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The Command and Query Responsibility Segregation (CQRS) Design Pattern

Introduction

The ‘Command and Query Responsibility Segregation’ (CQRS) design pattern is based on the “Command-Query Separation” (CQS) principle from the book “Object-Oriented Software Construction“ by Bertrand Meyer (1988) and was first mentioned by Greg Young and Udi Dahan in 2010.

This pattern has produced a lot of excitement and hype but the principle behind it is very simple.

As the demands on command methods (any method with write access that mutates state) are usually very different to those on query methods (any method with read access that returns a value), it makes sense to separate these two groups. This would naturally mean different services but might also mean different models.

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