Event sourcing is a valuable architectural pattern for building robust and scalable applications. It enables applications to store and replay events as the source of truth, ensuring reliable data synchronization and easy traceability. In this blog post, we will explore how to build event sourcing applications using GlassFish as the application server and Axon Framework in Java.

What is Event Sourcing?

Event sourcing is the practice of capturing and storing all changes to an application’s state as a sequence of events. Instead of persisting the current state, we persist the series of events that led to the current state. This approach provides a complete audit log of all actions and enables reconstructing the application’s state at any point in time.

GlassFish Application Server

GlassFish is an open-source application server that provides a robust and scalable platform for running Java applications. It supports Java EE (Enterprise Edition) specifications and provides features like clustering, load balancing, and automatic deployment.

To get started with GlassFish, you can download it from the official website and follow the installation instructions. Once installed, you can deploy your Java applications on the GlassFish server.

Axon Framework

Axon Framework is a powerful Java framework that simplifies the implementation of event-driven, scalable applications using the event sourcing pattern. It provides a set of building blocks and abstractions for event sourcing, command handling, and query handling.

To use Axon Framework in your Java application, you need to include the necessary dependencies in your project’s build file (e.g., Maven or Gradle). You can find the latest version of Axon Framework on Maven Central or the official Axon Framework website.

Building Event Sourcing Applications with GlassFish and Axon Framework

To build event sourcing applications with GlassFish and Axon Framework, follow these steps:

1. Define your events: Identify the key events in your domain that represent state changes. Create corresponding event classes in your Java application.

public class AccountCreatedEvent {
    // Define event properties
    private final String accountId;
    private final String accountName;
  
    // Constructor, getters, and setters
    // ...
}

public class MoneyDepositedEvent {
    // Define event properties
    private final String accountId;
    private final BigDecimal amount;
  
    // Constructor, getters, and setters
    // ...
}

// Define other events as needed

1. Implement your command handlers: Define the logic for handling commands that trigger state changes. Command handlers receive commands and emit events in response.

@CommandHandler
public void handle(CreateAccountCommand command) {
    // Validate command input and emit an event
    AccountCreatedEvent event = new AccountCreatedEvent(command.getAccountId(),
                                                        command.getAccountName());
    // Apply event to update the state
    // ...
}

@CommandHandler
public void handle(DepositMoneyCommand command) {
    // Validate command input and emit an event
    MoneyDepositedEvent event = new MoneyDepositedEvent(command.getAccountId(),
                                                       command.getAmount());
    // Apply event to update the state
    // ...
}

// Implement other command handlers as needed

1. Implement your event handlers: Define the logic for handling events and updating the application’s state.

@EventHandler
public void on(AccountCreatedEvent event) {
    // Update the state based on the event
    // ...
}

@EventHandler
public void on(MoneyDepositedEvent event) {
    // Update the state based on the event
    // ...
}

// Implement other event handlers as needed

1. Configure Axon Framework in your application: Set up the necessary configurations for Axon Framework, including the event store, command bus, and event bus.

@Configuration
public class AxonConfig {

    @Bean
    public EventStore eventStore() {
        // Configure your event store implementation (e.g., using JPA or MongoDB)
        // ...
    }

    @Bean
    public CommandBus commandBus() {
        // Configure your command bus implementation (e.g., using SimpleCommandBus)
        // ...
    }

    @Bean
    public EventBus eventBus() {
        // Configure your event bus implementation (e.g., using SimpleEventBus)
        // ...
    }

    // Other Axon Framework configurations
    // ...
}

1. Deploy your application on GlassFish: Build your Java application and deploy it on GlassFish using the provided deployment mechanism.

Conclusion

Event sourcing is a powerful pattern for building robust and scalable applications. By using GlassFish as the application server and Axon Framework in Java, you can easily implement event sourcing applications. Follow the steps outlined in this blog post to get started with building your own event sourcing application. #EventSourcing #Java