of 1. To change the TaskScheduler, create a bean of type TaskScheduler named with the ConsulConfigAutoConfiguration.CONFIG_WATCH_TASK_SCHEDULER_NAME constant.

YAML or Properties with Config

It may be more convenient to store a blob of properties in YAML or Properties format as opposed to individual key/value pairs. Set the spring.cloud.consul.config.format property to YAML or PROPERTIES. For example to use YAML:

spring:
  cloud:
    consul:
      config:
        format: YAML

YAML must be set in the appropriate data key in consul. Using the defaults above the keys would look like:

config/testApp,dev/data
config/testApp/data
config/application,dev/data
config/application/data

You could store a YAML document in any of the keys listed above.

You can change the data key using spring.cloud.consul.config.data-key.

git2consul with Config

git2consul is a Consul community project that loads files from a git repository to individual keys into Consul. By default the names of the keys are names of the files. YAML and Properties files are supported with file extensions of .yml and .properties respectively. Set the spring.cloud.consul.config.format property to FILES. For example:

bootstrap.yml

spring:
  cloud:
    consul:
      config:
        format: FILES

Given the following keys in /config, the development profile and an application name of foo:

.gitignore
application.yml
bar.properties
foo-development.properties
foo-production.yml
foo.properties
master.ref

the following property sources would be created:

config/foo-development.properties
config/foo.properties
config/application.yml

The value of each key needs to be a properly formatted YAML or Properties file.

Fail Fast

It may be convenient in certain circumstances (like local development or certain test scenarios) to not fail if consul isn’t available for configuration. Setting spring.cloud.consul.config.fail-fast=false will cause the configuration module to log a warning rather than throw an exception. This will allow the application to continue startup normally.