[OpenAjaxGadgets] property topic attribute

Howard Weingram weingram at tibco.com
Tue Apr 14 10:28:03 PDT 2009

I think the thing that bugs me the most about the widget.xml claiming
control over 

    <property topic="foo.bar">

is that the messages that the property management system publishes on
foo.bar have payloads that are completely proprietary to the property
management system. They are not specified in any standard, and widgets
therefore have no business subscribing to foo.bar or publishing on foo.bar.

And if the traffic on this topic is proprietary to the PMS, then it makes no
sense for the widget to demand control of the topic.

The topic attribute on property was originally intended to implicitly define
relationships between property values, in the same way that topics define
implicit relationships between publishers and subscribers. And it is a good
idea to do this. Standards bodies or widget family authors often do want to
define such implicit relationships, and something LIKE topic is the simplest
possible way to do so.

There is nothing wrong with using a topic-like scoped string to specify such
relationships, and an implementation of a mash-up or widget framework MIGHT
choose to IMPLEMENT the mechanism using hub topics. But it's wrong for
widget.xml to specify topics on which traffic flows that is part of the
widget framework implementation and is incomprehensible to the widget.

Maybe all we need is to either
1) rename the topic attribute (propEvent? propClass?) and remove from the
spec the explicit relationship between topic and property class. Or
2) we could keep the name topic but explicitly state that this is JUST A
SCOPED NAME, and that it:

* DOES NOT IMPLY that the hub is used
* if the hub is used, DOES NOT IMPLY that the topic is the specified name
    (it might be an extended or otherwise mutated version or
    something completely different)

#2 would not change the syntax of widget.xml, but it would introduce
confusion, because the intent of an attribute named topic SEEMS to be to
specify the exact topic to be used with the Hub.

There is another possibility, but I don't have time to write about it right


Howard Weingram      650.846.1000
Principal Architect  TIBCO Software Inc.

TIBCO PageBus(TM) delivers ultra-lightweight
publish-subscribe messaging for mash-ups.
Learn more at http://www.pagebus.org

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