[sr-dev] Throttling NOTIFY requests from presence

[Peter Dunkley]

Hi,

RFC 3856 section 6.10 states: "A PA SHOULD NOT generate notifications for
a single presentity at a rate of more than once every five seconds."

I would like to add this to the presence module (making the rate
configurable).

I have an idea as to how I would like to do it:
- Add a last notified time-stamp field to each presentity
- Add a updated since last notified flag field to each presentity
- Add a notify required flag field to each active_watcher

- When a presentity is updated the last notified time-stamp is checked. 
If the time is far enough in the past the notifies are sent and the
time-stamp is updated.  If enough time has not passed the updated since
flag is set for the presentity and the notify required flag is set for all
active_watchers of that presentity.
- When a presentity is subscribed to (this includes re-subscribes) the
last notified time-stamp is checked.  If the time is far enough in the
past the (single) notify is sent and the time-stamp is updated.  If enough
time has not passed the updated since flag is set for the presentity and
the notify required flag is set for this active_watcher record.
- A timer (the minimum time between subscribes - default 5 seconds) is
run.  On expiry a query is done on the presentity table for presentities
that have been updated _AND_ the last notified time is more than the
minimum time ago.  For each of these presentities, a query is done on the
active_watchers for watchers of that presentity that have the notify
required flag set.  Notifies containing the presentities are then sent to
the watchers waiting on them.

This should ensure that no presentity notifies more than it should, while
ensuring that all changes are (eventually) sent out, and all subscribes
result in a notify (eventually) being sent.  Eventually being typically
within 5 seconds and in under 10 seconds in the worst case (assuming the
default setting of 5 seconds).

Can anyone see any problems with this?

Are there any objections to me implementing this?

In theory running a (5 second) timer could make presence "lumpy" in the
same way RLS is (see my previous email).  However, if this proves to be
the case I believe the same mechanism I have proposed for RLS can be used
here.

Regards,

Peter

-- 
Peter Dunkley
Technical Director
Crocodile RCS Ltd