Issue 6917: preliminary proposal for Event Tags; description

Reviewing the comment that created this issue [1], it seems that what 
Ben is looking for is a way to "tag" events with a string/label and to 
be able to create a simple, efficient filter based on these 
strings/labels. He expresses this requirement in terms of "topics", but 
I'm going to avoid that term because of all the semantic baggage that 
comes from all the different messaging/queuing systems that use it to 
mean similar but different things.

So the first thing that occurs to me is that, if we are going to create 
a way to tag particular Event Types, we need a way to describe and 
advertise these tags. Similar to the XPATH filter, a Subscriber needs to 
know what the possible tags are and which tags apply to which Event 
Types if they are going to construct a tag filter that is at all useful. 
Naturally when I think of describing Event Types, I think of 
EventDescriptions. The following is an extended version of the 
"oceanevents.xml" file that I've been using as an example:

<wse:EventDescriptions 
targetNamespace="http://www.example.org/oceanwatch/notifications"
                       xmlns:wse="http://www.w3.org/2009/02/ws-evt"
                       xmlns:ow="http://www.example.org/oceanwatch"
                       xmlns:xs="http://www.w3.org/2001/XMLSchema">
  . . .
  <wse:eventType name="WindReport"
                 element="ow:WindReport"
                 action="http://www.example.org/oceanwatch/2003/WindReport">
*    <wse:eventTag>weather</wse:eventTag>
    <wse:eventTag>wind</wse:eventTag>*
  </wse:eventType>
  <wse:eventType name="RainReport"
                 element="ow:RainReport"
                 
action="http://www.example.org/oceanwatch/2003/RainReport"/>
*    <wse:eventTag>weather</wse:eventTag>
    <wse:eventTag>rain</wse:eventTag>*
  </wse:eventType>
  . . .
</wse:EventDescriptions>

As you can see, the wse:eventType has been extended to include 0 or more 
"eventTag" elements. A few points:

    * The presence of "eventTag" elements in an EventDescriptions has no
      effect upon the corresponding Event Sink WSDL.
    * As will be shown later, the value of the "eventTag" elements
      appear as SOAP headers in the Notifications.
    * The use of EventDesciptions is simply one way of advertising tags
      and their relationship to Event Types. There may be others.
    * This proposal highlights one of the strengths of the
      EventDescriptions proposal; because we control the schema for
      EventDescriptions it is easier to  modify/extend it  to support
      features like event tagging. I assert that it would be more
      difficult to support similar functionality using the
      "linked-WSDLs" approach.

- gp