CQRS stands for Command Query Responsibility Segregation. It is a software architectural pattern that separates the responsibilities for processing commands and retrieving data, so-called queries, projections or views, from each other. This means that (at least) two separate models are used: a command-model for processing and storing changes in the state and a query-model, also called read-model, view-model or projections, for retrieving data without changing it.

The advantage of CQRS is that it can make the application more scalable and flexible, because each model can be specifically optimized for its task. This pattern is often used in combination with Event Sourcing, where all changes in the application state are stored as a series of events. CQRS is popular in environments with high performance requirements and complex data models, such as with microservices and distributed systems.

In Domain Driven Design (DDD) the emphasis is on properly designing the business domains and associated systems and the separation between writing and reading strongly supports this.

References

For more information about CQRS (and Event Sourcing) and how it works, there are various sources you can consult:

  1. AxonIQ Docs: AxonIQ is the company behind the Axon Framework; a Java framework for building Event Sourcing systems. Read for example:

  2. Greg Young’s CQRS: Greg Young, one of the founders of CQRS, has various articles, videos and presentations on the subject. His explanation often goes deep into the philosophy behind the pattern:

    • Search for “Greg Young CQRS” on YouTube for videos and presentations.

  3. Marc Explains: Marc van Andel has created a series of videos to explain the various aspects of CQRS and Event Sourcing:

  4. Microsoft Docs (CQRS): Microsoft has detailed documentation about CQRS and Event Sourcing. They discuss the pattern, advantages, disadvantages and implementation details:

  5. Martin Fowler’s Blog: Martin Fowler, a renowned software architect, is an authority when it comes to patterns and concepts. We therefore cannot omit this reference:

  6. Books: There are some books that cover CQRS and Event Sourcing in broader contexts of distributed systems and microservices. A few recommended books are:

    • “Patterns, Principles, and Practices of Domain-Driven Design” by Scott Millett and Nick Tune
    • “Implementing Domain-Driven Design” by Vaughn Vernon

  7. Community Resources & GitHub: On GitHub you can also find example projects that implement CQRS and Event Sourcing in various languages (e.g. Java, C#, Node.js).

These resources can give you an in-depth understanding and help you with practical examples and best practices to apply CQRS in your projects.