[carterehsmith]

Caveat here. Sure, a mainframe/high end hardware & supervisor OS can run for 30 years... but the actual applications that users are facing... no, they cannot. You need to upgrade DB2 or IMS or whatever Java app you are using? There will be downtime.

[nickpsecurity]

Depends on the design. You have to plan for that stuff ahead of time. I'm not going to claim that's easy. It's just very helpful and there's companies that specialize in helping with it. Most common method was decomposing the app while running it on a cluster so parts of the app or nodes can be taken down. There's strategies for mainframes, too, but my experience was clusters.

Basic strategy was putting something in front of them that can redirect to the new system upon a trigger. Let's assume its functionality + tons of data. The new system first gets the data moved to it in batch form for efficiency reasons. Once it catches up enough, it starts syncing in a more online fashion until it gets to point that it's syncing in real-time. All kinds of tests are performed on that system throughout this process. Eventually, a change-over happens that should be barely perceptible. The inability to do this is usually due to fragile architecture or tightly-coupled implementations which are unfortunately all too common in enterprises.

Note: It can also help if your app was written in something like Common LISP or Erlang that supports live updates. That with the delta approach (version A->A/B->B) equals upgrades with no downtime. ;) Combining it with clustering approach is quite powerful but clustering approach is more applicable to tools majority uses.