Understanding Event Sourcing and CQRS with EventStoreDb: Projections Explained

Dive deep into Event Sourcing and CQRS with EventStoreDb, and learn how to effectively use projections to manage state.
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Understanding Event Sourcing and CQRS with EventStoreDb: Projections Explained

In the world of software architecture, concepts like Event Sourcing (ES) and Command Query Responsibility Segregation (CQRS) have become essential, especially when building scalable and maintainable applications. However, many developers often find themselves puzzled about the role of projections and how they integrate with these architectures. If you're one of those developers, you're in the right place! In this guide, we'll explore the nuances of Event Sourcing and CQRS, particularly focusing on the use of projections with EventStoreDb.

The Problem: Understanding Projections in Event Sourcing

You might be wondering how projections fit into Event Sourcing when using EventStoreDb. Here's a common scenario: you create commands and command handlers, but you're unsure how to manage the current state of your aggregates effectively using projections. Let's break it down.

The Key Questions:

What role do projections play in Event Sourcing?

Should we have a separate database like MongoDB to keep track of current states?

How does EventStoreDb handle projections?

The Solution: Integrating Projections with Event Sourcing and CQRS

What is Event Sourcing?

Event Sourcing is a design pattern that ensures that all changes to application state are stored as a sequence of events. In this approach:

Each action is captured as an immutable event in a store.

The current state of an application can be reconstructed by replaying these events.

What is CQRS?

Command Query Responsibility Segregation (CQRS) is a pattern that separates read and write operations. You utilize:

Commands: To change the state of the application (i.e., CreateUser commands).

Queries: To read data from the application (i.e., fetching user details).

Understanding Projections

To use projections effectively in Event Sourcing with EventStoreDb, follow these guidelines:

1. Event Storage and Retrieval

The aggregate (e.g., User) should be designed to restore its state from stored events. Every modification must read this event stream and apply events sequentially to update the current state.

2. Managing State Integrity

When working with aggregates, it's crucial to:

Implement version checks to handle optimistic concurrency.

Ensure that any new event is correctly created and associated with its aggregate.

For instance, if a CreateUserCommand is called twice with the same GUID, it could lead to inconsistent application states. This can be avoided with proper validation.

Code Best Practices

Refactoring Aggregate Creation

Instead of directly applying events within the constructor, events should be raised strictly within specific methods. Here’s a refactoring example:

[[See Video to Reveal this Text or Code Snippet]]

Command Handler Adjustments

Update your command handler to ensure that it retrieves the current state before proceeding with any modifications:

[[See Video to Reveal this Text or Code Snippet]]

Understanding Database Interactions with Projections

On the read side:

Use an out-of-the-box category projection of all users.

Write this data to MongoDB (or similar) for easier querying by different APIs. This separation ensures that the command side (which modifies data) stays distinct from the read side (which queries data).

Conclusion

Integrating projections into Event Sourcing with EventStoreDb may seem daunting at first, but with a solid understanding of the principles of CQRS, you can achieve a clean architecture that maintains separations of concerns. By applying the best practices outlined in this post, you can confidently manage your application's state with projections while ensuring robust command handling.

If you’re looking to implement or improve y

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