[Devel] Limiting number of parallel calls

[Andreas Granig]

Hi,

I'm planning to develop a module for limiting parallel calls per user. 
My deployment has some pre-requisites, and it would be rather easy in my 
case:

- Proxies are physically distributed
- A common MySQL cluster is used as DB backend
- Session-Timer is used for detecting lost BYEs.

This leads to a simple module built up on dialog support. For INVITEs, a 
DB transaction is started, like:

- BEGIN;
- SELECT count FROM call_table WHERE uuid = <uuid> FOR UPDATE;
- check in module if "count" is less than the allowed number of calls 
fetched from usr_preferences (passed to the function via AVP)
- UPDATE call_table SET count = count + 1 WHERE uuid = <uuid>;
- COMMIT;

So this would be an atomic procedure across all proxies. Decrementation 
is done on BYE, any error in call-establishment or on dialog timeout.

The problem is that this scenario is, well, a little bit specific :o)
So to be useful for others too, it either has to be more generic, or 
there have to be different implementations for different system designs. 
This leads to the question, how that could be done best.

On systems with only one proxy, it could be implemented in shared 
memory. Distributed systems using SIP-replication have a problem here. 
So are there any ideas?

Andy