Grokking the System Design Interview, Volume II
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Notification System: Failure Modes and Operations

Step 9: Failure Modes and Operations

Everything this system depends on is outside it. Apple, Google, an email provider and an SMS gateway all sit on the delivery path, and none of them is under our control. So the failures worth designing for are mostly other people's.

A provider is down or throttling

An SMS gateway starts returning 503, or a push service begins rate limiting. Workers retry, the retries fail too, and the queue for that channel grows.

Three things keep this from spreading.

**One queue per channel

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