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Subject: Action Item #90: Response to Issues 52 and 56
Dan Driscoll and I were asked by the TC (action item #90) to put
together a response to Issues 52 and 56. Here’s a statement that
reflects our perspective. We invite comments from others and hope that the TC
can reach consensus on this tomorrow. --
Toby Concerns are expressed in Issues 52 and 56
regarding the number and frequency of events that may be generated in
particular applications of DPWS, such as the WSD-Print and
WSD-Scan applications already in use and others that may be defined in the
future. In material submitted with Issues 52 and subsequently, Shin Ohtake suggested
defining a new event filter dialect based on severity levels, and in Issue 56
Alain Regnier suggested making the XPATH filter defined in WS-Eventing
mandatory for all DPWS implementations. We believe that DPWS is not
the right place to define either of the proposed solutions. Here’s why. Section
1.1 of DPWS says that it should “[c]onstrain Web services protocols and
formats so Web services can be implemented on peripheral-class and consumer
electronics-class hardware”, and that it should “[d]efine minimum
requirements for compliance without constraining richer implementations”.
This sets a high bar for what should be mandated in DPWS: mechanisms included
in DPWS need to be simple, lightweight, and very broadly applicable across many
device classes, while not limiting what can be added
to support particular applications. WS-Eventing
supports a broad range of applications beyond lightweight devices. The XPATH
filter defined in WS-Eventing is too resource-intensive
to be mandated across all DPWS device classes. An
event filter based on severity levels could be lightweight, but it’s not
broadly applicable across most device classes. Any
application of DPWS can define event filters specific to that application, such
as was done in WSD-Scan. Although a few printer vendors are represented in
WS-DD, we don’t have the expertise as a group necessary to make a
decision on what event filter would be best for the printing application. Even
if we were to add a filter to DPWS now, there’s no guarantee that it
would turn out to be the optimal way to solve the problem in WSD-Print. In
fact, it would be counterproductive for us to define another filter in DPWS
that turns out to not be right for WSD-Print. But since WSD-Print will need to
be updated to reference the standardized version of DPWS anyway, the right
solution is to bring the experts together and define an appropriate
application-specific filter as part of an updated WSD-Print specification. A
more serious matter is that Issue 52 brings into question the fundamental
design of WS-Eventing and the use of TCP and push-model event delivery. Alternative
event notification mechanisms such as pull-mode or connectionless notification
could be investigated, but this needs more careful study, which is not feasible
given the standardization timeline for DPWS 1.1. In the TC discussion of these issues, it’s clear that
there’s a strong desire to share information and best practices across
applications of DPWS. As the industry gains more experience with DPWS, we will
want to document the solutions developed so that subsequent efforts don’t
have to go through the same trial-and-error as earlier projects. We’ve
discussed and committed to forming a DPWS online community to share such information.
Rather than update the DPWS specification every time a new event filter is
defined for some application, the right solution will be to use this online
community to make information available about event filters along with other
design elements for DPWS applications. Dan Driscoll has published a paper
already that describes
additional ways to address the volume and frequency
of events; we support posting a more general version of this paper on the
community web site rather than incorporating it into DPWS. We look forward to working with the DPWS community to address
these concerns in the right place and time, but don’t believe that
mandating the XPATH filter, defining a severity level filter, or defining alternative event
notification mechanisms can be done in the current project to standardize DPWS 1.1. Toby Nixon
| Senior Standards Program Manager | Windows Device and
Storage Technologies | Microsoft Corporation toby.nixon@microsoft.com
| www.microsoft.com | V: +1 425
706 2792 | M: +1 206 790 6377 | F: +1 425 708 4811 |
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