DelayHandler messageGroupId

Question

From the Spring Integration documentation (https://docs.spring.io/spring-integration/docs/5.1.7.RELEASE/reference/html/#delayer) it is not clear to me what the messageGroupId in the DelayHandler means exactly and which value I have to set there exactly (is it arbitrary?). This value does not exist in the xml configuration, but does in the Java configuration.

@ServiceActivator(inputChannel = "input")
@Bean
public DelayHandler delayer() {
   DelayHandler handler = new DelayHandler("delayer.messageGroupId"); // THIS constructor parameter is not clear to me
   handler.setDefaultDelay(3_000L);
   handler.setDelayExpressionString("headers['delay']");
   handler.setOutputChannelName("output");
   return handler;
}

Answer 1

It is explained in the JavaDocs of that constructor:

/**
 * Create a DelayHandler with the given 'messageGroupId' that is used as 'key' for
 * {@link MessageGroup} to store delayed Messages in the {@link MessageGroupStore}.
 * The sending of Messages after the delay will be handled by registered in the
 * ApplicationContext default
 * {@link org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler}.
 * @param messageGroupId The message group identifier.
 * @see #getTaskScheduler()
 */
public DelayHandler(String messageGroupId) {

It is not required because the groupId is based on the required id attribute:

    String id = element.getAttribute(ID_ATTRIBUTE);
    if (!StringUtils.hasText(id)) {
        parserContext.getReaderContext().error("The 'id' attribute is required.", element);
    }
    ...
    builder.addConstructorArgValue(id + ".messageGroupId");

It is really mentioned and explained a little bit in the docs: https://docs.spring.io/spring-integration/docs/current/reference/html/messaging-endpoints.html#delayer-namespace.

The value indeed is arbitrary, but it must be unique per your application, so different delayers don't steal messages from each other.