Using event sourcing and CQRS with Apache Wicket

25 Sep 2023

Apache Wicket is a powerful Java web framework that allows developers to build enterprise-grade web applications. One popular architectural pattern for building scalable and robust applications is Event Sourcing coupled with Command Query Responsibility Segregation (CQRS). In this blog post, we will explore how to leverage these patterns in combination with Apache Wicket.

What is Event Sourcing?

Event Sourcing is an architectural pattern that models the state of an application by recording all changes as a sequence of immutable events. Instead of persisting the current state of an entity, we persist the events that led to that current state. This allows us to rebuild the current state by replaying the events in chronological order.

What is CQRS?

CQRS is another architectural pattern that separates the command (write) and query (read) responsibilities of an application. Instead of having a single model that handles both reading and writing, we split it into separate models. This allows us to optimize each model for its specific requirements, resulting in improved scalability and performance.

Integrating Event Sourcing and CQRS in Apache Wicket

Domain Modeling: Start by identifying the domain events in your application. These events should represent facts or changes in the domain. Using Apache Wicket, you can create event classes that encapsulate the necessary information.

Example:
```
public class UserRegisteredEvent implements Serializable {
    private String userId;
    private String name;
    // ...
}
```

Command Handling: Implement the command handlers that handle the user actions and generate the corresponding events. These command handlers should encapsulate the business logic and validation rules.

Example:

public class UserRegistrationCommandHandler {
    public void handle(RegisterUserCommand command) {
        // Validate command
        // Generate UserRegisteredEvent
        // Publish event to the event bus
    }
}

Event Persistence: Store the generated events in an event store. The event store should be capable of persisting and retrieving events based on their ordering and the aggregate they belong to.
Event Replay: Implement event replay mechanisms to rebuild the current state by replaying the events from the event store. Apache Wicket provides abstractions to hook into this process.
Query Models: Create separate query models optimized for reading data. These query models can be denormalized and stored in a suitable data store for efficient querying.

Example:
```
public class UserQueryModel implements Serializable {
    private String userId;
    private String name;
    // ...
}
```

Query Handling: Implement query handlers to fetch data from the query models. These query handlers will be responsible for handling read requests and returning the relevant data.

Example:

public class UserQueryHandler {
    public UserQueryModel getById(String userId) {
        // Retrieve data from query model based on userId
        // Return UserQueryModel
    }
}

By combining Event Sourcing and CQRS with Apache Wicket, you can build applications that are highly scalable, robust, and easy to maintain. The separation of concerns and the ability to replay events ensure data consistency and reliability. Apache Wicket provides the necessary tools and abstractions to implement these patterns efficiently.

#EventSourcing #CQRS #ApacheWicket