Notification system. Codes and campaigns

A shop’s notifications: OTP codes and promotional campaigns take separate paths (a bulkhead), so Black Friday does not hold up sign-ins, and a spare provider saves the codes when one goes down.

Diagram of a notification system: a bulkhead keeps urgent OTP codes apart from a huge campaign queue, and a spare provider takes over when one fails.

System Design — Load balancers, caches, queues and regions: build a system that holds the load