Aaron Bertrand hits an outage in both directions:
The Stack Overflow database has a long lineage, and a lot more warts than what’s exposed in Stack Exchange Data Explorer (SEDE). Many of the core tables were created a decade before I joined the staff in 2021, and they grew far larger than could have been envisioned back then.
One of those tables is
UserHistory. This table uses an identity column as the clustered index and primary key, and records all kinds of information about each user’s activity on the site. From changing your profile picture, to earning a new privilege, to changing a preference, to something as simple as logging in or out. Each event generates at least one new row inUserHistory. At peak popularity, this table would grow quite quickly and, as the application became more complex, more and more categories of activity and state would get written there.
Click through for the story of an outage, and then another outage with a very similar shape.