Running anything on a single server, however, is really a non starter for anything remotely serious. What do you do if you need to do an OS update? I suppose you could just never do those, like a former employer (1000+ day uptimes...)
You'd definitely need at least two servers. But I think you could surely just have simple master/slave replication and switch between them.
Personally I wouldn't run a critical service on only one server, but two servers? Definitely doable. I actually have a service running on two servers in different DCs 700 miles apart. Zero downtime in 9 years. :)
Source: https://foreignpolicy.com/2009/06/16/state-department-interv...
You absolutely would not. The cost of having developers put out extremely optimized code (due to the scaling limits) and cuddle that single server to never, ever fail easily eclipses the cost of a having a multiple servers by a few orders of magnitude.
EDIT: To the downvoters, I'd really love to see the calculation on how engineering time would be cheaper than buying a second server in any reasonable timeframe.
(You are right, servers are cheap compared to employee costs though.)
With a second OS partition, the server can alternate between the working copy and a copy that is updated in a VM. For a free service, customers cannot complain even if there is a reboot every day and the service is down for a couple of minutes.
Realistically, there would be a mirroring server to be prepared for hardware failures. One server can be restarted while the other is the main server.
Are you seriously suggesting that a service (the size of Twitter, no less) has an acceptable downtime of a few minutes a day?
> Realistically, there would be a mirroring server to be prepared for hardware failures. One server can be restarted while the other is the main server.
But for that mirroring, you need to replicate disk writes, databases, backups etc.. This additional load would easily bring the server to a point where a single server would no longer suffice, even an insanely spec'ed one.
We kinda know the answer to this: Twitter was struggling with harm to its reputation for a long time because of regular Fail Whales. It absolutely was a huge problem for them at the time.
Who needs Twitter to be a service without any downtime?
