Time-limited aggregation with publish-subscribe in Spring Integration - spring-integration

Question

I am trying to implement the following using Spring Integration with DSL and lambda:
Given a message, send it to N consumers (via publish-subscribe). Wait for limited time and return all results that have arrived form consumers (<= N) during that interval.
Here is an example configuration I have so far:
#Configuration
#EnableIntegration
#IntegrationComponentScan
#ComponentScan
public class ExampleConfiguration {
#Bean(name = PollerMetadata.DEFAULT_POLLER)
public PollerMetadata poller() {
return Pollers.fixedRate(1000).maxMessagesPerPoll(1).get();
}
#Bean
public MessageChannel publishSubscribeChannel() {
return MessageChannels.publishSubscribe(splitterExecutorService()).applySequence(true).get();
}
#Bean
public ThreadPoolTaskExecutor splitterExecutorService() {
final ThreadPoolTaskExecutor executorService = new ThreadPoolTaskExecutor();
executorService.setCorePoolSize(3);
executorService.setMaxPoolSize(10);
return executorService;
}
#Bean
public DirectChannel errorChannel() {
return new DirectChannel();
}
#Bean
public DirectChannel requestChannel() {
return new DirectChannel();
}
#Bean
public DirectChannel channel1() {
return new DirectChannel();
}
#Bean
public DirectChannel channel2() {
return new DirectChannel();
}
#Bean
public DirectChannel collectorChannel() {
return new DirectChannel();
}
#Bean
public TransformerChannel1 transformerChannel1() {
return new TransformerChannel1();
}
#Bean
public TransformerChannel2 transformerChannel2() {
return new TransformerChannel2();
}
#Bean
public IntegrationFlow errorFlow() {
return IntegrationFlows.from(errorChannel())
.handle(m -> System.err.println("[" + Thread.currentThread().getName() + "] " + m.getPayload()))
.get();
}
#Bean
public IntegrationFlow channel1Flow() {
return IntegrationFlows.from(publishSubscribeChannel())
.transform("1: "::concat)
.transform(transformerChannel1())
.channel(collectorChannel())
.get();
}
#Bean
public IntegrationFlow channel2Flow() {
return IntegrationFlows.from(publishSubscribeChannel())
.transform("2: "::concat)
.transform(transformerChannel2())
.channel(collectorChannel())
.get();
}
#Bean
public IntegrationFlow splitterFlow() {
return IntegrationFlows.from(requestChannel())
.channel(publishSubscribeChannel())
.get();
}
#Bean
public IntegrationFlow collectorFlow() {
return IntegrationFlows.from(collectorChannel())
.resequence(r -> r.releasePartialSequences(true),
null)
.aggregate(a ->
a.sendPartialResultOnExpiry(true)
.groupTimeout(500)
, null)
.get();
}
}
TransformerChannel1 and TransformerChannel2 are sample consumers and have been implemented with just a sleep to emulate delay.
The message flow is:
splitterFlow -> channel1Flow \
-> channel2Flow / -> collectorFlow
Everything seem to work as expected, but I see warnings like:
Reply message received but the receiving thread has already received a reply
which is to be expected, given that partial result was returned.
Questions:
Overall, is this a good approach?
What is the right way to gracefully service or discard those delayed messages?
How to deal with exceptions? Ideally I'd like to send them to errorChannel, but am not sure where to specify this.

Answer 1

Yes, the solution looks good. I guess it fits for the Scatter-Gather pattern. The implementation is provided since version 4.1.
From other side there is on more option for the aggregator since that version, too - expire-groups-upon-timeout, which is true for the aggregator by default. With this option as false you will be able to achieve your requirement to discard all those late messages. Unfortunately DSL doesn't support it yet. Hence it won't help even if you upgrade your project to use Spring Integration 4.1.
Another option for those "Reply message received but the receiving thread has already received a reply" is on the spring.integraton.messagingTemplate.throwExceptionOnLateReply = true option using spring.integration.properties file within the META-INF of one of jar.
Anyway I think that Scatter-Gather is the best solution for you use-case.
You can find here how to configure it from JavaConfig.
UPDATE
What about exceptions and error channel?
Since you get deal already with the throwExceptionOnLateReply I guess you send a message to the requestChannel via #MessagingGateway. The last one has errorChannel option. From other side the PublishSubscribeChannel has errorHandler option, for which you can use MessagePublishingErrorHandler with your errorChannel as a default one.
BTW, don't forget that Framework provides errorChannel bean and the endpoint on it for the LoggingHandler. So, think, please, if you really need to override that stuff. The default errorChannel is PublishSubscribeChannel, hence you can simply add your own subscribers to it.