[Serdev] usrloc loading

[Andreas Granig]

Hi all,

Greger V. Teigre wrote:
> Good for a start, some design-principles...  Do you take my challenge? ;-)

I dare to jump in here for a comment :o)
In my opinion, there are two reasonable concepts for scaling a system.

One is to use SIP proxies as balancers which route requests to the 
appropriate proxies/registrars. Each proxy/registrar only hosts a subset 
of the overall user-base, and the balancers know, which proxy/registrar 
is responsible for which users (e.g. by hashing the r-uri). This way, 
there's no need to share the location table. There must be a way though, 
to keep each proxy/registrar redundant, e.g by using a standby node. 
Nonetheless, the active node has to synchronize its location table to 
the standby node, e.g. by using database replication, in case the 
standby node has to take over.

The second possibility is to share a location table across all 
proxies/registrars, e.g. by using a database cluster. This way, a load 
balancer could route requests to any of the proxies, because each node 
knows the location of each user. There's no need for standby nodes, 
because the remaining nodes take over the load of the failed node.

Each of the solutions has its advantages and disadvantages, and both 
rely on external software to work. The first approach uses database 
replication, but needs some failover-management-software for node 
fail-over, it needs changes in the provisioning work-flow if a new 
proxy/registrar pair is added, each proxy/registrar needs a standby 
node, etc. The second approach heavily relies on some sort of database 
cluster and needs a cache-less usrloc, but is easier to scale and 
doesn't need standby nodes.

Jiri seems to go for the first solution, I like the second one very 
much. Both have proved to work, so I think it's just a matter of taste, 
and maybe also experience with one or another subject area the two 
solutions rely on (like experience with node-failover-software versus 
experience with mysql-cluster).

Regards,
Andreas