[sr-dev] Exposing additional dialog functions through API

[Timo Reimann]

Hey Jason,


On 12.08.2011 13:50, Jason Penton wrote:
> Ok, I agree with you on the reference counting - this can be avoided by
> keeping the h_entry:h_id pair instead of a pointer to dlg. The reason I
> was doing the ref was to make sure that the dialog module does not
> delete a dialog under a modules feet (in which case a module would hold
> a pointer to memory that has been freed). However, to avoid this we can
> just call lookup_dlg passing in the entry:id pair. (another reason why
> we would need lookup_dlg to be exposed ;)

A much easier approach IMHO would be to register a callback to
DLGCB_DESTROY or DLGCB_TERMINATE. That way, you'll be notified
automatically when the dialog is destroyed/terminated and don't need to
deal with implementation details such as hash table keys.

You could also use other dialog callbacks to react to specific dialog
lifetime phases. See the docs for details.


> If we just use as one example the Ro interface we have built.
> Effectivley Ro is used in the IMS world for online charging (i.e.
> realtime charing during the call). So naturally, this module is dialog
> aware. What we do is keep a mapping between the dialog and the
> particular Ro session (Ro session exists between Kamailio and an OCS
> (online charging system). This is the reason for storing the dialog
> pointer or id pairs. Now, when we run out of credit - the OCS will deny
> a new batch of requested credit. In this case we lookup the
> corresponding dialog associated to the Ro session and tear it down,
> using terminate_dlg function

If you really need to terminate calls proxy-wise, I agree you need some
terminate function. It's usefulness might be restricted in your case as
mischievious clients may just ignore your BYE request. I don't know your
exact setup, however, so this objection might not count.

Assuming that it holds I think dialog callbacks, again, are the way to
hook into the dialog module. Just keep registering for new dialogs
(possibly "confirmed" ones only) and make your module logic keep track
of credits during the course of the call. Should the account drop to
zero while the dialog is still active, force termination.

Termination, by the way, could also be implemented by letting your
module run a particular Kamailio route on zero credits which, in turn,
could call dlg_end_dlg(). That way, you wouldn't need to export another
function. I am not strictly against exporting the termination function
on C level though, just wanted to mention the route approach.



> I really think there are a number of scenarios where these extended API
> functions could be used so as to ensure modules don't have to replicate
> what alrady exists in the dialog module, from both memory and processing
> perspective.

I'd be interested to know whether all these scenarios can be covered by
means of using the dialog callbacks as they nicely isolate
implementation details from dialog usage. If there are cases where
callbacks don't suffice, we can think about ways to work around. In my
opinion, that should go by enhancing the callback mechanism accordingly.


Cheers,

--Timo



> On Fri, Aug 12, 2011 at 12:58 PM, Timo Reimann <timo.reimann at 1und1.de
> <mailto:timo.reimann at 1und1.de>> wrote:
> 
>     Hey Jason,
> 
> 
>     On 12.08.2011 12:54, Jason Penton wrote:
>     > this wont be available to configuration users but to other modules
>     > through API.
> 
>     Ok, thanks for clarifying this. Still, allowing other modules to fiddle
>     with referencing counting is a no-go IMHO.
> 
> 
>     > On phone now so will respond to use cases when I'm back at my PC
> 
>     Sounds good!
> 
> 
>     Cheers,
> 
>     --Timo
> 
> 
> 
>     > On Aug 12, 2011 12:48 PM, "Timo Reimann" <timo.reimann at 1und1.de
>     <mailto:timo.reimann at 1und1.de>
>     > <mailto:timo.reimann at 1und1.de <mailto:timo.reimann at 1und1.de>>> wrote:
>     >> Hey,
>     >>
>     >>
>     >> On 12.08.2011 12:33, Jason Penton wrote:
>     >>> We are currently refactoring and cleaning the various IMS
>     modules for
>     >>> inclusion into SR, diameter_rx, diameter_cxdx, diameter_ro, etc.
>     >>>
>     >>> One thing we have noticed is that the use of dialog module functions
>     >>> would make the code alot better and cleaner, so 2 questions:
>     >>>
>     >>> 1. why is the Dialog module not exposing more if its methods?
>     >>> 2. Can we put in a patch to expose the ones we require.
>     >>>
>     >>> Currently, we have exposed and are using the following:
>     >>>
>     >>> lookup_dlg;
>     >>> terminate_dlg;
>     >>> get_dlg;
>     >>> unref_dlg;
>     >>> ref_dlg;
>     >>
>     >> I strongly opt against exporting any functions related to reference
>     >> management. It's already hard to handle reference counting properly
>     >> inside the module; allowing configuration users to touch that part of
>     >> the module will likely result in all kinds of ugly bugs. IMHO,
>     it's best
>     >> to keep it internal and provide functions to whatever feature you
>     like.
>     >> There's already a bunch of dialog PVs and (more recently) the very
>     >> generic dialog variable mechanism which allows you to do a series of
>     > things.
>     >>
>     >> Regarding the other functions you mentioned, can you outline what
>     your
>     >> use case for those is?
>     >>
>     >>
>     >> Cheers,
>     >>
>     >> --Timo