Command and Query Responsibility Segregation

Command and Query Responsibility Segregation (CQRS) is a simple pattern to define. Objects are split into two new objects:

• One is responsible for commands. 

• One is responsible for queries.

The above figure demonstrates just how simple the concept might be, but it’s important to observe its practical details. The definitions for command and query are the same as they are for Command-Query Separation (CQS):

• Command: Performs a mutation of the application state.

• Query: Returns application state to the caller.

Note: In CQRS, just as it is in CQS, an action can either be a command or a query but not both.

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