CQS Architecture
CQS (Command-Query Separation) Architecture is a software design principle that distinguishes between operations that modify state (commands) and those that return data without side effects (queries). It enforces that methods should either be commands that perform actions and change system state, or queries that return data without altering anything, but never both. This separation improves code clarity, testability, and maintainability by making side effects explicit and predictable.
Developers should learn and apply CQS Architecture when building systems where clear separation of concerns, predictable behavior, and ease of testing are priorities, such as in domain-driven design (DDD), event-sourced systems, or complex business applications. It is particularly useful in scenarios involving concurrency, auditing, or undo/redo functionality, as it helps prevent unintended side effects and simplifies reasoning about code flow.