prasobhpk / spring-domain-events

Spring (reliable) Domain Events

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.