Projector @EventHandler

Chapter: Reacting to Events in Projectors - The @EventHandler Annotation

Projectors, the backbone of materializing domain state in CQRS, rely on event handlers to react to domain events and update the corresponding projection data. This chapter delves into the @EventHandler annotation used within projectors to define these event handling methods.

Understanding @EventHandler

The @EventHandler annotation marks a method within your projector class as an event handler. This method is responsible for processing a specific type of domain event and performing the necessary updates on the projection state stored in the database.

Here's a breakdown of the annotation's definition:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface EventHandler {
    public int retry() default -1;
    public int retryDelay() default 1000;
}

• @Retention(RetentionPolicy.RUNTIME): Ensures the annotation information is retained at runtime, allowing Evento to access it during application execution.

• @Target(ElementType.METHOD): Specifies that the annotation can only be applied to method declarations within your projector class.

Optional Retry Functionality:

• public int retry() default -1;: This attribute empowers you to define the number of retries the framework should attempt if an exception occurs while executing the event handler. By default, the retry count is set to -1, indicating no specific limit on retries.

• public int retryDelay() default 1000;: This attribute allows you to specify the delay (in milliseconds) between each retry attempt. The default delay is 1000 milliseconds (1 second). These optional features enhance the robustness of your event handling logic by enabling automatic retries in case of transient errors.

Handling Persistent Errors: Dead Event Queues (DEQs)

While retries can help overcome temporary hiccups, there may be situations where event handling consistently fails even after retry attempts. To prevent such events from getting stuck in limbo, the Evento Framework utilizes Dead Event Queues (DEQs).

• After exceeding the configured retry limit, the event is automatically routed to a dedicated DLQ.

• DLQs act as a safety net, storing these unprocessed events for potential future intervention.

By employing DLQs, Evento ensures efficient processing of most events while providing a mechanism to handle persistent failures and prevent data loss.

Implementing @EventHandler Methods

An @EventHandler method adheres to a specific structure for effective event handling:

• Event as the First Parameter: The first parameter of the method must be a subtype of the Event class representing the specific event being handled (e.g., DemoCreatedEvent).

• Optional Additional Parameters: Depending on your specific needs, you might include additional parameters like:

  • QueryGateway: This allows the implementation of some cross-domain materialization.

  • EventMessage<?>: This provides access to details about the received event, including metadata and timestamp.

  • Metadata: This represents metadata associated with the event (often already included in the EventMessage object).

  • Instant: This represents the timestamp of the event.

• Void Return Type: @EventHandler methods typically do not have a return value. Their primary purpose is to update the projection state based on the received event.

While exploring event handlers in projectors (@EventHandler), it's essential to understand the limitations regarding CommandGateway. Projectors, by design, focus on materializing the domain state based on events, not modifying the system state directly through commands.

Here's why CommandGateway is not typically used within @EventHandler methods:

• Read Model Focus: Projectors primarily deal with updating the read model (projection state) in the database. This read model serves as the source of data for queries.

• Event-Driven Updates: @EventHandler methods react to domain events, which represent past changes within the system. These events are not intended to trigger further commands that could modify the current state.

• Data Consistency: Sending commands from within event handlers could potentially lead to inconsistencies between the event stream and the actual system state.

Processing Flow within an @EventHandler Method

Here's a breakdown of the typical processing flow within an @EventHandler method:

1. Projection State Update: The core logic of the event handler lies in modifying the projection state in the database based on the information contained in the domain event. This might involve creating new entries, updating existing data, or deleting data from the projection.

2. Optional Additional Logic: In some scenarios, your event handler might perform additional tasks like logging or sending notifications after updating the projection state.

Example: @EventHandler in Action

@Projector(version = 3)
public class DemoProjector {

    private final DemoRepository demoRepository;

    public DemoProjector(DemoRepository demoRepository) {
       this.demoRepository = demoRepository;
    }

    @EventHandler
    void on(DemoCreatedEvent event,
          QueryGateway queryGateway,
          EventMessage<?> eventMessage,
          Metadata metadata,
          Instant instant) {
       Utils.logMethodFlow(this, "on", event, "BEGIN");
       var now = Instant.now();
       demoRepository.save(new Demo(event.getDemoId(), event.getName(),
             event.getValue(), now, now, null));
       Utils.logMethodFlow(this, "on", event, "END");
    }
 }

The provided code example showcases an @EventHandler method within a DemoProjector class:

• The method is annotated with @EventHandler.

• It handles the DemoCreatedEvent event.

• It utilizes the demoRepository to save a new Demo object to the database, reflecting the creation of the Demo Aggregate.

• It logs the processing flow for debugging purposes.

Key Takeaways

• @EventHandler empowers you to define methods within projectors that listen for specific domain events.

• These methods are responsible for updating the projection state in the database to reflect the changes represented by the events.

• Understanding @EventHandler is crucial for building effective projectors that maintain consistency between domain events and the materialized projection state.

In the next chapter, we'll explore different strategies for handling complex event processing scenarios within projectors.