The True Meaning of Service
by Kendall Grant Clark
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Pages: 1, 2
What Does DAML-S Include?
There is a lot to DAML-S already and there is more to come. I can't describe it all, much less go into much detail. In what remains, I want to point out the big chunks of DAML-S and point to sources for additional information.
Service Profiles, Models, and Groundings
The top level of the service ontology revolves around the
Service class. A Service is a kind of web
resource. One needs to know at least three things about a Service:
what it does, how it works, and how one might access it. Corresponding to these
questions, DAML-S offers three classes: ServiceProfile,
ServiceModel, and ServiceGrounding. DAML-S declares
three properties, presents, describedBy,
supports, which have as their range these three classes. So,
abstractly considered, DAML-S allows one to declare, in a machine-digestible
form, that a Service presents a
ServiceProfile, is describedBy a
ServiceModel, and supports a
ServiceGrounding. In practice, child classes of Service will
have corresponding child classes of ServiceProfile,
ServiceModel, and ServiceGrounding.
Process Models
In order to declare or describe how a service works, one must have a way to
talk about processes, actions, and modes of execution. So DAML-S provides an
ontology of processes, modeled by a child class of ServiceModel,
the ProcessModel class, which is composed by the process ontology
and the process control ontology. The former allows one to describe the inputs,
outputs, preconditions, and effects of a service. The latter allows one to
describe the state of a process, including how it is invoked, the paths of its
execution, and the conditions of its completion. DAML-S also includes simple ontologies of resources and of time.
The Process class is the basic element of DAML-S's process
ontology. A Process can have inputs, outputs, preconditions, and
effects -- of any cardinality. DAML-S divides Process into three
kinds: AtomicProcess, a process that takes all of its inputs at
invocation time, executes, returns all of its upon completion, and which has a
mandatory instance of ServiceGrounding;
CompositeProcess, a process that is composed of atomic or other
composite processes, and which can be decomposed into its constitutive parts
using one or more of DAML-S's control constructs (Sequence,
Concurrent, Split, Split+Join,
Unordered, Choice, If-Then-Else,
Repeat-Until, Repeat-While); and
SimpleProcess, an abstract process that provides an alternative
view of an AtomicProcess or an abstraction of a
CompositeProcess, and which is either realizedBy an
AtomicProcess or expands to a
CompositeProcess.
DAML-S, WSDL, and the "Real World"
Eventually, of course, all this ontological declaration and high-level classification has to bottom out in real, working code. DAML-S calls this a "grounding", represented in DAML-S by the ServiceGrounding class.
The ServiceModel and ServiceProfile parts of DAML-S
are abstract representations of concrete services, and it is the
ServiceGrounding which ties these two together. What is especially
interesting is the relation between DAML-S's ServiceGrounding and
WSDL. The DAML-S architects have discovered several important pathways of
continuity between DAML-S and WSDL. First, what DAML-S conceptualizes as a
grounding corresponds neatly with WSDL's notion of binding. Second, there is
multipart correspondence of AtomicProcess to a WSDL operation
(that is, variants of AtomicResponse correspond to the
request-response, one-way, notification, and solicit-response variants of WSDL
operation). Third, WSDL's abstract types, specified in XML Schema, overlap with
the DAML-S declaration of service inputs and outputs, which can be specified by
description-logic DAML+OIL classes.
Servicing the Future
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Also in XML-Deviant | |
By my lights it is the idea of grounding in DAML-S, and the likelihood of that idea being widely realized by WSDL, which propels DAML-S from the category of "interesting academic curiosity" to "something which may become part of my toolkit". Curiously, however, there is not too much that is Web-specific in DAML-S, which I count as another of its virtues. So even while the WSDL grounding will likely prove to be the most important, I haven't yet found any reason to think there could not be groundings based on high-level programming languages like Perl, Python, Lisp, Smalltalk, and so on. And if that's the case, it may well be possible to apply the advantages of DAML-S for web services to component programming in these languages. For example, it appears to be possible to use DAML-S to describe web server plugins such that, with the right infrastructure, one could do discovery, invocation, verification, composition, monitoring, and interoperation of these plugins. In a world where XML-RPC is being used as an intra-component server protocol to good effect, it's interesting to think about CPAN or an Apache module repository smoking DAML-S crack.
One thing DAML-S proves is that, as with nearly all comforting fictions, the "Services versus Semantics" debate turns out to be a big lie. Well, okay, maybe not a lie, but it's neither very accurate nor very enlightening. We would do well to cease believing in and fighting over and about it. Though we don't yet have any another story as stark or dramatic to replace it with, we as developers should do our best to learn to live without such stories. I suspect the future of the Web will be much like it is today, only different. Insofar as things like DAML-S succeed, that difference will be a pleasant and a happy one.
DAML-S Resources
- The DAML-S site
- W3C's Web Services list archives -- DAML-S is a frequent topic of conversation here
- The Parts of DAML-S:
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Comment on this Article
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- former vs. latter
2002-07-30 09:25:04 Mike Dean [Reply]
Thanks for an informative and helpful article.
In
[[[
a struggle between "Web Services" and the "Semantic Web". The former was thought to be rooted in the W3C and academia, the latter in IBM-Microsoft-Sun and industry.
]]]
I believe that "former" and "latter" (or their referents) are reversed.
- former vs. latter
2002-07-31 12:18:36 Kendall Clark [Reply]
And so they are (reversed, that is), and I've fixed
the article.
Thanks for the catch.
Kendall
- former vs. latter
- Crack?
2002-07-20 02:04:01 Ian Young [Reply]
Kendall writes that "it's interesting to think about CPAN or an Apache module repository smoking DAML-S crack". This appears to be a reference to the drug 'crack' cocaine. Having no personal experience of the drug, I am at something of a loss to make the connection with DAML-S. Can someone help?
- Crack?
2002-07-21 20:15:33 Kendall Clark [Reply]
Hmm, okay, so it's not the most felicitious metaphor, and probably inappropriate. All I meant to say was that one might think about CPAN (or the Python stdlib or a Squeak image or...) as a collection of things ("modules") which provide services, and about which it would be terribly useful to have semantic descriptions.
In other words, if CPAN modules had DAML-S descriptions, that might lead to strange, unexpected, and good things.
Hope that clear it up a bit.
Kendall
- Crack?
- is there really so much confusion?
2002-07-18 09:07:16 dude mc dude [Reply]
I have never thought in terms of services vs. semantic web - they are so obvioulsy different, how could anyone confuse them? It was always apparent that web services would allow semantic operations on their meta data. But then again, how much meta data is there to an online car reservation?
Are there really so many people out there debating web services vs. semantic web?
- is there really so much confusion?
2002-07-21 20:18:55 Kendall Clark [Reply]
--I have never thought in terms of services vs. semantic web - they are so obvioulsy different, how could anyone confuse them?--
Who confused them? Or, rather, who said that anyone confused them? If I said that in my piece, then I misspoke.
--It was always apparent that web services would allow semantic operations on their meta data. But then again, how much meta data is there to an online car reservation?--
A lot of it, I suspect, but that's not a very interesting example, in my view.
--Are there really so many people out there debating web services vs. semantic web?--
Well, sure, if you read xml-dev, this debate is often an overtone of other, more specific ones. Plus, it's not so much that in specific forums people are lined up debating this exact question but that these *are* the two "big visions" for the future of the Web, and they are often presented as mutually exclusive or competitive, and, yes, lots of XML people think of them as such.
I could tell some tales out of school about the corporate sponsors of the W3C *not* funding "Semantic Web" stuff precisely because it gets in the way of "Web Services", but that would just be too tacky! :>
Kendall
- is there really so much confusion?
