Sync vs. Async

1. The Definitions

Synchronous communication means the caller sends a request and then waits. It does nothing else until the response arrives. An HTTP call is the standard example. Checkout calls Payments and holds the connection open until Payments answers or the call times out. Think of it like calling a function, except the function runs on another machine.

While that call is open, the caller's fate is tied to the callee's. If Payments hangs, Checkout hangs with it.

Asynchronous communication means the caller sends a message and moves on right away