Why are websites (even this one) sometimes "Down for Maintenance"?

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No server NEEDS to go down for maintenance. You can avoid doing so for anything, at any scale, DB change, server updates, etc.

The problem is that a 0-downtime system, at a certain scale, is very costly to create and maintain. You need redundancy everywhere, load balancing everywhere, data replication, synchronization. Those are hard problems.

Basically you need to arrive to the level of being able to release the Netflix Chaos Monkey in prod to be sure it works even if part of your system is busy with the update, or just out of sync. This is certainly doable. It's also very expensive, requires a lot of time and many experts to work on the problem.

Putting a site on maintenance mode can be a middle ground you choose, because you don't want to invest that much just to avoid taking down you site for a little time once in a while.

Economics.

Of course, if you do choose the road of 0down time, you site will gain more than just availability, it will gain reliability as well, since those best practices serve both purposes.

Source: Software Engineering Stack Exchange