"From the point of view of the service"

The use of the phrase (see subject header) caught my attention over the weekend, and out of idle interest and a desire to stir up the trout pond, I investigated.

On investigation, it seems to me that the only reason that we need to say that a WSDL instance describes interactions "from the point of view of the service" is because of the <input> and <output> elements.  Without this "point of view" context, "input" and "output" are problematic.

Suppose, instead, that operation looked something like this?

<operation mep="http://www.w3.org/2003/13/wsa/meps/request-response"
           xmlns:rr="http://www.w3.org/2003/13/wsa/meps/request-response">
  <message name="rr:request" direction="toService" [other attrs] />
  <message name="rr:response" direction="fromService" [other attrs] />
  <fault name="rr:xyzzy" direction="fromService" [other attrs] />
</operation>

MEPs are already defined in terms of the service (that is, because they use "input" and "output" they are also required to state that the MEP is from the point of view of a service).  This linguistic adjustment, using "toService" and "fromService", makes that explicit.  An additional benefit of placing direction into an attribute (no matter what the content of the attribute; one could imagine continuing to use the ambiguous "input|output" pair) is that fault directionality can now be described as well.

This may annoy some folks who like the mirror model, but I submit that there are a fair number of MEPs in which the capabilities of the service and the capabilities of the service partner are clearly distinguished, so that it is not inadmissable to designate them server and client.  The requirement that every MEP has a mirror presumes a service-to-service (or peer-to-peer) model, in my opinion, and unnecessarily restricts the universe of services that may be described.

However, please take this as observation, rather than a proposal.  Should someone find this interesting enough to make a proposal of it, I'll back it, but I don't feel strongly enough to want to argue about (err, I mean "discuss") it.

Amy!
-- 
Amelia A. Lewis
Architect, TIBCO/Extensibility, Inc.
alewis@tibco.com

Received on Monday, 27 January 2003 10:17:12 UTC