What a Postmortem Owes You

1. The claim

Ask of any postmortem: what is now different? Not “what did we learn”, which is unfalsifiable, and not “what will we do”, which is a forecast. What is different, today, in a way another engineer would notice without reading this document.

If the honest answer is nothing, the incident is unresolved regardless of how good the write-up is. This is uncomfortable because the write-up is often genuinely excellent. Timeline theatre is the most convincing artefact in engineering: precise, chronological, full of real detail, and structurally incapable of changing anything.

2. Three failure modes

2.1. The weather report. A minute-by-minute account, accurate throughout, ending in a paragraph of reflection. It documents that a thing happened. It commits to nothing. It is the most common form and the hardest to criticise, because every individual sentence is true.

2.2. Root-cause singularity. The search terminates at the first satisfying explanation, usually the last change before the alert. Incidents in systems of any size are conjunctions: a latent defect, a configuration drift, a missing signal, and a human decision made with the information available at the time. Stopping at one of the four means the other three are still armed.

2.3. Action items with no owner, no date and no test. “Improve monitoring.” “Consider adding a circuit breaker.” An item that cannot fail cannot be completed. Six months later the list is still open and nobody can say whether that matters.

  incident
     |
     v
  narrative -------------> archive  (weather report)
     |
     +--> contributing factors
              |
              +--> decision
                      |
                      +--> owner
                      +--> verification date
                             |
                             v
                    something a future
                    engineer trips over

3. What the document owes the reader

Four obligations. They are ordered by how often they are skipped.

4. The part that is culturally expensive

A postmortem should name the moment of the missed decision, including where it was missed by the reviewer rather than the author. This is where blamelessness is most often misapplied. Blamelessness means the consequence to the individual is zero. It does not mean the record is vague. A document that will not say “this was approved in review, and the review did not ask about the retry budget” has removed the only detail from which the review process could learn.

Erratum 7.3 on this site is that exact case, written about me, and it is the reason I hold this claim at 0.90 rather than lower. I have watched the vague version fail and the specific version work, in the same organisation, eight months apart.

Figure 2. The inversion this paper asks for. The optional row is the one most templates make mandatory.

5. The strongest objection

6. What this paper does not claim

Timelines are not useless. They are the evidence the decision rests on, and a decision without them is an assertion. Nothing here argues for shorter documents, or for writing one after every incident. The four obligations in Section 3 are not sufficient either. My claim about them is weaker than it may read: a document missing any of the four has, in my experience, failed to change the system it describes, which is not the same as saying that a document containing all four succeeds.