Incident Postmortem: Catalog Latency Spike

On June 9 the catalog service experienced elevated p95 latency and intermittent 502s for approximately 47 minutes during peak traffic. This document captures the timeline, root cause, and the follow-up actions we committed to.

Summary

A configuration change to the catalog read-replica routing shifted a disproportionate share of traffic to a single replica that was mid-reindex. The replica's connection pool saturated, queue depth grew, and downstream callers began timing out and retrying, amplifying load.

Timeline (UTC)

• 13:02 — Config change merged: replica weighting adjusted from 4×25% to 70/10/10/10.
• 13:18 — p95 latency on catalog crosses 150 ms; first 502s observed.
• 13:24 — On-call paged; incident opened (INC-2284).
• 13:41 — Replica reindex identified as concurrent load source.
• 13:55 — Weighting reverted; latency begins recovering.
• 14:05 — p95 back under 130 ms; incident downgraded.

Root cause

The routing weights were changed without accounting for the replica's scheduled reindex window. The change passed CI because the staging environment does not run reindex jobs on the same schedule, so the interaction was never exercised before production.

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What went well

• Detection was fast — alerting fired within 16 minutes of the change.
• The revert path was clean and well-understood; recovery took under 12 minutes.

Follow-ups

• Add a pre-merge check that blocks replica-weight changes during scheduled reindex windows.
• Mirror the reindex schedule into staging so the interaction is exercised in CI.
• Add per-replica connection-pool saturation alerts.

Generated from the Reliability workspace · for internal review.