[OpenAjaxGadgets] [OpenAjaxIDE] Property onChange notification events

Jon Ferraiolo jferrai at us.ibm.com
Mon Apr 6 10:42:27 PDT 2009

Regarding explicit mention of OpenAjax Hub in the spec, it seems like we
should be able to find a way to describe the necessary behavior (i..e,
message transmission and reception) without having to mention a particular
technology (i.e., the Hub).


             BM at IBMUS                                                   To 
             Sent by:                  "gadgets at openajax.org"              
             gadgets-bounces at o         <gadgets at openajax.org>,             
             penajax.org               "ide at openajax.org"                  
                                       <ide at openajax.org>                  
             04/06/2009 06:12                                              
             AM                                                    Subject 
                                       Re: [OpenAjaxGadgets] [OpenAjaxIDE] 
                                       Property onChange notification      

I agree that a widget only has its subscribe handler invoked if someone
outside the widget causes the property value to be updated (actual value
change). Likewise, an event is only published if the widget causes the
property value to change. It also is reasonable to have an overall
namespace for topics with the default being the widget's namespace.

Reading through the quoted portions of the spec was tough with my spec
writer hat on. The reason is that I firmly believe specs should be loosely
coupled unless the specified functionality can not be achieved with a loose
coupling (rare). This section explicitly requires use of the Hub spec
without any real need for that requirement (i.e. any event distribution
mechanism would be fine, the Hub is an example). Is there a reason for such
tight coupling other than both are being defined under the OAA umbrella??


 From:  Howard Weingram <weingram at tibco.com>                               
 To:    "gadgets at openajax.org" <gadgets at openajax.org>, "ide at openajax.org"  
        <ide at openajax.org>                                                 
 Date:  04/06/2009 06:55 AM                                                
 Subjec [OpenAjaxIDE] Property onChange notification events                

I have some questions, comments and proposals.

The IDE spec currently says the following:

The publish attribute. Boolean. Default value is 'false'. If true,
that the user agent should publish an event on the OpenAjax Hub whenever
value of the property changes. Attribute 'topic' indicates the name of the
topic that is used when that event is published. 2009-02-23 Jon: I updated
the description [MASHUP]
The subscribe attribute. Boolean. Default value is 'false'. If true,
indicates that the user agent should subscribe to property change events on
the OpenAjax Hub for the topic indicated by the 'topic' attribute, and
update this property to the new value that is received. 2009-02-23 Jon: I
updated the description [MASHUP]
The topic attribute. String. Defaults to NULL. Identifies the topic name
that is published to (if 'publish' is true) and/or subscribed to (if
'subscribe' is true) on the OpenAjax Hub. If the attribute is not
or is NULL or the empty string, and if 'publish' is true or 'subscribe' is
true, then the value of the 'name' attribute is used as the topic name.
(Note that providing a value to 'topic' has no effect if 'publish' is false
and 'subscribe' is false.) [MASHUP]


1. This appears to state that the property name must be used as
   the default topic.

   Topics SHOULD be namespaced. Namespacing ensures that different
   organizations can specify their own sets of topics without
   causing endless collisions.

   Using a single-token topic like "accountNo," as the spec
   appears to require, is GUARANTEED to result in topic name
   collisions between different families of widgets, balkanizing
   the application space, causing a lot of trouble when widgets
   receive incompatible data on overly generic topics, and greatly
   reducing the usefulness of the standard.


   (If property names are used to implicitly define topics),
   widget.xml MUST specify a topic prefix that namespaces
   these implicitly defined topics.

2. Does a change notification get sent each time setPropertyValue
   is called, even if this does not change the property value?
   (i.e. can SetPropertyValue be used to affirm an existing value?)
   Or does a change notification only get sent if the value has
   actually changed?

   To determine whether the new value is different from the old
   value, setPropertyValue would need to traverse the value, which
   might be a complex object, and directly compare old and new

   PROPOSAL: setPropertyValue ALWAYS forces a change notification
   to be published, even if the data did not actually change.

3. If subscribe and publish are both true, we need to specify the

   a. Does the widget see its own updates?

       PROPOSAL: NO.

   b. If the answer to (a) is "yes", then does the widget
       distinguish its own updates from others' in some way?
       If not, then because of #2, there will be infinite loops.

4. If an update event arrives and updates the property value, this
   change MUST NOT cause another update notification to be
   published. Update notifications are published only when the
   setPropertyValue function is called explicitly.

The expected behavior of properties & their update events needs to be
clearly defined in the specification to avoid different vendors from
implementing mutually incompatible behavior.

Your thoughts welcome.


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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