Gary Pigott wrote:
> Round robin DNS is the way to go for load balancing, but it's not a way
> of implementing redundancy. If one of your servers goes down, the DNS
> server will still hand out it's IP as before. TTLs & DNS caching, and
> propagation delays ensure that there's no way you can change the DNS
> records fast enough in the event of an outage to redirect traffic only
> to the live server.
It was sort of implied, but I guess not obvious, that the monitoring
system would update the DNS in the case of a switchover.
Obviously a DNS based switchover is not instantaneous, but it is
pretty easy to implement compared to multihoming.
> Multihoming may be the way to go, although I'm not sure if you can
> multihome an IP address to two sites. We multihome 1 IP address down
> multiple pipes (from multiple providers) to the same location using BGP.
> That way our clients never notice a link go down and always get the
> lowest hop count when connecting. I suppose there's nothing stopping one
> of the links pointing to a separate site, giving you multi-site load
> balancing. There are others on this list that know this kind of thing
> way better than I do (and do contract work for a reasonable fee).
I don't see why it shouldn't be possible to set things up this way. How
quickly does a BGP routing change propagate in the case of a switchover?
> The one tricky thing is reconciling transactions after an outage.....
> it's entirely possible that the link between both sites can go down, and
> both accept traffic assuming the other is dead.... this can lead to
> duplicate transaction IDs etc. You need to have some mechanism in place
> to either tolerate this within the application or to reconcile them later.
That's why the monitoring + switchover system really needs to live in a
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