GitHub Availability Report: April 2023

In April, we experienced four incidents that resulted in degraded performance across GitHub services. This report also sheds light into three March incidents that resulted in degraded performance across GitHub services.

March 27 12:25 UTC (lasting 1 hour and 33 minutes)

On March 27 at 12:14 UTC, users began to see degraded experience with Git Operations, GitHub Issues, pull requests, GitHub Actions, GitHub API requests, GitHub Codespaces, and GitHub Pages. We publicly statused Git Operations 11 minutes later, initially to yellow, followed by red for other impacted services. Full functionality was restored at 13:17 UTC.

The cause was traced to a change in a frequently-used database query. The query alteration was part of a larger infrastructure change that had been rolled out gradually, starting in October 2022, then more quickly beginning February 2023, completing on March 20, 2023. The change increased the chance of lock contention, leading to increased query times and eventual resource exhaustion during brief load spikes, which caused the database to crash. An initial automatic failover solved this seamlessly, but the slow query continued to cause lock contention and resource exhaustion, leading to a second failover that did not complete. Mitigation took longer than usual because manual intervention was required to fully recover.

The query causing lock tension was disabled via feature flag, and then refactored. We have added additional monitoring of relevant database resources so as not to reach resource exhaustion, and detect similar issues earlier in our staged rollout process. Additionally, we have enhanced our query evaluation procedures related to database lock contention, along with improved documentation and training material.

March 29 14:21 UTC (lasting 4 hour and 57 minutes)

On March 29 at 14:10 UTC, users began to see a degraded experience with GitHub Actions with their workflows not progressing. Engineers initially statused GitHub Actions nine minutes later. GitHub Actions started recovering between 14:57 UTC and 16:47 UTC before degrading again. GitHub Actions fully recovered the queue of backlogged workflows at 19:03 UTC.

We determined the cause of the impact to be a degraded database cluster. Contributing factors included a new load source from a background job querying that database cluster, maxed out database transaction pools, and underprovisioning of vtgate proxy instances that are responsible for query routing, load balancing, and sharding. The incident was mitigated through throttling of job processing and adding capacity, including overprovisioning to speed processing of the backlogged jobs.

After the incident, we identified that the pool, found_rows_pool managed by the vtgate layer, was overwhelmed and unresponsive. This pool became flooded and stuck due to contention between inserting data into and reading data from the tables in the database. This contention led to us being unable to progress any new queries across our database cluster.

The health of our database clusters is a top priority for us and we have taken steps to reduce contentious queries on our cluster over the last few weeks. We also have taken multiple actions from what we learned in this incident to improve our telemetry and alerting to allow us to identify and act on blocking queries faster. We are carefully monitoring the cluster health and are taking a close look into each component to identify any additional repair items or adjustments we can make to improve long-term stability.

March 31 01:07 UTC (lasting 2 hours)

On March 31 at 00:06 UTC, a small percentage of users started to receive consistent 500 error responses on pull request files pages. At 01:07 UTC, the support team escalated reports from customers to engineering who identified the cause and statused yellow nine minutes later. The fix was deployed to all production hosts by 02:07 UTC.

We determined the source of the bug to be a notification to promote a new feature. Only repository admins who had not enabled the new feature or dismissed the notification were impacted. An expiry date in the configuration of this notification was set incorrectly, which caused a constant that was still referenced in code to no longer be available.

We have taken steps to avoid similar issues in the future by auditing the expiry dates of existing notices, preventing future invalid configurations, and improving test coverage.

April 18 09:28 UTC (lasting 11 minutes)

On April 18 at 09:22 UTC, users accessing any issues or pull request related entities experienced consistent 5xx responses. Engineers publicly statused pull requests to red and issues six minutes later. At 09:33 UTC, the root cause self-healed and traffic recovered. The impact resulted in an 11 minute outage of access to issues and pull request related artifacts. After fully validating traffic recovery, we statused green for issues at 09:42 UTC.

The root cause of this incident was a planned change in our database infrastructure to minimize the impact of unsuccessful deployments. As part of the progressive rollout of this change, we deleted nodes that were taking live traffic. When these nodes were deleted, there was an 11 minute window where requests to this database cluster failed. The incident was resolved when traffic automatically switched back to the existing nodes.

This planned rollout was a rare event. In order to avoid similar incidents, we have taken steps to review and improve our change management process. We are updating our monitoring and observability guidelines to check for traffic patterns prior to disruptive actions. Furthermore, we’re adding additional review steps for disruptive actions. We have also implemented a new checklist for change management for these types of infrequent administrative changes that will prompt the primary operator to document the change and associated risks along with mitigation strategies.

April 26 23:26 UTC (lasting 1 hour and 04 minutes)

On April 26 at 23:26 UTC, we were notified of an outage with GitHub Copilot. We resolved the incident at 00:29 UTC.

Due to the recency of this incident, we are still investigating the contributing factors and will provide a more detailed update in next month’s report.

April 27 08:59 UTC (lasting 57 minutes)

On April 26 at 08:59 UTC, we were notified of an outage with GitHub Packages. We resolved the incident at 09:56 UTC.

Due to the recency of this incident, we are still investigating the contributing factors and will provide a more detailed update in next month’s report.

April 28 12:26 UTC (lasting 19 minutes)

On April 28 at 12:26 UTC, we were notified of degraded availability for GitHub Codespaces. We resolved the incident at 12:45 UTC.

Due to the recency of this incident, we are still investigating the contributing factors and will provide a more detailed update in next month’s report.

Please follow our status page for real-time updates on status changes. To learn more about what we’re working on, check out the GitHub Engineering Blog.

Written by

Jakub Oleksy

@jakuboleksy