Retrying Failed Dead Letter Sequences

The dead letter queue is a stop-gap. It’s important to strive to have the queue empty as soon as possible. Ideally by retrying the events, and removing them from the queue. You can retrieve a SequencedDeadLetterProcessor from the EventProcessingConfiguration based on a processing group name. Additionally, it’s also possible to use AxonIQ Console and process a queue manually.

To schedule processing dead letter sequences you can do something like:

@Component
public class ProcessDeadLetterComponent {

    private final EventProcessingConfiguration config;

    ProcessDeadLetterComponent(EventProcessingConfiguration config) {
        this.config = config;
    }

    @Scheduled(fixedDelay = 30_000, initialDelay = 30_000) (1)
    public void process() {
        for(String processingGroup : config.eventProcessors().keySet()) { (2)
            config.sequencedDeadLetterProcessor(processingGroup).ifPresent(
                    SequencedDeadLetterProcessor::processAny (3)
            );
        }
    }
}

1	Spring-specific way to wait 30 seconds, then invoke the method every 30 seconds. You want a large interval to not stress the system too much. Especially as you don’t know what’s going wrong, it might be the application is already having a hard time.
2	A way to get all the processing groups. You might also use different intervals, depending on the processing group. Or don’t have automatic processing for some groups.
3	The `processAny()` rotates the sequence to try based on when it was last tried. Once there are multiple queues present, it might take some time before they’re all processed with this approach. You could also get the whole queue from the configuration, and process each existing sequence.

By default, the framework enqueues a letter again, unless a processor can process it successfully. How to change this, is part of the next session.