Interested in a concrete example of where you see a migration being impossible without downtime; I'm not aware of Amazon or Google taking their APIs down for maintenance, which suggests that you can solve these problems with enough engineering spend.
depending on... how coupled everything is
If you're just saying here that some systems aren't currently able to migrate in a hitless way, rather than claiming that it would never be possible for some systems, then I fully agree. Hitless migrations require a lot of work, and probably aren't worth the cost for many applications/systems.
Relational databases are awesome, but it's much harder to achieve 100% uptime compared to schemaless, distributed, replicated data stores. You easily can take down your system with one innocuous-looking DDL migration that works great in test but grinds the machine to a halt on a production dataset.
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Interested in a concrete example of where you see a migration being impossible without downtime; I'm not aware of Amazon or Google taking their APIs down for maintenance, which suggests that you can solve these problems with enough engineering spend.
If you're just saying here that some systems aren't currently able to migrate in a hitless way, rather than claiming that it would never be possible for some systems, then I fully agree. Hitless migrations require a lot of work, and probably aren't worth the cost for many applications/systems.
Neither Amazon nor Google run off of an RDBMS.
Relational databases are awesome, but it's much harder to achieve 100% uptime compared to schemaless, distributed, replicated data stores. You easily can take down your system with one innocuous-looking DDL migration that works great in test but grinds the machine to a halt on a production dataset.
That's a good general point, but Youtube does use MySQL:
https://www.mysql.com/customers/view/?id=750 http://highscalability.com/youtube-architecture