Command–query separation

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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. In other words, Asking a question should not change the answer.[1] More formally, methods should return a value only if they are referentially transparent and hence possess no side effects.

Connection with design by contract[edit]

Command–query separation is particularly well suited to a design by contract (DbC) methodology, in which the design of a program is expressed as assertions embedded in the source code, describing the state of the program at certain critical times. In DbC, assertions are considered design annotations – not program logic – and as such, their execution should not affect the program state. CQS is beneficial to DbC because any value-returning method (any query) can be called by any assertion without fear of modifying program state.

In theoretical terms, this establishes a measure of sanity, whereby one can reason about a program's state without simultaneously modifying that state. In practical terms, CQS allows all assertion checks to be bypassed in a working system to improve its performance without inadvertently modifying its behaviour. CQS may also prevent the occurrence of certain kinds of heisenbugs.

Broader impact on software engineering[edit]

Even beyond the connection with design by contract, CQS is considered by its adherents to have a simplifying effect on a program, making its states (via queries) and state changes (via commands) more comprehensible.[citation needed]

CQS is well-suited to the object-oriented methodology, but can also be applied outside of object-oriented programming. Since the separation of side effects and return values is not inherently object-oriented, CQS can be profitably applied to any programming paradigm that requires reasoning about side effects.[citation needed]

Command query responsibility segregation[edit]

Command query responsibility segregation (CQRS) applies the CQS principle by using separate Query and Command objects to retrieve and modify data, respectively.[2][3]


CQS can make it more difficult to implement reentrant and multithreaded software correctly. This usually occurs when a non-thread-safe pattern is used to implement the command–query separation.

Here is a simple example of a pattern that breaks CQS principles but is useful for multi-threaded software. It breaks CQS principles because the function both mutates state and returns it:

private int x;
public int increment_and_return_x() {
  lock x;   // by some mechanism
  x = x + 1;
  int x_copy = x;
  unlock x; // by some mechanism
  return x_copy;

Here is a CQS-compliant pattern. However, it is safely usable only in single-threaded applications. In a multithreaded program, there is a race condition in the caller, between where increment() and value() would be called:

private int x;
public int value() {
  return x;
void increment() {
  x = x + 1;

Even in single-threaded programs, it is sometimes arguably significantly more convenient to have a method that is a combined query and command. Martin Fowler cites the pop() method of a stack as an example.[4]

See also[edit]


  1. ^ Meyer, Bertrand. "Eiffel: a language for software engineering" (PDF). p. 22. Retrieved 16 December 2014.
  2. ^ Young, Greg. "CQRS Documents" (PDF). Retrieved 2012-12-28.
  3. ^ Fowler, Martin. "CQRS". Retrieved 2011-07-14.
  4. ^ Fowler, Martin. "CommandQuerySeparation". Retrieved 5 December 2005.

Further reading[edit]

External links[edit]