Incident with Pull Requests

Updates timeline

1. Investigating Jul 24, 07:37 PM UTC

   We are investigating reports of degraded performance for Pull Requests

2. Investigating Jul 24, 07:40 PM UTC

   We are investigating errors creating pull requests

3. Investigating Jul 24, 07:43 PM UTC

   Pull Requests is experiencing degraded availability. We are continuing to investigate.

4. Investigating Jul 24, 07:59 PM UTC

   We have applied a mitigation and are monitoring for recovery

5. Monitoring Jul 24, 08:02 PM UTC

   The degradation affecting Pull Requests has been mitigated. We are monitoring to ensure stability.

6. Resolved Jul 24, 08:23 PM UTC

   Between July 24, 19:17 UTC and July 24, 20:02 UTC, users were unable to create pull requests due to a database schema change. In total, 113,930 pull request creation attempts were impacted across 50,904 users, with an average error rate of 1.75% and a maximum error rate of 2.25% for all requests to Pull Requests service. Existing pull requests and other GitHub functionality were not affected. The issue was resolved by reverting the change to the affected database, upon which pull request creation immediately resumed.<br /><br />The root cause was related to a backfill workflow into the Vitess keyspace hosting Pull Request data. The backfill Vitess command encountered errors and increased VReplication lag, and the workflow was canceled at 19:17 UTC. The cancellation executed a misunderstood Vitess codepath that dropped the backing table to the target keyspace, leaving a non-existent reference that resulted in errors creating Pull Requests. The mitigation was executing a command to drop the vschema reference to the dropped table, allowing Pull Request creation to resume.<br /><br />We are adding stronger pre-flight validation to our tooling to prevent similar issues and expanding lower-environment support to provide better test coverage end-to-end before promoting them to production. We're also fixing our backfill migration tooling to protect from this specific codepath.