Hey Sarah, thanks for writing up a synopsis about the eventful day that we had today! I just wanted to offer a few corrections, as user trust is incredibly important, and we went into all changes today with user trust at the front of our minds.

WordPress’ Core systems team had an eventful Friday when an error in the auto-update system caused sites to update to WordPress 5.5.3-alpha-49449, including live production sites with no auto-update constants defined.

Auto-updates do not need constants defined, or special settings to receive auto-updates on minor releases. This is a core feature of WordPress that makes it possible to dole out features to millions of sites over the hundreds of releases over the last few years. With the 5.5.2 release on Thursday, we auto-updated code to 19 versions of WordPress. All of the way back to 3.7!

The primary issue that led to today’s release stems from the way that WordPress handles database connections. We added a change to the 5.5.2 release, and then after that release, an issue was reported that installations were failing. With failing installations in mind, the release team jumped into action knowing that if installs are prevented, that's obviously a huge issue, and we aimed to resolve this as soon as possible.

To slow the spread of this problematic update that was already rolling out, we wanted to change the API to prevent new downloads while we fixed the problem and rolled out a new minor release. The update API was rolled back by the systems team to 5.5.1 as a stop-gap.

Unexpectedly, it caused this 5.5.3 alpha state for a small subset of sites that were connected to the update server during this window of about 24 minutes. After realizing the problem, a change was made to pin the version and stop the updates.

It was after this change that the reports started coming into Trac and the WordPress.org slack. Obviously, this was an unintended result of the change. Triaging the issues, both in the forums and in Trac, was carried out quickly.

The minor release team returned focus to the pressing issue around installations and they were able to pull the release together. The change was backported from WordPress 5.5 to WordPress 5.1, fixing all sites that would be affected, both with the alpha status, and the installation issue.

Ideally in testing, both user and automated, we would have caught this issue ahead of the release. That's not what happened, but I am incredibly proud of how the WordPress core team was able to roll out a fix successfully to the millions of sites that connect for updates.