[Users] clustering

[Andreas Granig]

Edson wrote:
> I use this kind of clustering, but there is a performance penalty. Not so
> high, but...

Right, but most importantly it scales. If you observe performance 
degradation, you can just add another host without needing to touch the 
other instances. You just have to add the new instance to DNS-SRV or to 
a load-balancer.

If performance becomes an issue in a t_replicate environment, adding 
another host doesn't help you much, because each registration has to be 
handled at every single host of the cluster. And registrations are 
typically the most frequent requests in a system.

That's just beside the fact that if one host goes down, you will always 
get an inconsistent usrloc cache among the hosts, because if the host 
comes up again, you have to mysql-dump the location table from another 
host and start the proxy. Between the moment of the dump and the moment 
the proxy finishes reading the location table into the cache, you will 
miss all registrations. This becomes a problem if you have a large 
number of actively registering subscribers.

These two issues are the main reasons why I'm very happy with a 
cache-less solution.

Andy