What is CQRS?

CQRS stands for Command Query Responsibility Segregation. The idea is simple: Separate the part of your system that changes data from the part that reads it.

Traditionally, most applications use a single model or service to handle both reading and writing – typically in the form of CRUD (Create, Read, Update, Delete). With CQRS, you break this pattern in favor of clear responsibilities:

• Commands change state.

  They represent actions, such as "BorrowBook" or "ExtendLoan".

• Queries read state.

  They return data, but never modify anything.

This distinction isn't just about code structure – it's about reflecting intent.

Why Separate Commands and Queries?

In the real world, reading and writing follow different rules:

• Reads are frequent, fast, and optimized for display.
• Writes are often more complex: they trigger business logic, validations, and workflows.

By separating them, you can optimize each side independently:

• Read models can be designed for fast UI access.
• Write models can focus on enforcing rules and expressing domain logic.

CQRS is About Behavior, Not Storage

CQRS does not require multiple databases or any particular infrastructure. It’s a design principle that helps you model your system in a way that matches the mental model of your users.

When you think in terms of commands ("What is the user trying to do?") and queries ("What does the user need to know?"), your system becomes easier to reason about – and better aligned with the real world.

Ready to go further? Next, learn about Event Sourcing – a natural fit for systems built on CQRS.