Re: [wsn] Use of WS-RF and WS-N for a simple use case I have

[Peter Niblett <peter_niblett@uk.ibm.com>]

Prasad

You can do exactly what you describe using WS-BaseNotification. In fact it
is a good use case for the "implied resource pattern" about which there has
been a lot of recent discussion.

The implied resource pattern involves a single web service that fronts a
number of "instances" or "resources". Each instance has a different End
Point Reference (EPR). These EPRs all share a common address (the address
of the fronting web service) but also contain an opaque element (or set of
elements) that act as an identifier for the instance in question.

The fronting web service can permit clients to subscribe to notifications
from these underlying instances by supporting the message exchanges defined
in the WS-BaseNotification's NotificationProducer interface. A client
constructs a subscribe request in the usual fashion (i.e. as if there were
only one resource) and sends it to the fronting service, but it inserts the
identifier element(s) - called ReferenceProperties - from the EPR of the
intended target resource into the (SOAP) header of the request message.

This way

a) Each instance has its own set of subscriptions, independent of the other
instances that are managed by the same service (this is the function you
are asking for)
b) The instances are free to choose whatever format they like for their
identifiers
c) The body of the WS-BaseNotification subscribe request message does not
have to contain an "instance identifier".

It's because of c) that we call this the "implied resource pattern" (an
explicit resource pattern would be one where the request body contained an
explicit identifier).

Separating the identifier from the request message body has two advantages
i)  It means we don't have to prescribe a particular element name or type
for the identifier in the WS-BaseNotification spec
ii) The message body looks the same whether the NoticationProducer is
fronting multiple dynamic resources, or whether it only ever has one
resource (the so-called Singleton resource pattern)

In a similar fashion WS-ResourceProperties allows to query or set the
properties of a given instance by inserting the identifier element(s) from
the EPR into the header of the appropriate request message.

Regards

Peter Niblett



                                                                           
             Prasad Yendluri                                               
             <pyendluri@webmet                                             
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                                                                   Subject 
                                       [wsn] Use of WS-RF and WS-N for a   
                                       simple use case I have              
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           




Hi,

My apologies for cross-posting here but, it seemed this question is
relevant to both teams.

I have a use case where the resources come into existence dynamically
and terminate after sometime (e.g. like an instance of a class, a unix
process etc.). I would like to host a Web service that would front these
instances and permits clients to subscribe to notifications on the
status of these instances. All instances share common property types
including an "instance-id" type. However the number and actual
instance-ids of these is not known in advance, and change with time.
The Notifications should identify the instance-id of the entity the
notification is for, and a list of properties (types) and their values.

The clients also need to have the option to subscribe to notifications
on a specific instance(s) by supplying their instance-ids (e.g. for long
running ones) (based on previous call like getInstanceList()).

Subscribing to Notifications via Topics does not seem to give me the
granularity or of an instance-id type dynamism that I am looking for
above. Is that correct or did I not understand certain aspect of WS-RF
or WS-N that facilitates this?

To utilize WS-RF and WS-N for my use case how would I go about defining
the resource properties and the topics (and the associated
messageTypes?). How do I specify an instance-id in my subscription?

Thanks, Prasad