ID generator: what it does in a system design

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ID generator

Generates short IDs for writes.

Strategy: How unique ids are made. UUID and snowflake need no coordination and are nearly free. Counter uses one central service that can become a bottleneck. Hash-based retries on collisions.
Max QPS (counter): Counter only. How many ids per second the central counter can hand out. Calls beyond it queue and add delay, and a backlog over one second's worth is refused.
Codes per call (counter ranges): Counter only. How many ids a caller takes per trip to the counter and then hands out itself. Bigger blocks cut load on the counter, so it saturates much later.
Collision probability (hash_based): Hash-based only. Chance that a generated id is already taken and must be retried, from 0 to under 1. Higher adds delay per retry and can fail a request after 1000 tries.
Crashes: Scheduled crashes to test failure handling. While one is active, the target refuses requests, and anything it was holding is lost.