Question 1

How to implement CQRS with Event Sourcing in Java?

Accepted Answer

This project provides a step-by-step guide with Java code examples, starting from no CQRS and evolving to separate commands, queries, models, storage, and event sourcing. It includes visual diagrams to illustrate each stage, making it practical for hands-on learning.

Question 2

What is the difference between CQRS and Event Sourcing?

Accepted Answer

CQRS separates command (write) and query (read) responsibilities, while Event Sourcing stores state changes as a sequence of events. This project explains both with diagrams and shows how they can be combined for more maintainable systems, as seen in the evolutionary examples.

Question 3

When should I use CQRS and Event Sourcing in my project?

Accepted Answer

Use CQRS when read and write operations have different scalability or complexity needs, and Event Sourcing for auditability, replayability, or complex domain logic. The project's evolutionary approach helps identify when these patterns are beneficial based on practical scenarios.

Question 4

Java CQRS frameworks: Axon vs this educational project?

Accepted Answer

Axon is a comprehensive framework for building CQRS and Event Sourcing applications in Java with production features like event handling and sagas. This project is an introduction focused on concepts and basic implementation, serving as a learning tool rather than a ready-to-use framework.

Question 5

How to migrate an existing Java application to CQRS?

Accepted Answer

The project demonstrates an evolutionary migration path, starting with separating commands and queries, then models, storage, and finally adding event sourcing. It provides practical examples and diagrams, such as the 'Separate Storage Engines' stage, to guide refactoring.

Question 6

Best practices for implementing Event Sourcing in Java?

Accepted Answer

This project covers fundamentals like event storage and state rebuilding. For advanced practices, you'll need to explore topics like snapshotting for performance, event versioning for schema evolution, and handling concurrency conflicts, which are beyond its introductory scope.

Java CQRS Intro

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