Update 2021-10-31T14:19:00Z: We are done. Thank you for your patience and all your supportive tweets.
Update 2021-10-31T13:22:00Z: Today's work will be starting shortly. https://twitter.com/StackStatus/status/1454800691898339329 and https://stackstatus.net/post/666565626004979712/maintenance-beginning
Update 2021-10-30T17:07:00Z: The site is back. It took a lot longer than expected (our apologies) but we decided to take the safe/slow route rather than take some risks. Tomorrow's maintenance window is still required.
Update 2021-10-30T14:00:00Z: This will be starting soon. Once the website go into read-only mode it will last 1-2 hours. See https://twitter.com/StackStatus until we are back.
Due to ongoing planned network upgrades, all public Stack Exchange properties (stackoverflow.com, stackexchange.com, and so on) will be in read-only mode during two maintenance windows: Saturday, October 30 and Sunday, October 31, 2021.
While in read-only mode, the public Q&A network sites will be accessible, but no new content may be added or modified (questions, answers, comments, votes, etc.). Also, Teams will not be accessible as it requires authentication that is paused in read-only mode.
Once read-only mode starts, it is expected to last no more than 2 hours, though our goal is much shorter.
What is happening?
This is the last of 4 major network hardware upgrades planned for 2021. While all of our network equipment is redundant and can usually be upgraded without downtime, there are a few rare procedures that are high risk or require us to switch to read-only mode to avoid actual downtime.
While these upgrades involve many days of actual work, we have minimized the user-visible interruption and isolated it outside of peak usage times.
Expect a blog post with the exciting details after we've had time to recover. This has been quite exhausting.