CQRS

Definition

CQRS stands for Command Query Responsibility Segregation. It's a pattern. At its heart is the notion that you can use a different model to update information than the model you use to read information. For some situations, this separation can be valuable, but beware that for most systems CQRS adds risky complexity.

CRUD

CQRS

Fits

CQRS naturally fits with some other architectural patterns.

• As we move away from a single representation that we interact with via CRUD, we can easily move to a task-based UI.
• CQRS fits well with event-based programming models. It's common to see CQRS system split into separate services communicating with Event Collaboration. This allows these services to easily take advantage of Event Sourcing.
• Having separate models raises questions about how hard to keep those models consistent, which raises the likelihood of using eventual consistency.
• For many domains, much of the logic is needed when you're updating, so it may make sense to use EagerReadDerivation to simplify your query-side models.
• If the write model generates events for all updates, you can structure read models as EventPosters, allowing them to be MemoryImages and thus avoiding a lot of database interactions.
• CQRS is suited to complex domains, the kind that also benefit from Domain-Driven Design.