Categories of Web Service messages: data-oriented vs action-oriented

["Roger L. Costello" <costello@mitre.org>]

Subject: Approaches to designing XML messages

I have convinced myself that there are two fundamentally different
approaches to designing Web service XML messages.  They are:

1. The data-oriented approach

2. The action-oriented approach

I believe that which approach a person takes will have serious
implications on the nature of Web services.

Below I elaborate upon each approach.  Before doing so, however, I
present the 3 questions that I have:

a. Has someone already articulated the two approaches?

b. What are the pros/cons of the two approaches, especially as applied
to Web services?

c. Intuitively, I think that the data-oriented approach is the right
approach.  I believe that XML messages should not be simply procedure
calls disguished in XML (which is how I see the action-oriented
approach). Data-oriented XML messages requires a different mindset, and
will result in a different way of building Web services.  However, I do
not have clarity on how to create services that are data-oriented. 
After all, isn't the purpose of a service is to "do some action"?  Can
you provide clarity?


1. The data-oriented XML message approach

With this approach a message is simply comprised of data.  The tags
denote data, not actions.   For example, below is an XML message that is
to be sent to a travel agent service: 

<itinerary>
    <departureInfo>
        <departingCity>Boston</departingCity>
        <destination>Washington, D.C.</destination>
        <departureDate>2002-2-4</departureDate>
        <departureTime>late afternoon</departureTime>
        <seatPreference>aisle</seatPreference>
    </departureInfo>
    <returnInfo>
        <departingCity>Washington, D.C.</departingCity>
        <destination>Boston</destination>
        <departureDate>2002-2-10</departureDate>
        <departureTime>mid morning</departureTime>
        <seatPreference/>
    </returnInfo>
</itinerary>

Notice that this XML message just contains data about a person's
itinerary.  Since this is simply data, not targeted to any particular
application, I can imagine that it could be used by a variety of Web
services, not just a travel agent service.

2. The action-oriented XML message approach

With this approach a message is basically a procedure call disguised in
XML:

<getFlights>  
    <departureInfo>
        <departingCity>Boston</departingCity>
        <destination>Washington, D.C.</destination>
        <departureDate>2002-2-4</departureDate>
        <departureTime>late afternoon</departureTime>
        <seatPreference>aisle</seatPreference>
    </departureInfo>
    <returnInfo>
        <departingCity>Washington, D.C.</departingCity>
        <destination>Boston</destination>
        <departureDate>2002-2-10</departureDate>
        <departureTime>mid morning</departureTime>
        <seatPreference/>
    </returnInfo>
</getFlights>

This XML message is identical to the earlier XML message except for the
root element.  The root element implies action.  The root element is
directly correlated to a method name.  Its child elements are the
parameters.  

I contend that this action-oriented root element gives the message a
very different feel.  I contend that it tightly couples the message to a
particular kind of service - one that returns flight information.  The
data-oriented approach, on the other hand, is not coupled to any
service, so it could be used by a variety of services.

I appreciate any clarity you can bring to this topic.  /Roger