[standards-jig] Re: [JDEV] PubSub (0060) Max_items -> Range?

Bob Wyman bob at wyman.us
Tue May 20 21:34:41 UTC 2003


Timothy Carpenter wrote:
> Sequencing per node can work if a node cannot be caused to 
> be intermittently delivered due to subscription filtering 
> (some updates not of interest).

	I believe you should assume that filtering will be used. Thus,
many clients will not see all messages sent to a particular node. This
implies a requirement for sequence numbers which are specific to a
subscription. If it is desired to allow inter-client references to
messages (and it should be), there needs to be at least a second series
of sequence numbers which is independent of subscriptions.

		bob wyman

-----Original Message-----
From: standards-jig-admin at jabber.org
[mailto:standards-jig-admin at jabber.org] On Behalf Of Timothy Carpenter
Sent: Tuesday, May 20, 2003 4:22 PM
To: standards-jig at jabber.org
Subject: [standards-jig] Re: [JDEV] PubSub (0060) Max_items -> Range?



>A similar solution could be crafted for pubsub.

The example I have given in JEP0032 for sequence was not just plucked
out of the air, but the mechanism is drawn from three long implemented
highly robust pub sub protocols including Tibco's TIB and Reuters'
Triarch.

Tibco's founder, Vivek Ranadive invented the concept of pub sub, so I
think this is not  a bad place to start... :-)

>I think of the IMAP protocol which has addressed a similar problem.  In

>IMAP you have "mailboxes" (think nodes) and messages (think items).

Email is not quite the same beast as a real time datastream. Nodes do
not map to mailboxes, as a node contains a static or expanding or
contracting set of items that can change their value over time. Items to
not map to messages as messages do not necessarily have linkage and do
not form part of a time series (unless it is in such a forum as this :-)
).

The analogy would be more like: a mailbox is like a circuit between pub
and discrete sub, with mail messages being the events. This is as close
as it gets as far as I can see and it is not a good comparison due to
the lack of linkage between mail messages.

For robust pub sub, you need to have the concept of sequence numbering
at different levels. It may be overkill to have separate sequences for
node, channel (i.e. Circuit between pub and sub) and source  (the
overall stream from a pub) but there are many gotchas out there and
making the intermediate pub server efficient and light whilst allowing
for gap filling is a good test of elegant design.

The pub in Jabber terms is a server, not the actual source of the data.
Should a request for data fall outside the cache of the pub server, then
the pub server may need to revert back to the originator. This happens.

The originator (true publisher) does not know about who has subscribed
and what sequencing they are using across their channel. You cannot give
the subscriber the originator's sequencing as the subscriber may not be
getting every event (due to filtering via their subscription). The pub
server has to perform the mapping and management of that, as it is the
one that knows who is where from both sides and who wants to see what.

Sequencing per node can work if a node cannot be caused to be
intermittently delivered due to subscription filtering (some updates not
of interest). If we cannot rely on this, we may have to use curcuit
and/or source sequence numbering, if not both.

Brgds
Tim

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