A nuclear blast in northern Virginia can take out all the availability zones for AWS US-East-1 - that doesn’t invalidate the availability claims of a system that is distributed across three AZs, it just puts an upper limit on it.
If so, I agree - in that case it would be similar to running two severs in the same AZ. Which provides a certain form of availability.
Otherwise I disagree, because as soon as you have a single point of failure, your system is limited by the availability of that specific part, whereas in a distributed system you can by design increase availability by adding more nodes, at least up to a certain point.
Think about it: I can run the two servers I mentioned about in the same AZ. But I can also move them to a separate AZ each. For a distributed system it doesn't matter conceptually. That doesn't mean that there is no impact on the availability or performance, it just means that I don't have to change the logic of my system.
For a mainframe however that's not true - and that is what makes the charm of it: because I don't have (and don't want) to care, which simplifies things a lot; at the expensive of (the option of) the availability of a distributed system.
> One, quite commonly known, is that two separate mainframes can run completely in lock-step if they are less than ≈ 50 km / 30 mi apart
Since they are 50km apart, they are connected to each other by one or multiple cables. I assume that they are also both connected to the internet (or something else that matters) so that if one burns down the other can take over. Correct so far?
If so, then please answer this questions: how does the system behave if all the cables between the two machines are severed, for an unknown time?
The ultra-scalable techniques that Google, Facebook, ect, use are great, but most applications do not need that kind of scalability. Throwing a little more money on a fancy mainframe is much, much cheaper than the programmer time needed to build a distributed system.
Which is odd, when you think about it, since you allegedly made this choice in order to get better availability.
Though, thinking about it more, what it also tells us is that we need to be clear about what "the system" is. There are, after all, some systems that _do_ take nuclear blasts into account, or systems that take "physical breach by a hostile actor" into account.
Less dramatically, we make trade-offs all the time as to what we consider "in" system vs "out", and it's good to be conscious and explicit about them. We see this a lot in UI, especially web UI in terms of things like what browsers to support, whether we care about users' battery life or data cap limits, if we foresee ourselves running the system in a jurisdiction with different regulations around data collection and usage, etc.