How Postgres FOR UPDATE SKIP LOCKED solves concurrent resource allocation

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A developer building StashMe, a digital cloakroom management system for venues, encountered a race condition where multiple simultaneous check-ins could assign the same coat hanger to different guests. The naive SQL approach of reading then writing allowed two transactions to claim the same row before either could lock it. Postgres's FOR UPDATE SKIP LOCKED clause, available since version 9.5, resolves this by having concurrent queries skip rows already locked by other transactions rather than waiting on them. This allows multiple check-ins to proceed in parallel, each claiming a distinct free hanger with no queuing or deadlocks. The same pattern underpins popular Postgres-based job queue libraries and can be applied broadly to any system distributing finite resources under concurrent load.

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