Wouldn’t it be a simpler solution to
disallow the Producer changing the metadata on a leased instance?
This could simplify the protocol because the
changed metadata could be refreshed by the Consumer on each new lease.
In this case we could even tot to define
the “metadata changed” – style events at all…
From: Spector, Artem
Sent: Sunday, March 13, 2005 10:24
Subject: RE: [wsrp] Spec defined
Yes, but the question is:
Can the Producer throw a “related
fault” before sending the proactive notification to the Consumer?
If the answer is no, the Producer will
have to maintain the previous version of a Portlet until the next call.
If the answer is yes, the Consumer must be
ready to get a fault before (or instead of?) the notification.
From: Rich Thompson [mailto:email@example.com]
Sent: Thursday, March 10, 2005
Subject: RE: [wsrp] Spec defined
I think the spec would need to have a paragraph describing the
difference between these proactive notifications and related faults. It may
also be worth having the notifications carry a flag indicating whether or not
the next use of the relevant resource is likely to result in a fault if the
notification is ignored.
RE: [wsrp] Spec defined events
Is it assumed that all these events express
backward-compatible changes on the Producer side? I mean if the Consumer ignores
these “proactive notifications” and keeps invoking the Portlets in
the same way, they keep working?
If an event expresses a non-backward-compatible change on
some Portlet and the Consumer ignores this event, then on the next invocation
of this Portlet an exception will be thrown. This exception could be for
example OperationFailed, and it will not be easy to the Consumer to understand
the reason of the fault and to recover.
Do you think that the backward-compatibility semantics should
be defined for each spec defined event, and the Consumer should be responsible
to treat the “severe” events before the next invocations?
Since the Producer may notify the Consumer only on pbia or
handleEvent invocation, there always can be a situation when:
1. Portlet P1 is changed and the producer is waiting for the
next invocation to inform the Consumer
2. The Consumer invokes P1 and gets an exception
So if the backward-compatibility is really relevant here, we
have to admit that sometimes exceptions should be used for passing information
to the Consumer.
From: Rich Thompson
Sent: Thursday, March 10, 2005 7:53 PM
Subject: Re: [wsrp] Spec defined events
Interesting questions. Other than the last one, they all seem predicated on an
implementation style that tracks changes and does a computation upon
interaction to determine whether or not to generate one of these events.
Another implementation choice (that happens to avoid the bulk of these issues)
is to persistently store the need to provide one of these notifications to each
known Consumer and then delete the record saying the need exists when it is
delivered. In addition to removing datetime computation while processing a
request, this also permits notification by means other than event delivery. An
example of this would be to consider a getServiceDescription invocation as
removing any Producer/Portlet metadata change notifications as the response
will already update the Consumer.
The other point about these event definitions is that their purpose would not
be to require any particular functionality, but rather to provide a
standardized means by which a Producer could proactively inform a Consumer
about changes. Whether or not a particular Producer or Consumer implementation
makes use of this means to reduce polling for metadata would still be entirely
On the last question, I do not think it is up to the spec to define what a change
is that should trigger any of these notifications. Rather they are defined as a
means for carrying a notification that a particular form of metadata has
changed. As a result of this, I would resist optimizations such as having the
ProducerMetadataChanged event payload carry the new ServiceDescription.
Instead, the event is just a notification and it is up to the Consumer to
decide when and how to respond to the information.
Subbu Allamaraju <firstname.lastname@example.org>
Re: [wsrp] Spec defined events
> As to other options to carry such notifications by other means, why
> would we add such dependencies when we already have a channel that could
> easily carry the information. I agree that the Consumer is not likely to
> display the fact that it received such an event to normal end-users, but
> it certainly would be valuable info to display to system admins (or
> queue for display to them).
Thanks Rich. You make some good arguments. So, let me counter with few
o Producers don't necessarily know whether the Consumer's version of the
Producer's metadata is current or not, unless it starts to keep track of
all metadata requests.
o The Producer has to keep track of metadata changes persistently and
make sure to cleanup those changes periodically.
o The only time a Producer could return events is via handldEvent(s) and
pbia responses (excluding fault conditions). This may be acceptable.
o Assuming that the Producer recognized a change, how would it know
whether a given Consumer should be notified or not? Should it start
sending notifications to all Consumers (including those that already
have the current metadata)? Such Consumers will have to do some extra
processing before ignoring the event.
o Should the Producer keep sending events for ever or just once? It
can't be the latter since the Producer does not always know about
Consumers. So, it does not know which Consumer was already notified.
I think, a more fundamental question how would the spec define what a
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