Imagine a payments service that starts timing out at 14:02. Your app keeps sending requests. Each one waits two seconds and fails. By 14:04 the worker pool is full, and checkout is down because one dependency is sick.

A circuit breaker stops the sending. After a set number of failures it returns an error without making the call. Your app keeps the capacity it would have spent on calls that cannot succeed.

The calls that cannot succeed

A breaker wraps a remote call and watches the result. Past a threshold it stops calling. Azure's architecture guide states the purpose: the pattern "helps prevent an application from repeatedly trying to run an operation that's likely to fail."

Martin Fowler credits Michael Nygard's book Release It! with popularizing the pattern. In Fowler's account, the breaker trips once failures reach a threshold. Then "all further calls to the circuit breaker return with an error, without the protected call being made at all."

You save the wait, not the error.…