What is a query command?

Question

Command-query separation (CQS) is a principle of imperative computer programming. It was devised by Bertrand Meyer as part of his pioneering work on the Eiffel programming language. It states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both.

What is the difference between a command and a query?

Queries: Return a result and do not change the observable state of the system (are free of side effects). Commands: Change the state of a system but do not return a value.

Why is CQS important?

The main reason to follow the CQS pattern is to keep code readable and reusable. Your code gets more trustworthy as it ensures certain code will not cause unexpected side effects. A Query that changes state would be a clear and serious violation and I would never allow that.

What mean CQRS?

CQRS stands for Command and Query Responsibility Segregation, a pattern that separates read and update operations for a data store.

What is command in Microservices?

A Command is a message directed at a receiver to get something done.

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