Introduction of CQS

Reference: https://dundung.tistory.com/183, https://hardlearner.tistory.com/383, https://www.inflearn.com/questions/35961

Command Query Separation (CQS) is all about separating command and query. The reason to separate them is when there is an issue regarding unintentional changes made in data, we can just look at the methods that cause change.

Command

Command, also known as modifiers or mutators in other contexts, changes state of a system, but do not return a value.

Query

Query returns a result and do not change the observable state of the system (free of side effects)

If it does not cause change in internal state of system, it is free of side effects

Example of CQS in JPA

Follow this rule:

insert는 id만 반환하고 (아무것도 없으면 조회가 안되니), update는 아무것도 반환하지 않고, select (조회)는 내부의 변경이 없는 메서드로 설계하면 좋습니다

Let’s look at it one by one.

insert returns only the id ```java private EntityManager em;

@Transactional
public Long save(Item item){
    em.persist(item);
    Item item = em.find(Item.class, id);
    return item.getId();   } ```

insert causes change in state of the DB BUT at the same time, returns the value of id. Thus, it is both a command and a query. It breaks the CQS rule but is needed.

update does not return anything (query)

private EntityManager em;
    @Transactional
    public  void update(Item item){
        em.persist(item);
  }

update causes change in state of the DB but does not return anything (void). So it is a command.

select(조회) returns result without chaning state of system (query)

@Transactional
    puiblic Item findOne(Long id){
        Item item = em.find(Item.class,id);
        return item;
    }

select(조회) does not change state of system and just returns the result of sql query so it is a query.

Exception to CQS

A perfect exception to CQS is stack.pop(). pop()’s command removes the top value from the stack, thus causing change in stack’s state whilst pop()’s query is getting that popped value. Pop() is both a command and query, which is an exception to CQS.

insert id in JPA is also an exception.