[Pub-Sub-Task] Re: Some thoughts on wsdl pub/sub

Amelia A Lewis <alewis@tibco.com> writes:
> Is it truly possible to require and describe subscription?  At the level
> of an advanced flow language such as BPEL, I think it is.  At the level
> of WSDL, I don't believe so.
> 
> The subscription might be modelled as an operation.  But what if there
> *is* no operation?

The operation concept in WSDL is purely logical. There is no
assumption whatsoever that the service is implementing the
different "operations" as methods of an object for example. All
messages could be being delivered to one big assembly language
program and I, the user of the service, really don't know or
care.

> A specific example: netnews (usenet, NNTP server-to-server transport
> with variable client access) is a publish/subscribe model (with some odd
> pieces; it's also store-and-forward, which has a big impact).  A
> publisher does not know, and cannot know, who will read any given
> message.  Subscribers do not have to use nntp for access of any sort
> (they may use a local news spool, for instance), and in any event are
> not required to perform any operation that looks like "subscribe" to the
> service.
> 
> This is not particularly uncommon in messaging systems.

Let's be precise- netnews is not all magic like that. The model is
the following isn't it?

    - author of news article (event source) writes the article
    - (s)he publishes the article to a local NNTP server
    - local NNTP server upstreams it to one or more other
      servers it knows about
    - servers publish that article (event) to other servers
      that are subscribed to it (typically via configuration)
    - eventually it finds its way to my NNTP server (where I am the
      event sync)
    - I read the article

So in this model too I believe there is indeed a subscription
mechanism. However, its between the different NNTP servers
and not between the actual event source (the article author)
and the actual event sync (the article reader). 

The only thing the reader cares about in NNTP is that there's
an NNTP server to which (s)he goes to to download news. That
service would just have a bunch of request-response operations.

> This is not particularly uncommon in messaging systems.

I'm not a messaging expert, but I imagine that's while applications
may not directly do a subscribe, the underlying middleware does.

Now, going back to WSDL- the only thing that should be described
in WSDL is everything the client (the service user/requestor)
needs to use the service. If it doesn't need to know that the
underlying info is delivered via a pub sub mechanism to the service
provider then there's no event there .. it just sees (probably)
a request-response operation which it uses to ask the provider for
the information.

Sanjiva.

Received on Thursday, 31 October 2002 19:15:14 UTC