Asynchronous communication

Synchronous and asynchronous communication are two approaches to interaction in software design. Each approach has its own advantages and disadvantages, and each has different use cases.

In asynchronous communication (aka. asynchronous messaging), the sender and receiver do not interact in real-time. The sender may or may not expect a response from the receiver. But if a response is expected, the sender can continue with other tasks while waiting for it. The interaction is therefore said to be non-blocking.

Asynchronous communication is often used in distributed software, and typically implemented using message-driven or event-driven architectural patterns to distribute processing across multiple nodes. Asynchronous communication increases the [fault tolerance] of distributed systems by decreasing temporal coupling between nodes. Temporal coupling is a form of coupling where one service is dependent on the immediate availability of another service. Removing temporal coupling using asynchronous, [message-driven] communication patterns reduces the risk of cascading failures, where a drop in performance or outright failure in one component causes a chain reaction of failures in other components. Reducing temporal coupling between services solves the problem of each service and network call being a potential [point of failure], so improving the [fault tolerance] of the overall system.

Asynchronous communication patterns have a number of other advantages in the context of distributed software systems. For example, responsibility for the implementation of [retries] and [circuit breakers] can be offloaded from domain services to dedicated infrastructure such as [message queues], reducing the [accidental complexity] of individual services (but at the cost of increasing the accidental complexity of the overall system). Message-driven communication patterns also better support [evolutionary] [microservice] designs, because the system is less likely to incrementally degrade into "API hell" where the number of direct service-to-service calls becomes unmanageable and where anti-patterns like [circular service calls] may emerge.

Asynchronous communication should not be confused with asynchronous programming, which is a programming paradigm that allows for non-blocking execution of tasks using concurrent or parallel processing, and implemented using constructs such as promises and callbacks.