[Kamailio-Users] Dispatcher hash algorithms.

[Henning Westerholt]

On Tuesday 30 September 2008, Daniel-Constantin Mierla wrote:
> > [..]
> > Sure, it's great if my registrants always go to the same location
> > server, but if 90% of my users end up going to one location server
> > because of the distribution that the variance of their From URIs
> > provides, what does this really give me except a predictable route?
> > It's not as if I can use the hash to "find" a user's location server
> > -- unless the location server was determined using the same hash also.
> > What's the point?
>
> The hash function was tested to get pretty fair distribution for AoR,
> most of the hashed values respect this format. If 90% of your users end
> up to same server, then you may need to code a bit :-) and add an
> alternative hash function to the module. For me the existing one seems
> good so far.
>
> I do not need to know where a user call is going. Practically, they
> could share the same db backend for auth, but the location and other
> user profiles details may be in memory for speed purposes. In what I am
> doing, all the servers in a group have same config, if i add a new one,
> I get a new dispersion of the users across servers.
>
> Getting the distribution is quite simple, take the has function and make
> a simple app that takes as parameter a string and outputs the hash
> value. Knowing your subscriber base ids, you can estimate the results of
> dispatching.

Hi Alex,

don't that much about the dispatcher internals, but i think it also uses the 
CRC32 hash. In my tests with the carrierroute module that uses this too, 
requests over a random callid/ from user are distributed equally with about 
+/- 5% accuracy, which i think is ok. If you need to adjust the target host 
distribution, take a look at the hash_id functionality (carrierroute config 
file mode), here you can specify the exact destination. We use also the this 
approach of pre-calculating the hash value that Daniel described.

Cheers,

Henning