Circuit Breaker

1. The Incident

It is 2:15 PM on Black Friday. Your checkout page calls a recommendation service to show "customers also bought" items. It is an optional widget. Nobody's purchase depends on it.

The recommendation database begins failing, and queries that normally take 20 ms now require 30 seconds or never return.

Although recommendations are optional, the slow dependency makes checkout unavailable.

The Timeout calculation explains the failure