I couldn't save this to docs/postmortems/2026-02-10-checkout-api-500-errors.md because I have no file-write tool in this session. The full document is below, ready to paste into that path.

Owners and due dates are placeholders. The skill requires a named person for each action, and I only know Maya's name. Due dates are proposed, so please confirm them.

Post-Mortem: Checkout API 500 Errors

Date: 2026-02-10
Severity: P2
Duration: 1 hour 15 minutes (14:05 UTC – 15:20 UTC, detection to resolution)
Incident Commander: @Maya
Status: Resolved

Summary

After the v2.41 deploy at 13:58 UTC, the checkout API began returning 500 errors. A new query in the release loaded full result sets into memory, which caused pods to run out of memory and crash-loop. At peak, about 12% of requests failed, against a 0.1% baseline. Rolling back to the previous version at 15:00 restored service, and recovery was confirmed at 15:20. No data was lost and no SLA was breached.

Impact

Duration note: The impact window began at the 13:58 deploy. Detection to resolution was 1h15m. Deploy to resolution was about 1h22m.

Timeline

All times UTC, 2026-02-10.

Gaps to verify from logs, alerts and Slack:

• When the first 500s appeared. The alert fired 7 minutes after the deploy, but the first symptom may have been earlier.
• What happened between 14:35 and 15:00. Root cause was known, but the rollback took 25 minutes. This could have been decision time, the rollback process, or the deploy pipeline.
• Whether any customer or internal communication was sent, and when.
• Whether the incident was formally declared, and at what time.

Root Cause

The release shipped a query that loaded entire result sets into memory instead of bounding or streaming them. For accounts with large result sets, memory use exceeded pod limits, and the pods crashed and restarted repeatedly. This produced the 500s.

The underlying gap is that the pre-release process had no way to catch this. Load tests covered only median accounts, so memory behavior for large accounts was never exercised before production.

5 Whys

• Why did users see 500s? The checkout API pods were crash-looping.
• Why were they crash-looping? They exceeded their memory limits.
• Why was memory exceeded? The new query loaded full result sets into memory.
• Why wasn't this caught before deploy? Load tests covered only median accounts.
• Why? There were no test fixtures or scenarios for large accounts, so a memory-heavy query path looked fine under test.

Contributing Factors

• Memory alert threshold at 90%. Pods can reach OOM before a 90% alert gives useful lead time, so the memory alert did not give an early signal.
• No runbook for OOM pods. Responders had no documented path for diagnosing memory crash-loops or deciding on a rollback.
• Load tests covered only median accounts. This is the same gap as the root cause, and it also meant the release had no large-account validation.

What Went Well

• The alert fired 7 minutes after the deploy.
• The on-call was paged within 5 minutes of the alert.
• Root cause was identified about 30 minutes after the page.
• Rollback resolved the issue, and recovery was explicitly confirmed.
• No data was lost and no SLA was breached.

What Could Have Gone Better

• 25 minutes passed between identifying root cause and starting the rollback.
• The release had no staged rollout or canary that might have limited exposure to the failing path.
• Memory pressure was not visible early, because the alert threshold left little warning.
• Responders had no runbook to speed up diagnosis and the rollback decision.

Action Items

Lessons Learned

• Tests built around typical accounts can hide failures that only appear at the tail. Memory-bound code paths need large-data fixtures.
• An alert threshold that is only reached when a pod is about to die gives little warning. OOM kills and restart counts are better early signals.
• Once the cause is known, rollback speed matters as much as diagnosis speed. A runbook and clear rollback criteria would shorten the gap between the two.

Next steps:

• Fill in owners for the TBD items.
• Check the flagged timeline gaps against logs and Slack, especially 14:35–15:00 and any customer communication.
• Add a user-count estimate if one is available.