= Spring (reliable) Domain Events
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== The Problem
Spring allows applications to publish events via its `ApplicationEventPublisher` API.
With a transaction in place, these events can either be consumed within the transaction (using `@EventListener`) or in a dedicated transaction completion phase (using `@TransactionalEventListener`, defaulting to after transaction commit).
While an application failure for an in-transaction event listener is not a problem as the transaction has not been committed, a failure during the publication of events to transactional event listeners will mean that the event is lost and the notification of those listeners cannot be guaranteed.
== The idea
As we already have a transactional datastore in place, we could also store publication information about all transactional event listeners with the transaction that publishes one or more events.
We can then wrap the transactional event listeners to be able to remove those registrations on successful listener invocation and completion.
This allows us to re-publish the events to transactional listeners that either haven't been notified yet or didn't successfully complete the message handling in case of an application failure (on either a restart or in a scheduled way).
== Building blocks of the prototype
* The `EventPublicationRegistry` -- the core interface to register publications and mark them completed. It allows different implementations (JPA, JDBC).
* The `EventSerializer` -- a component to serialize the actual domain event so that it can be kept around in the publication. Again, to allow pluggable implementations (Jackson etc.)
* `PersistentApplicationEventMulticaster` -- a replacement for Spring's default `ApplicationEventMulticaster` that stores publications via the `EventPublicationRegistry`.
* `CompletionRegisteringBeanPostProcessor` -- a `BeanPostProcessor` that wraps `@TransactionalEventListener` instances with an interceptor to mark publications as completed.
* `@EnablePersistentDomainEvents` -- registers the multicaster and includes configuration classes for `EventPublicationConfigurationExtension` (to register the registry) and `EventSerializationConfigurationExtension` (to register an `EventSerializer`) via `spring.factories`.
=== Implementation modules
* `core` -- multicaster implementation, general and configuration infrastructure and SPI interfaces.
* `jackson` -- a rudimentary Jackson-based `EventSerializer` implementation.
* `jpa` -- a JPA-based `EventPublicationRegistry`.
* `test` -- a sample integration test featuring two successful and one failing listener to show the registry exposes the publication of the failed listener after the failure.
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