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Subject: RE: [obix-xml] Issues

3. Polling

- Since we can't add/remove points from a subscription, the likely
implementation scenario will be a subscription for every container.  This
could lead to a client with many threads polling a server.  I would like to
see the pollReq allow multiple subscription id's.

While Polling is normal in Control Apps, it is usually considered *bad* in
the wide area internet world.  I believe the preferred WS model (common to
WS-Events, WS-Eventing, and WS-Notification) is a Subscription model.  

We so not need to solve this before BuilConn, as we do not anticipate "many
points" in that time frame. 

Long term, I acknowledge that some sort of polling will be required. Perhaps
we need to suggest that best practices would be subscription based and some
sort of polling, if required, be restricted polling the overall service.

The model then would be:

"Mr. oBIX ERP:, tell me if anything noteworthy happens. By that I mean
points 1, 2, 3, 4, 5, 6, 7 ship to me every 5 minutes and tell me if points
A, B, C, D ever get outside of bounds." (A normal subscription request)

Client: "I will check of Mr. ERP is alive (one point, essentially a high-end
PING) every n minutes.  As long as he replies (or perhaps indicates good
health), then I will assume that no news is good news on the
subscription/notification events." 

I believe that such an approach will substantially expand scalability of an
oBIX solution as compared to other solutions that merely re-factor control
architectures into XML while better aligning us with best practices in Web

BTW - Each of the Subscription models has a TTL component.  We will need to
address algorithms for this, or means to parameterize/reacquire/... TTL
subscriptions before we are done.


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