30 Mar
2012
30 Mar
'12
11:01 a.m.
On 29.03.2012 23:19, Peter Dunkley wrote:
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 wonder if this is useful. E.g. a user tries to call somebody, but the
target is busy. Thus, the call will last maybe 3 seconds having the
dialog states: trying, proceeding, early, terminated.
How is it supposed to work? It just sends trying, but no the others? Or
will they be queued, so only trying, and 5 seconds later, terminated is
sent?
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.
RFC3265 states that a NOTIFY MUST sent immediately after every SUBSCRIBE.
- 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?
I wonder why you delay NOTIFYs for (re)SUBSCRIBEs? On (re)SUBSCRIBEs
just NOTIFY with last published state and do not set the "notify
required" flag. Then it may happen that the minimum NOTIFY interval is
not maintained the first few seconds after a (re)SUBSCRIBE but IMO this
is not that bad and enables immediate notification after every
(re)SUBSCRIBE.
Are there any objections to me implementing this?
Make it configurable :-) E.g. setting "minimum_notification_delay" to 0
disables the feature.
It may also increase DB load and cause more race-conditions (to be
resolved with DB-transactions) due to more DB lookups.
E.g. what if the timer finds a presentity for which NOTIFYs needs to be
sent and - while iterating over all the presentities - a new PUBLISH is
received which triggers immediate NOTIFY. It may happen that both
processes manipulate the active watchers table at the same time (notify
required flag).
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.
Why run the timer only every 5 seconds? Run it every second (or 100ms)
then it should not be lumpy anymore.
regards
Klaus