High availability replicating every fifteen minutes? I see several problems with this:
1) Session information is probably impossible to replicate; anyone who has an open session on a server that goes down will lose his/her session.
2) Generally IP# changes will fail for someone who has accessed a page recently, even without sessions, meaning viewers currently on a site when one server goes down will probably lose their access to the site unless they know to clear all DNS cached anywhere between them and the authoritative servers (which may not be possible).
And then there's the issue of replicating your MySQL across various servers. Have you figured out how to do that across the 'net betweeen datacenters?
Because if you're not in multiple datacenters, then you're not truly high availability; what happens if connectivity to the datacenter goes down (or as has happened within the past year) if the entire datacenter goes down?
So the right way to implement high availability is by running the front end to the site on multiple servers in multiple datacenters, with the database managed on a separate high-availability server. with multiple connections, RAID drives with hot-swap or a storage-area-network, and redundant power supplies.
That said, we recently had a hard machine failure (our first in three years). It was easy enough to quickly swap the drives to another server, and get the sites up and running again.
In this case it didn't happen as quickly as we'd liked because the swap-server wasn't in the datacenter, and there was a major traffic accident on the road to the datacenter (they closed it in both directions for over an hour; they had to do helicopter-evacuation), and the same morning many streets in Los Angeles (within a quarter-mile of the datacenter) were closed for a parade honoring the Los Angeles Lakers basketball team.
But it could have, if we'd had the server already in the datacenter. Which we should have had; and we now know better.
If you've got an extra server on standby in the datacenter and standby staff (most datacenters offer it at a cost) you can have the drives swapped rapidly at a lot lower cost than true redundancy.
Note there are still issues; for example you should NOT have the NIC IDs in your network configuration files; if they're wrong the network won't start (and they will be wrong after a drive swap) but if they're absent the network will start just fine.
We're currently working on a combination of email redundancy from a different datacenter and redundant server availability in the hosting datacenter; that should satisfy most clients in a cost-effective manner.
Jeff
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