Postmortem: Web Stack Outage [EXAMPLE]

Postmortem: Web Stack Outage [EXAMPLE]

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Issue Summary:

Duration: 4 hours (May 15, 2023, 10:00 AM - 2:00 PM GMT)

Impact: Slow response times and intermittent service disruptions experienced by users. Approximately 30% of users were affected during the outage.


  • 10:00 AM: The issue was detected when the monitoring system generated multiple alerts for high latency and increased error rates.

  • Actions Taken:

    • The operations team initiated an investigation into the web stack components, including web servers, load balancers, and databases.

    • The initial assumption was that the database servers might be overloaded or experiencing performance issues.

  • Misleading Investigation/Debugging Paths:

    • Extensive database performance analysis was conducted, including index optimizations and query tuning, but no significant issues were found.

    • Load balancers were also thoroughly examined for misconfigurations or network bottlenecks, but no abnormalities were detected.

  • Escalation:

    • As the issue persisted, the incident was escalated to the development team and system administrators for further analysis and resolution.
  • Incident Resolution:

    • After reviewing system logs and analyzing network traffic, it was discovered that a recent deployment had introduced a misconfiguration in the web server's caching layer.

    • The misconfiguration caused excessive caching, resulting in stale content being served to users and increased load on the servers.

    • The misconfiguration was corrected, and the web servers were restarted to apply the changes.

Root Cause and Resolution:

  • Root Cause:

    • The root cause of the issue was identified as a misconfiguration in the web server's caching layer.

    • Specifically, the cache expiration settings were set too high, leading to stale content being served to users.

  • Resolution:

    • The misconfiguration was fixed by adjusting the cache expiration settings to appropriate values.

    • The web servers were restarted to apply the changes and ensure the correct caching behavior.

Corrective and Preventative Measures:

  • Improvements/Fixes:

    • Implement automated configuration validation checks for the web server's caching layer during the deployment process to prevent misconfigurations.

    • Enhance monitoring and alerting capabilities to promptly detect and notify about abnormal caching behavior or high latency.

  • Tasks:

    • Conduct a thorough review of the deployment process to ensure proper configuration management and validation.

    • Develop and implement a comprehensive caching strategy, including proper cache invalidation mechanisms and appropriate cache expiration settings.

    • Enhance system monitoring by adding specific metrics for cache hit rates, cache expiration, and latency.

    • Conduct regular performance testing and load testing to identify potential bottlenecks or misconfigurations before they impact users.

This postmortem provides an overview of a web stack outage that occurred due to a misconfiguration in the caching layer. It highlights the timeline, actions are taken, misleading investigation paths, root cause, resolution, and the corrective/preventative measures to be implemented. By addressing these issues and implementing the suggested tasks, the aim is to improve system stability, enhance performance, and minimize the chances of similar outages in the future.