Command–query separation

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

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

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.

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.

Command Query Responsibility Segregation

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

Drawbacks

CQS can make it more difficult to implement re-entrant and multi-threaded 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 multi-threaded 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;
}

Finally, here is a thread-safe CQS pattern. This preserves the separation between the mutation of state and reading the resulting value, without introducing a race condition in the caller:

private int x;
private int _x;
public int value()
{
  return x;
}
void increment()
{
  lock _x;   // by some mechanism
  _x = _x + 1;
  x = _x;
  unlock _x; // by some mechanism
}

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]

References

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

External links

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