pgstore: advisory lock hash collisions can serialize unrelated streams #160

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opened 2026-03-01 12:35:29 +00:00 by ash · 0 comments

ash commented

2026-03-01 12:35:29 +00:00

Owner

Problem

Red team found: hashtext(streamID) maps stream IDs to int32 advisory locks. Two different streams can hash to the same value, causing one to block while the other holds the lock. Under heavy load with many streams, this reduces throughput.

Options

Use two advisory locks (high + low 32 bits of a 64-bit hash) — reduces collision probability to ~1 in 4 billion
Use pg_advisory_xact_lock(bigint) with a 64-bit hash
Accept the collision rate and document it — at 10K concurrent streams, collision probability is ~0.001%

Priority

Medium — causes throughput degradation, not data corruption

## Problem Red team found: `hashtext(streamID)` maps stream IDs to int32 advisory locks. Two different streams can hash to the same value, causing one to block while the other holds the lock. Under heavy load with many streams, this reduces throughput. ## Options 1. Use two advisory locks (high + low 32 bits of a 64-bit hash) — reduces collision probability to ~1 in 4 billion 2. Use `pg_advisory_xact_lock(bigint)` with a 64-bit hash 3. Accept the collision rate and document it — at 10K concurrent streams, collision probability is ~0.001% ## Priority Medium — causes throughput degradation, not data corruption