Guidance on using Kamailio as Presence Server and Scalability issues for millions of subsribers
We're trying to determine how best to design/architect a solution where we use Kamailio as a Presence/XCAP server and be scalable to support millions of subscribers. I see the Dispatcher module, but it appears that requires having dedicated instances of the server running just in Dispatcher mode - then you can load balance across the actual instances of the presence servers. It's not clear if a server can be configured to work as a dispatcher and a presence server at the same time.
Does anyone have any high-level guidance how to approach this problem?
Thank you,
Tom Hoffman
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On 15.10.2012 23:36, Thomas Hoffman wrote:
We’re trying to determine how best to design/architect a solution where we use Kamailio as a Presence/XCAP server and be scalable to support millions of subscribers.
I see the Dispatcher module, but it appears that requires having dedicated instances of the server running just in Dispatcher mode – then you can load balance across the actual instances of the presence servers.
Yes, it is typical to use a dispatcher to dispatch the load over several servers which do the real stuff (presence, gateways ...), eg:
clients <--Internet-----> dispatcher <------- server 1 ------- server 2 ------ server X
Dispatching (eg. Kamailio with dispatcher module) is much less resource intensive (e.g. no DB lookups, just stateless forwarding), thus a single dispatcher can handle the load for several backend servers.
If the load is too big for a single dispatcher node, then you need more dispatcher nodes. You can use for example multiple SRV records for the SIP domain, or provision the clients with different IP addresses (of the respective dispatcher nodes). Then, every dispatcher sends the traffic to a certain partition of the backend servers, or you distribute the traffic over all backend servers.
It’s not clear if a server can be configured to work as a dispatcher and a presence server at the same time.
Yes you can but it does not make sense - if a node itself is a presence server it does not make sense to dispatch the request to other presence servers. One if the main advantages of having a dispatcher in front of the cluster is to have a single border element (single IP address) which eases problems like NAT traversal.
Does anyone have any high-level guidance how to approach this problem?
Presence requires, that a presence server which receives a subscription for a presentity, also knows the status of this presentity. There are 2 approaches to handle this problem:
a) make sure to send PUBLISH requests from a certain presentity, and the SUBSCRIBE requests to this presentity, to the same presence backend. In this setup, every presence server uses its own database (or just memory without a DB). In case a presence server is removed (crash, maintenace), all traffic needs to be moved to other nodes and the current state is usually lost. Further you have to be sure to have the XCAP data available to all nodes.
b) all presence servers use the same database. In this case, the presence servers can not do any caching and have to query the DB for every operation. This setup simplifies the SIP setup, but increases the load on the DB and you may need a fast DB cluster. There were recently much changes to the presence modules for such a DB-only mode.
regards Klaus
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Klaus Darilion -
Thomas Hoffman