Incident postmortem

Incident postmortem

The purpose is a system that cannot fail this way again: not a culprit. The moment a postmortem blames a person, everyone stops telling the truth in the next one.

Structure

1. Summary: three sentences: what broke, who felt it and for how long, and the one-line cause. Written last, placed first.
2. Impact: numbers, not adjectives: users affected, requests failed, data lost or at risk, duration, money if known. "Significant impact" is a tell that nobody measured.
3. Timeline: timestamped facts from first trigger to full resolution: what happened, what people saw, what they did. Include the wrong turns; the 40 minutes spent on the wrong hypothesis is signal about your observability, not a person's failure.
4. Root cause analysis: walk the why-chain past the trigger to the conditions: the deploy was the spark; the missing rate limit, the silent fallback, and the alert that pages nobody are the fuel. Most incidents need three to five "why"s before reaching a cause you can fix in a system. Name mechanisms ("retries amplified load 40×"), never persons ("X pushed bad config" → "config validation accepted a value that cannot work in production").
5. What went well / what was lucky: the alert that fired correctly; the fact it happened Tuesday 10:00 and not Saturday 03:00. Luck listed honestly today is a to-do item, not a defense, tomorrow.
6. Action items: each with an owner, a date, and a mechanism: prevention (make the failure impossible), detection (find it in one minute, not forty), mitigation (halve the blast radius). "Be more careful" is not an action item; "reject configs that fail validation at deploy time" is. Three completed actions beat twelve open ones.

Rules

• Facts and interpretation stay visibly separate; the timeline is facts only.
• Write for the engineer who joins next year: no unexpanded acronyms, no tribal context assumed.
• If the same class of incident has happened before, say so and link it , a repeat is a statement about the previous action items.

Litmus test

If every person involved left the company tomorrow, does the document still prevent the next occurrence?