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1. IL-0385 GitHub MySQL read replicas hit a semaphore deadlock during a schema migration's table rename and kept crash-recovering

   GitHub2021

   Symptoms

   On 27 November 2021 at 20:40 UTC, a significant part of GitHub's MySQL read replicas entered a semaphore deadlock while a schema migration on a large MySQL table ran its final step, the rename that moves the updated table into place. Core GitHub services, including GitHub Actions, API Requests, Codespaces, Git Operations, Issues, GitHub Packages, GitHub Pages, Pull Requests and Webhooks, became degraded.

   Lock contention or deadlockDatabase overload

   Trigger

   The final rename step of a schema migration on a large MySQL table. Schema migrations are a common task at GitHub and often take weeks to complete.

   Root cause

   Software defect

   Component

   DatabaseSchema migration

   Contributing factor

   No headroomNo staged rollout

   Remediation

   • Promoted every healthy internal replica into the production path to add read capacity
   • Removed production traffic from the broken replicas until each had processed the table rename
   • Moved the recovered replicas back into production to restore capacity

   Action items

   • Continue functional partitioning so migrations can run in canary mode on a single shard
   • Update internal procedures to increase how much each cluster is over-provisioned
   • Pause schema migrations until it is known how to safeguard against this failure
   • Keep investigating the specific failure scenario
   • Test the migration tooling and classify ways to improve it for this kind of scenario

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