Incident documentation/20200625-caching-sessions

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document status: draft

Summary

See also <https://lists.wikimedia.org/pipermail/wikitech-ambassadors/2020-June/002316.html>

What happened, in one paragraph or less. Avoid assuming deep knowledge of the systems here, and try to differentiate between proximate causes and root causes.

Impact: Who was affected and how? In one paragraph or less. For user-facing outages, estimate: How many queries were lost? How many users were affected, or which populations (editors? readers? particular geographies?), etc. Do not assume the reader knows what your service is or who uses it.

Timeline

Write a step by step outline of what happened to cause the incident, and how it was remedied. Include the lead-up to the incident, and any epilogue.

Consider including a graphs of the error rate or other surrogate.

Link to a specific offset in SAL using the SAL tool: https://tools.wmflabs.org/sal/, For example: https://tools.wmflabs.org/sal/production?q=synchronized&d=2012-01-01

All times in UTC.

  • 00:00 (TODO) OUTAGE BEGINS
  • 00:04 (Something something)
  • 00:06 (Voila) OUTAGE ENDS
  • 00:15 (post-outage cleanup finished)

TODO: Clearly indicate when the user-visible outage began and ended.

Detection

Write how the issue was first detected. Was automated monitoring first to detect it? Or a human reporting an error?

Copy the relevant alerts that fired in this section.

TODO: If human only, an actionable should probably be to "add alerting".

Conclusions

What weaknesses did we learn about and how can we address them?

What went well?

  • (Use bullet points) for example: automated monitoring detected the incident, outage was root-caused quickly, etc

What went poorly?

  • (Use bullet points) for example: documentation on the affected service was unhelpful, communication difficulties, etc

Where did we get lucky?

  • (Use bullet points) for example: user's error report was exceptionally detailed, incident occurred when the most people were online to assist, etc

How many people were involved in the remediation?

  • (Use bullet points) for example: 2 SREs and 1 software engineer troubleshooting the issue plus 1 incident commander

Links to relevant documentation

Add links to information that someone responding to this alert should have (runbook, plus supporting docs). If that documentation does not exist, add an action item to create it.

Actionables

Create a list of action items that will help prevent this from happening again as much as possible. Link to or create a Phabricator task for every step.

TODO: Add the #Wikimedia-Incident-Prevention Phabricator tag to these tasks.