Are you saying no downtime is impossible or merely expensive?
Erlang/OTP with relups is an example of a system that provides zero downtime online upgrade and rollback with zero downtime.
Erlang/OTP with relups is an example of a system that provides zero downtime online upgrade and rollback with zero downtime.
> 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.
https://www.mysql.com/customers/view/?id=750 http://highscalability.com/youtube-architecture