Blizzard provided some details on why maintenance got extended today as the team worked through some unexpected critical bugs related to the season transition.
(Source)
A little while ago, our Executive Producer asked our Lead Live Operations Producer to distill the answer to the question “Why?”. This is what he said,
We have the goal of performing Season transitions while keeping realms online for all regions. Due to how a number of systems were originally architected, we’ve had challenges in the past, and we iterate through issues each time. We work through expected issues quickly, but there are always unexpected issues that we can only deal with by testing on the realms in a hidden state. (Hidden because we didn’t want all players to feel the impact of our fixes – some require large data jobs). A number of issues we discover during maintenance require fixing while keeping realms offline to avoid in-game economy damage or players encountering bugs because they happened to be online just before we fixed them.
Overall, we’re able to minimize future downtime by finding and fixing “last minute” issues like this. As a result of today’s work, we hope that we will not need to do anything other than normal restarts for the remaining regions.
We’re finished with today’s maintenance. Each one-hour extension of the maintenance period was because we were fixing critical bugs related to the season transition, and the list of those grew smaller with each hour until it vanished.
