CQS

Command Query Separation

Don't return the object created or updated

Do query separately

Important Principle of Imperative Programming

Conceived in Bertrand Meyer's Eiffel programming language

Every method should perform only one action: either a command that changes state or a query that returns data

Don't modify the answer when performing an action on a request

In other words, state modification and data return should be implemented separately - this is the theoretical foundation of get and set

Methods should be referentially transparent when returning values (a function is referentially transparent when it doesn't change the system's state) and should not cause side effects

While widely used, the most common problem arising from this is

Race Condition

Query - Returns a result value and does not change the observable state of the system. Therefore, it is free of side effects.

Command - Does not return a result, but instead changes the state of the system.

Code should be standardized through interfaces, and

CQS separation is important for extension. What's important at the code level is to prevent it from being too easy to extend. Make the architecture not easy to create a similar function that does the same thing. This enables command query separation, and separating mutation and query files is also a good practice.

Command와 Query, CQS 알아보기

아는 사람은 알지만, 나는 프로그래밍 교육 기관에서 조교, 코칭 일을 적잖이 했다. 아무래도 요즘 프로그래밍 입문을 파이썬으로 많이 하다보니 파이썬을 공부하는 내가 설 자리가 있었던 것 같다. 그래서 프로그래밍을 처음 하는 사람들에게 변수, 할당, 함수 등 초보적인 개념부터 설명할 일이 많았는데 특히 함수를 공부할 때 list.sort()와 sorted()처럼 비슷한 일을 하는데 구체적인 동작은 다른 형태에 대해 많은 코칭이 필요했다.

https://shoark7.github.io/programming/knowledge/command-and-query-method