commandbus

A command bus is a software component that decouples the place where a request is issued from the place where it is executed. It is commonly used in CQRS (Command Query Responsibility Segregation) and mediator-style architectures to route commands to their designated handlers without requiring the caller to know how a command is processed.

Typically a command is a plain data object that encodes an intent to perform a change, such as CreateUser or UpdateOrderStatus. The command bus maintains a registry or mapping from command type to a specific handler and invokes that handler when a command is dispatched. This separation promotes single-responsibility design and simplifies testing and orchestration.

Dispatch can be synchronous, with the caller awaiting completion and possible return values, or asynchronous, with the command handed off to a queue for later processing. Most implementations support a pipeline of middleware or decorators that perform cross-cutting concerns such as validation, authorization, auditing, logging, retry policy, and transaction handling.

Typically a command has a single, explicit handler. Errors raised by a handler may be translated into domain exceptions or failure results by the bus. In practice, command buses are part of a larger messaging architecture that also includes query buses for reads and event buses for notifications.

See also: CQRS, mediator pattern, and message bus.