Grokking the System Design Interview, Volume II
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Unique ID Generator: Failure Modes and Operations

Step 8: Failure Modes and Operations

Most failure questions in an interview are about data: what gets lost, what goes stale, what has to be repaired.

This system stores almost nothing, so most of those questions do not apply. It has one way to break badly, and it is the clock. The picture below lists the five failures and the one rule that covers them.

The clock goes backwards

Snowflake puts the current time in the top bits of every id. So an id is unique only while time moves forward on that node. If a node's clock jumps backwards, it reaches a millisecond it has already used.

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