[standards-jig] JEP-0024: Publish/Subscribe - Topic based messaging

Piers Harding piers at ompa.net
Mon Apr 29 06:59:20 UTC 2002

Comments within:

On Wed, Apr 24, 2002 at 11:13:26AM +0100, Dave Turner wrote:
> Hi,
> I have a few comments/ideas to add regarding the pub/sub proposal.
> I have experience with IBM's MQ and MQe middleware products, specifically
> in a SCADA [1] context, and I draw my ideas mainly from this.  Forgive me
> if I make some point that is specific to SCADA; I have tried to generalise
> the points to those that I think a good pub/sub system should provide but
> if you think that something is too specific let me know.
> I pretty much agree with the handling of the publish and subscribe events
> as they are defined in the JEP, however I think that there is room for
> extra levarage.
> 1.  I realised that subscribing to all messages from all publishers
>     can generate a lot of traffic but I don't think that it is the place of the
>     protocol to restrict the ability to do this.  There may be some valid
>     reason to want to subscribe to everything, a logging application for example.
>     It should be down to the programmer of the application to take care not
>     to do this if the system isn't going to be able to cope.

Well - perhaps that is so - but everything is very "relative" in the
context of any pubsub application.  What passes thru a pubsub component
is only by virtue of who subscribes/publishes to that particular
instance of a component.
2ndly - where logging is concerned - that I would have thought was an
implementation detail of a pubsub component, as it is in a prime
position to do it.
And lastly - other than your logging example - I can not think of any
example of an application that would beable to deal with all the
possible application specific issues that would arise from a client
receiving ( essentially ) random pieces of data.

> 2.  I think that the notion of using namespaces for published messages puts
>     restrictions on the protocol.  Maybe it's just my misunderstanding of how
>     it would work.  I'll describe my ideas for topics and then let's see
>     if we can clear the picture up afterwards.  I use the term topic to
>     refer to the same part of the <publish> tag but with different semantics
>     which I will now describe.
>     I think that topics should be heirachical in the same way as a directory
>     name, for example.  So I could publish messages on topics such as:
>         /london/office/temperature
>         /london/apartment/temperature
>         /boston/office/temperature
>         /boston/office/humidity
 namespace is an arbitrary string of data ( admitedly with restrictions
on what you can use to creat ethat string ) - it is entirely up to your
imagination how you want to construct that string.  The meaning of that
string is an "agreement" between the publisher anf the subscriber.
for example:


>     I think that these topics are pretty self-explanatory in that they are
>     for an imaginary environmental system.  The publishing event is
>     fairly straightforward to understand.  By publishing on a particular
>     topic we describe what the data represents in some structured way.  An
>     example message would be (following the JEP):
>         <iq type='set' to='pubsub.localhost' from='publisher.localhost' id='s1'>
>           <query xmlns='jabber:iq:pubsub'>
>             <publish topic='/london/office/temperature'>14</publish>
>           </query>
>         </iq>
>     As a side note, in this sort of situation it is almost certainly beneficial
>     to acutally publish XML data rather than raw data, such as:
>         <temperature scale="centigrade">14</temperature>
>     so as not to lose meaning in the data.  But that's not a pub/sub thing so
>     I'll ignore that from now on.
>     With structured topics the subscription process is then more interesting.
>     Firstly, I think that with topic based pub/sub the need to subscribe to
>     a particular publisher is diminished but the JEP allows for this.  The
>     biggest advantage of topics is wildcard subscriptions.

The use of wild card subscriptions is interesting, but it is still an
implementation issue.
For "subscription realm" ( or topic I guess - for want of a better name
), the idea that we had was that the client woud luse IQ Browse to
discover a list of things to subscribe to.  It would be a feature of the
browsing client to allow wild card selection of items to subscribe too.
JEP0024 allows for multiple subscription requests in a single packet.

In a business applicaiton, subscription to a particular publisher is not
diminshed at all - If I was implementing a business hub for
broadcasting catalog and pricing information, subscriptions may very
well be based around formal business agreements.

>     a.  My subscription client may be interested in only the temparature in the
>         London office and would subscribe to the /london/office/temparature
>         topic.  This is the boring case.
>     b.  The client may be interested in all data from London and, using wildcard
>         subscriptions, could subscribe to the topic /london/*.  It would then
>         receive messages published on any topic that starts with /london/.
>     c.  The client may be interested in temperature data from london only and
>         subscribe to /london/*/temperature.  (This is probably the hardest
>         case to implement efficiently.  FSAs are you friend.)
>     Ok, I think I've made the point that I wanted to about wildcards.  Now, this
>     may well be possible using the namespaces and I just didn't see it right.
>     Feel free to shoot me down... just don't use real flames..
> 3.  On subscription to a topic it would be convenient to elect to receive
>     the last message that was published on that topic, if one is available.

Why does historical information have to be bound up in the same protocol
as pubsub?  Why wouldnt that be yet mor einformation exposed by the
publisher thru a browse function?

>     eg.
>         <iq type='set' from='subscriber' to='pubsub' id='s1'>
>           <query xmlns='jabber:iq:pubsub'>
>             <subscribe>
>               <topic last='1'>/london/office/temperature</topic>
>               <topic>/boston/office/temperature</topic>
>             </subscribe>
>           </query>
>         </iq>
>         would subscribe to the two topics but request that the last message
>         from /london/office/temperature be sent back.
>     This is important when considering the premis of pub/sub that messages
>     are only sent when something interesting has happened or the data needs
>     updating.  When subscribing one presumes that the last message is still
>     relavent and interesting so it would be good to send it to the subscriber
>     and get them up-to-date as quickly as possible rather than waiting around
>     for the next message.
>     This should also be complemented by an attribute in the publish message
>     to tell the broker NOT to honour requests for the last message.
> 4.  My last idea, and this might take some waffling to make my point.
>     Simply put it is handy to be able to set a 'Will' message that a publisher
>     can have published on their behalf (so that it seems to have come from
>     them) if they have stopped responding.  Let me explain why with a SCADA
>     example.
>     The publisher is usually a very lightweight device, running in a remote
>     location with a possibly intermittant connection to the broker.  If that
>     publisher is monitoring some critical metric in a system it is usually
>     important to be able to define the failsafe situation.
>     A somewhat contrived example would be for the temperature monitor in
>     the London office, I'll skip the actually message content until I've
>     convinced people that this is really useful... ;o)
>     Publisher connects to the broker and gives it a message to publish
>     in the event of the publisher's untimely death -- a will message.
>     With this message it will also be necessary to specify a timeout
>     that if the broker hasn't heard from the publisher within that time
>     then it publishes the will message.

It sounds quite specific - I would have thought that this could be
achieved thru TTLs on subscriptions.  A client re negotiates it's
subscription - when it browses to the publisher it finds it unavailable.

>     Now, the publisher can operate as normal, publishing messages when
>     something interesting happends.  However it must be careful that it
>     sends messages to the broker with an interval shorter than the timeout
>     period.  If there is no useful data to publish this could be satisified
>     with a ping message just to inform the broker that it is still alive.
>     So the publisher connects and sets a will message with a timeout of
>     60 seconds to publish the content 'n/a' on the topic /london/office/temperature.
>     It then happily publishes temperature readings everytime the temperature
>     changes by more than 0.5 degrees.  If the reading doesn't change
>     within 60 seconds then it sends a ping message (or just resends the
>     current reading.. although if the message content is large this wastes
>     bandwidth).
>     If someone disconnects this publisher, the broker will timeout for
>     that publisher and publish 'n/a' as though it came from the publisher.
>     All subscribers will show sensible data.  Of course, if this were in
>     a nuclear reactor and the temperature reading was no longer available
>     the controlrod subscriber could then shut down the reactor as a failsafe.
>     (told you it was a little contrived)
>     The timeout probably wants to be time + x for the broker to account
>     for network lag and the like.  Not really thought about that bit.
> I have a few more points but they generally depend on understanding of
> these first.  So let me know if they make sense or are just pure fantasy.
> Cheers.
> [1] http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?SCADA
> -- 
> Dave Turner
> http://figroll.com/
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