What is a postmortem and what makes one blameless?

A postmortem (also called an incident review or retrospective) is a structured document written after a significant incident. Its purpose is to understand what happened, why it happened, what impact it had, and how to prevent recurrence. In SRE culture, postmortems are treated as a learning opportunity, not a blame-assignment exercise.

A typical postmortem includes:

• Incident summary: What broke, when, and for how long.
• Impact: Number of affected users, revenue or SLO impact, error budget burned.
• Timeline: Precise chronology of detection, escalation, diagnosis steps, mitigation, and resolution.
• Root cause analysis: The chain of contributing factors (using 5 Whys, fishbone diagrams, or similar).
• Action items: Specific, assigned, and time-bound follow-up tasks to prevent recurrence.

A blameless postmortem operates under the assumption that engineers make reasonable decisions given the information and tools available to them at the time. Rather than asking "Who caused the outage?", it asks "What conditions made this mistake possible?" and "How do we remove those conditions?" This approach, championed by John Allspaw at Etsy and codified in Google's SRE book, creates a psychologically safe environment where engineers honestly report their actions without fear of punishment.

Blameless postmortems produce higher-quality information because engineers do not hide or sanitize their actions. The result is better action items targeting systemic fixes (tooling, automation, process) rather than individual performance reviews.

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