Why do servers “go down” for routine maintenance? Is there not a backup where traffic can route?
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Usually a website has a database with all the important information about users and everything you see on the site (posts / comments / videos / ads for a social media website, or name / address / accounts / debit cards / transactions for a banking website).
Having multiple copies gives you a backup, but it also raises the possibility the website gets “confused” if the multiple databases have different data.
So to make it simpler, they often only have one copy of the database, and take it offline. They upgrade the software, and maybe run some special software to convert database records from an old format to a new format (called “database migration”).
Things get really confusing if your software has to be coded to be dealing with changes simultaneously being made by an older / newer version of itself, or database records that could be in either an old or new format.
Some companies like Google pay enormous amounts of money to have large, high-skill engineering teams, but a lot of other companies are unwilling or unable to accept the costs, headaches and risks.
Even if you’re willing to do it, it’s often not really even possible if your software wasn’t originally designed to be upgraded while online. Basically not just the software, but your entire company needs to build from the ground up with a mentality of “we’re online 24/7” and be willing / able to write its own software when other companies’ software doesn’t meet those standards.
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