REQREP

REQREP is a basic messaging pattern used in message-oriented middleware and libraries such as ZeroMQ and nanomsg to implement synchronous request-reply interactions. In this pattern, a requester (REQ) sends a single request to a responder (REP) and waits for a corresponding reply. The communication is intended to be strictly alternating: a REQ socket must perform a send operation followed by a receive operation, and a REP socket must perform a receive followed by a send. This makes the pair behave like a simple RPC channel, where requests are matched to replies in order.

Semantics and behavior: A REQ socket cannot issue a new request until the previous response has been received; a REP must not emit a reply until it has received a request. If the reply is delayed or lost, the peer may block on its next operation, potentially causing deadlocks in simple pipelines. The pattern is therefore easy to reason about but limits in-flight requests.

Applications and limitations: REQ/REP works well for straightforward synchronous services, such as a small number of clients querying a service or a microservice that requires a tight request-response contract. For higher throughput or asynchronous workloads, patterns such as DEALER/ROUTER or a brokered queue are preferred, as they allow multiple in-flight requests and more flexible routing.

Limitations: It is not suited for out-of-order delivery, streaming, or asynchronous callbacks without additional framing or adapters. In practice, developers use REQ/REP for simple RPC-like endpoints, while using more advanced patterns for scalability.