Making Postgres Queues Scale

Scaling Postgres-backed task queues for durable workflows presents challenges, particularly as workloads grow to tens of billions of workflows per month. A major issue is contention between multiple workers attempting to dequeue the same tasks simultaneously, which can be mitigated using Postgres's SKIP LOCKED feature, allowing workers to select and lock only those tasks not already being processed. However, transaction isolation levels, specifically REPEATABLE READ, can cause serialization failures at high concurrency, which can be alleviated by using READ COMMITTED for queues without global flow control. Additionally, inefficient indexes can lead to high CPU usage, necessitating more selective indexing that optimizes query performance and reduces maintenance costs, ultimately allowing the system to scale to over 30,000 workflows per second. These optimizations significantly enhance throughput and efficiency, demonstrating the potential for Postgres-backed systems to handle massive scales with proper tuning and adjustments.