Request-response

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Request-response, also known as request-reply, is a message exchange pattern in which a requestor sends a request message to a replier system which receives and processes the request, ultimately returning a message in response. This is a simple, but powerful messaging pattern which allows two applications to have a two-way conversation with one another over a channel. This pattern is especially common in client-server architectures.^[1]

For simplicity's sake, this pattern is typically implemented in a purely synchronous fashion, as in web service calls over HTTP, which holds a connection open and waits until the response is delivered or the timeout period expires. However, request-response may also be implemented asynchronously, with a response being returned at some unknown later time. This is often referred to as "sync over async", or "sync/async", and is common in EAI implemetations where slow aggregations, time-intensive functions, or human workflow must be performed before a response can be constructed and delivered.