What is Event Sourcing Design Pattern in Java Microservices? Example

Hello Java programmers, if you are tired of constantly trying to keep track of your application's current state using a traditional relational database then the Event Sourcing pattern might be just what you need. Event Sourcing is a popular pattern in Microservice architecture that allows services to persist and query the state of an application using a sequence of events instead of storing the current state. This pattern has gained popularity because it helps to build complex systems with a clear audit trail and support for long-term scalability. By storing a sequence of events, rather than just the latest state, developers can reconstruct the application state at any point in time, even after failures or bugs. It's also one of the 10 essential Microservice patterns I have shared earlier and it's one of the top 3 along with Saga and CQRS patterns which we have seen in my earlier articles.  

The Event Sourcing pattern involves storing the complete history of an application's state as a sequence of events. Each event represents a change in the application's state, such as the creation of a new record or the modification of an existing one.

Event Sourcing Design Pattern in Microservice Architecture

The advantage of using the Event Sourcing pattern is that it allows you to reconstruct the current state of the application at any given point in time by replaying the events in chronological order. This means you can easily track changes, revert to previous states, and audit your application's activity.

So, how do you implement the Event Sourcing pattern in Java? Here's a high-level overview of the steps you'll need to take:

1. Create an event store to store the events and provide a way to retrieve them. There are a few different options for implementing an event store, including using a database, a file system, or a message queue.
2. Define your events and create event handlers to handle the events. The event handlers should be responsible for applying the events to the current state of the application.
3. Use the event store and event handlers to implement the main logic of your application. When something changes in the application's state, create an event and store it in the event store. When you need to retrieve the current state of the application, retrieve the events from the event store and apply them using the event handlers.

Let's take a closer look at each of these steps.

1. Event Store

The event store is the central component of the Event Sourcing pattern. It is responsible for storing the events and providing a way to retrieve them.

Here's an example of how to implement an event store using a database:

public class DatabaseEventStore implements EventStore {

  private final Connection connection;

  public DatabaseEventStore(Connection connection) {
    this.connection = connection;
  }

  @Override
  public void store(Event event) {
    String sql = "INSERT INTO events (type, data) VALUES (?, ?)";
    try (PreparedStatement statement = connection.prepareStatement(sql)) {
      statement.setString(1, event.getType());
      statement.setString(2, event.getData());
      statement.executeUpdate();
    } catch (SQLException e) {
      throw new RuntimeException(e);
    }
  }

  @Override
  public List<Event> getEvents(long startId, long endId) {
    String sql = "SELECT id, type, data FROM events WHERE id >= ? AND id < ?";
    try (Prepared Statement statement = connection.prepareStatement(sql)) {
      statement.setLong(1, startId);
      statement.setLong(2, endId);
      ResultSet resultSet = statement.executeQuery();
      List<Event> events = new ArrayList<>();
      while (resultSet.next()) {
        long id = resultSet.getLong("id");
        String type = resultSet.getString("type");
        String data = resultSet.getString("data");
        Event event = new Event(id, type, data);
        events.add(event)
   }catch(Exception e) ()
}
}

2. Events and Event Handlers

Next, you'll need to define your events and create event handlers to handle them. The events should represent the changes in the application's state, and the event handlers should be responsible for applying those changes to the current state of the application.

Here's an example of how to define an event and an event handler in Java:

public class UserCreatedEvent implements Event {
  private final String username;
  private final String email;

  public UserCreatedEvent(String username, String email) {
    this.username = username;
    this.email = email;
  }

  public String getUsername() {
    return username;
  }

  public String getEmail() {
    return email;
  }
}

public class UserCreatedEventHandler implements EventHandler<UserCreatedEvent> {
  private final UserRepository userRepository;

  public UserCreatedEventHandler(UserRepository userRepository) {
    this.userRepository = userRepository;
  }

  @Override
  public void handle(UserCreatedEvent event) {
    User user = new User()'
}
}

3. Main Logic

Now that you have your event store and event handlers set up, you can use them to implement the main logic of your application.

Here's an example of how to use the event store and event handlers to create and retrieve a user in an application:

EventStore eventStore = new DatabaseEventStore(connection);
EventHandler<UserCreatedEvent> userCreatedEventHandler 
   = new UserCreatedEventHandler(userRepository);

// create a user
User user = new User("john.doe", "john.doe@example.com");
Event userCreatedEvent = new UserCreatedEvent(user.getUsername(), user.getEmail());
eventStore.store(userCreatedEvent);
userCreatedEventHandler.handle(userCreatedEvent);

// retrieve the user
long userId = 1;
List<Event> events = eventStore.getEvents(userId, userId + 1);
UserState userState = new UserState();
for (Event event : events) {
  if (event instanceof UserCreatedEvent) {
    userCreatedEventHandler.handle((UserCreatedEvent) event);
  }
}
User user = userState.getUser();

Event Sourcing Microservices Pattern Frequently Asked Questions

1. What is the Event Sourcing pattern?

The Event Sourcing pattern is a software design pattern for Microservice architecture that involves storing the complete history of an application's state as a sequence of events. Each event represents a change in the application's state, such as the creation of a new record or the modification of an existing one. You can use Event Sourcing pattern for distributed transaction management along with 2 Phase commit and SAGA Pattern. 

2. What are the advantages of using the Event Sourcing pattern?

The main advantage of the Event Sourcing pattern is that it allows you to reconstruct the current state of the application at any given point in time by replaying the events in chronological order. This means you can easily track changes, revert to previous states, and audit your application's activity.

You can also use Event Sourcing Pattern for distributed transaction management in Microservice architecture. 

3. How do you implement the Event Sourcing pattern in Java?

To implement the Event Sourcing pattern in Java, you'll need to create an event store to store the events and a mechanism to apply the events to the current state of the application. You'll also need to define your events and create event handlers to handle the events.

Conclusion

The Event Sourcing design pattern is a powerful tool for managing distributed transaction, storing and tracking the complete history of an application's state. It allows you to reconstruct the current state of the application at any given point in time, track changes, revert to previous states, and audit your application's activity.

By implementing an event store and event handlers, you can easily incorporate the Event Sourcing pattern into your Java applications. So give it a try and see how it can improve the performance and scalability of your applications.

Overall, Event Sourcing in Microservices is a powerful pattern that can help build resilient, scalable microservices. By using an event stream to represent the application state, developers can build complex systems that are more robust and fault-tolerant than traditional architectures.

I hope this article on the Event Sourcing pattern was helpful and gave you a good understanding of how to implement it in Java. As always, if you have any questions or need further clarification, don't hesitate to ask!

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P. S. - If you are new to Microservice architecture and want to learn more about Microservice Architecture and solutions from scratch and looking for free resources then I highly recommend you to check out my post about 7 free Microservice courses. It contains free Udemy and Coursera and courses to learn Microservice architecture from scratch.