Dead-Letter Queue for Event Processors

[smcvb]

Feature Description

It would be beneficial if EventProcessors would allow the configuration of a Dead Letter Queue.
This queue should be filled with EventMessages for which event handling failed.

Furthermore, it should consider that events are handled in sequence, based on the SequencingPolicy.
As such, events that return a SequencingPolicy for a dead-lettered event should not be handled and added to the queue instead.
Doing so will allow for retries of the entire event order starting from a given point.
This should ensure that event execution, e.g., for query model updates, doesn't get mixed up with failed and successful operations.

The DeadLetterQueue implementation should provide several implementations, differing per storage medium of the letters.
Logical implementations are in-memory, JPA, JDBC, Mongo, and Axon Server.

Lastly, several triggers upon which to reevaluate the letters should be supported.
The above does not mean we should provide several approaches directly, but only that there's space to offer other triggers.
Upon such a trigger, the dead-letters should be injected as a regular event stream into the processor.

Current Behaviour

There's no support for dead letters on the Event Processor.

Wanted Behaviour

Axon Framework should provide dead letter support for Event Processors.

Possible Workarounds

Entirely custom implementations done by the end user.

[smcvb]

Happy to announce, as follows from the label changes, that the base pull request introducing the Dead-Letter Queue is finalized and merged into the main branch.

The JPA implementation of the Dead-Letter Queue will follow soon.
Furthermore, it'll be included in 4.6.0.

Note that there are still future additions we will make to the Dead-Letter Queue support, with most notably the introduction of a JDBC implementation of the Dead-Letter Queue.
Other interesting ideas we'll cover are:

• Message Handler (parameter) support for Dead Letters  #2351 - Provide a form of parameter support within Message Handlers, to differentiate between regular handling and dead-lettered handling.
• Think about supporting dedicated MessageHandlerInterceptors for dead-letters.