CREATING A META-MODEL FOR SEMANTIC WEB SERVICE

STANDARDS

Florian Lautenbacher and Bernhard Bauer

Programming Distributed Systems Lab, University of Augsburg, Universitaetsstr. 14, Augsburg, Germany

Keywords: Semantic Web Services, OWL-S, WSDL-S, WSMO, SWSF, UML-profile, ODM, MDA.

Abstract: Annotating web services with semantic information is a tremendous piece of work for persons who are not

familiar with the languages and underlying logic. However, a graphical model can assist users to achieve

this goal easily. Therefore, we reviewed existing standards and developed a meta-model and UML profile

for semantic web services based on the current W3C submissions to enable an automatic generation of code

and reduce this normally time-consuming task.

1 INTRODUCTION

Service-oriented architectures (SOA) have gained a

lot of attention over the last years. As the number of

available web services is steadily increasing,

companies realize the need for automatically

discovering web services and having an automated

composition. Several organizations have already

proposed web service composition languages (like

WSBPEL (Alves et al., 2006) or XPDL (WfMC,

2005)), but these standards lack a semantic

description of services which can be automatically

interpreted by machines and can not be used to make

an automatic discovery or composition. But,

semantic interoperability of web services is needed.

To solve these issues several organizations and

companies focus currently on the development of

semantic-enabled web service technologies (or buy

companies which have knowledge in this area).

Several approaches have been submitted to the

World Wide Web Consortium (W3C) – each one

focusing on different aspects as the other. As the

number of “standards” is getting higher, the need for

an independent way of describing semantic web

services increases, too, because the learning curve

for the average service developer can be steep for

such languages and the creation of ontologies

(especially for web services) is a huge amount of

work.

The Object Management Group (OMG)

promotes the model-driven architecture, shortly

MDA (OMG, 2003), approach towards the analysis,

design and implementation of systems. One of the

key aspects is the usage of the Unified Modeling

Language UML (OMG, 2005) to model all kinds of

aspects. UML (currently version 2.1 is under

development) is established in the section of

computer science, but also easily understandable for

business experts (e.g. business process models as

activity diagrams, see e.g. Lautenbacher, F. and

Bauer, B., 2006) and supports with its well-defined

meta-model (building on MOF, OMG, 2006a) model

transformation and code generation.

To fulfil the need of an independent way of

describing semantic web services we developed a

meta-model and UML-profile for semantic web

services. Additionally, we specified informal

transformation rules to generate code from the meta-

model and implemented these rules using the

openArchitectureWare-language XPand (Efftinge,

C. and Kadura, C., 2006). Our UML-profile has

been implemented in a UML tool-suite

(innovatorAOX 2006 from MID Enterprise Software

Solutions GmbH, Nuremberg) and a well-known

example has been modeled as a case study.

This paper is organized as follows: In the next

section we compare already existing standards (of

semantic web services and others) which we use for

our meta-model and give a short explanation about

each standard. We introduce our meta-model and

explain each concept in section 3. Related work,

other profiles and their shortcomings are described

in section 4, before we conclude in the last section

where we summarize the contributions of this

approach and describe future work.

376

Lautenbacher F. and Bauer B. (2007).

CREATING A META-MODEL FOR SEMANTIC WEB SERVICE STANDARDS.

In Proceedings of the Third International Conference on Web Information Systems and Technologies - Web Interfaces and Applications, pages 376-381

DOI: 10.5220/0001266203760381

 SciTePress

2 A SURVEY OF EXISTING

STANDARDS

This chapter gives a short introduction to the current

semantic web service (SWS) submissions and other

approaches and compares them.

2.1 OWL-S

OWL-S (Martin, D. et al., 2004) (currently version

1.2 is under development) enables the discovery,

invocation, interoperation, composition and

verification of services. It builds on the formerly

developed DAML-S and was the first submission of

a semantic web service to the W3C.

Each Semantic Web Service in OWL-S consists

of a service profile, a service model and grounding.

The service profile describes what the service does

and is used to advertise the service. The service

model answers the question “how is it used?” and

describes how the service works internally. Finally,

the service grounding specifies how to access the

service.

To give a detailed perspective on how to interact

with a service, it can be viewed as a process. OWL-

S distinguishes three kinds of processes: Atomic

processes are directly callable and correspond to the

actions a service can perform by engaging it in a

single interaction; Composite processes correspond

to actions that require multi-step protocols and/or

multiple server actions; finally, Simple processes are

not callable and not associated with a grounding and

only provide an abstraction mechanism to enable

multiple views of the same process.

OWL-S is based on the Web Ontology Language

OWL and supplies web service providers with a core

set of markup language constructs for describing the

properties and capabilities of their web services in

an unambiguous, computer-interpretable form.

2.2 WSMO

The Web Service Modeling Ontology (WSMO)

(Lausen, H. et al., 2005) is a formal ontology and

language that consists of four different main

elements for describing semantic web services:

Ontologies provide the terminology used by

other elements to describe the relevant aspects of the

domains of discourse.

Goals state the intentions that should be solved

by web services and are representations of one or

more objectives which need to be fulfilled.

Web Services: A Web Service is a computational

entity which is able to achieve a part of or the

complete goal a user seeks to fulfil. WSMO web

service descriptions describe various aspects of a

service and consist of functional, non-functional and

the behavioural aspects of a web service.

Mediators resolve interoperability problems and

describe elements to overcome incompatibility

problems between different elements on data,

process and protocol level.

2.3 WSDL-S

WSDL-S (Akkiraju, R. et al., 2005) is another

semantic web service approach submitted to the

W3C in November 2005 and extends the WSDL

standard with semantic information. Currently it

only considers single-step services and there is no

specification how a composition of several web

services should be handled. However, there are

already efforts to annotate web service choreography

languages like WS-BPEL similar to the annotations

made in WSDL-S (see e.g. Pistore, M. et al., 2006).

WSDL-S uses the extensibility elements of WSDL

and introduces new elements to describe the

semantics of a service, of its inputs and outputs and

of preconditions and effects.

The advantages of this approach can be

summarized as follows:

- it builds on existing web services standards

- it supports the user’s choice of the semantic

representation language

- it allows the association of multiple annotations

- it supports semantic annotation of web services

whose data types are described in XML schema

(therefore e.g. GRDDL – see Hassael-Massieux,

D. and Connolly, D., 2005 – could be used)

2.4 SWSF

Another W3C submission (from September 2005) is

the Semantic Web Services Framework (SWSF)

(Battle, S. et al, 2005) which represents an attempt

to extend the work of OWL-S and consists of two

major parts: the Semantic Web Service Language

(SWSL) and the ontology SWSO above.

SWSO can itself be divided in two formats:

FLOWS, the first-order logic ontology for web

services and ROWS, the rules ontology for web

services.

SWSF emerged from the work in service

composition which might require more expressivity

than is available in OWL and is therefore based on

logic programming, first-order logic and policy

research. It builds on DAML-S, OWL-S and WSMO

and provides rich semantics for greater automation

of discovery, selection and invocation, content

transformation, composition, monitoring and

recovery and verification. Laying the focus on

CREATING A META-MODEL FOR SEMANTIC WEB SERVICE STANDARDS

377

messages (similar to WSDL 2.0) it introduces the

concepts of Channels and Messages which can be

created and modified using several specialized

actions.

2.5 Ontology Definition Metamodel

Based on the ideas and work of Stephen Cranefield,

Dragan Gašević and others, the Object Management

Group (OMG) works on a specification for the

interoperability between ontologies (and its

underlying description logic), UML diagrams, ER-

diagrams, Topic Maps and common logic. The

Ontology Definition Metamodel (ODM) (OMG,

2006b) consists of four platform independent models

(PIM: Common logic, Topic Maps, RDF and OWL)

and informative models like the one for Description

logic. It defines a meta-model and UML-profile for

RDF(S) and OWL-ontologies and mappings

between the meta-model and these semantic web

standards.

2.6 Comparison of SWS Approaches

Figure 1: Categorization of Semantic Web Service

submissions.

Figure 1 shows how the semantic web service

submissions might be categorized. WSDL-S does

not force to use a specific ontology language and

therefore it does not define any underlying logic. It

is only usable for single-step processes whereas the

other standards can handle multi-step processes, too.

OWL-S builds on OWL which is based on

description logic (OWL DL). OWL-S is widely

common in projects and research, because it was the

first SWS submission. SWSF uses first-order logic

and rules languages to model the ontology in its

underlying language SWSL. WSMO offers both

logic layers for the modeling of ontologies in

WSML. SWSF and WSMO offer with their first-

order logic and rule support a more comprehensive

way of modeling, which might be problematic for

reasoning.

3 A META-MODEL FOR

SEMANTIC WEB SERVICES

Based on the above described standards and seeing

the need for an independent way of modeling

semantic web services, we designed a meta-model

which can be applied and transformed into all of the

mentioned W3C submissions. shows the

dependencies from our meta-model to the standards

introduced above.

Our semantic web service meta-model is based on the

Ontology Definition Metamodel for modeling ontologies

(in form of a UML-profile) and on the above mentioned

semantic web service submissions. WSDL-S is

independent on the underlying ontology modeling

language, but OWL-S, WSMO and SWSF require a

specific ontology language (OWL, WSML and SWSL).

Figure 2: Profile dependencies.

It is difficult to integrate our meta-model into the

layers of MDA. It is platform independent in

principal, but also includes constructs for each

specific platform and semantic web service language

and one can directly generate code from the meta-

model. But, this matches to our own experiences that

business users prefer one meta-model avoiding

model transformations where possible and use

different views on a meta-model instead. This also

conforms to current discussions about the future of

the MDA at the OMG where e.g. Stan Hendryx,

Chairman of the OMG Business Rules Special

Interest Group, pleaded for a weakening of the

current layers.

Figure 3: Package overview for our meta-model.

WSDL-S

OWL-S WSMO SWSF

no ontology language and

logic pre-defined

describtion logic

(DL)

first-order logic /

higher order logic

(FOL / HOL)

single-step processesmulti-step processes

WSDL-S

OWL-S WSMO SWSF

no ontology language and

logic pre-defined

describtion logic

(DL)

first-order logic /

higher order logic

(FOL / HOL)

single-step processesmulti-step processes

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Our profile consists of five packages which

interact with each other. The Ontology-package

contains all concepts that are needed to model an

ontology (similar to ODM). The Interfaces-package

provides all elements to model a WSDL service and

to describe it with semantics (like in WSDL-S). The

ServiceProvider-package includes all aspects to

model one or more semantic web services with non-

functional descriptions and using the elements of the

ProcessFlow-package the functional elements and

composition of multi-steps can be modeled. Every

single step can be annotated with functional

descriptions as described in the Functional-package.

The Interfaces-package, the ProcessFlow-package

and the Functional-package access elements of the

ontology and therefore import the Ontology-

package. The ProcessFlow-package extends

concepts defined in the ServiceProvider-package and

therefore merges this package. The Functional-

packages extends some concepts of the

ProcessFlow-package and therefore merges this, too.

Figure 4: SWS meta-model for the ontology.

At the bottom of our meta-model are elements to

model the constructs in an ontology (whereby it

doesn’t matter whether this ontology needs to be in

OWL or in WSML). We are conform to the ODM

standard in this package, but since we also need

elements which are not semantically described (e.g.

in a XSD-file) the DataElement (compare) is

specialized in two ways: It can be an element

specified in an XSD-file or it can be a

SemanticElement. This might be an RDFSResource

which itself is then part of an ontology. A resource

can either be a class (or concept as it is called in

WSML) or a property. RDFSClasses can be

connected to other classed via RDFProperties which

contain the domain and range of the property. Every

class (resp. property) can be generalized and there

exist specializations for OWL classes which are the

mostly used presentation form of current ontologies.

ODM defines many more constructs, but these are

the most basic ones to model an ontology.

Figure 5: SWS meta-model for interfaces, operations and

messages.

Having finished the ontology grounding, the next

step would be to develop elements for modeling the

infrastructure of the service(s) (). Every web service

(as defined in WSDL) has an interface which

includes a number of operations. Every operation

can have one input and output message and zero or

more fault messages. Each operation might be

described with a semantic element from the ontology

(similar to WSDL-S). A message contains one or

more documents which can also be described with

semantic elements and which can be structured with

several attributes which might themselves be

documents again or simple data types (like String,

int, etc.).

Figure 6: SWS meta-model for a service provider.

With these basic constructs for describing the

interfaces, operations, messages and the underlying

ontology we can now start the modeling of one or

more semantic web services. A web service is a

Process with a specified Behavior. A

ServiceProvider (an organization or institution)

offers a number of processes which can be executed.

Every process can be categorized (according to

OWL-S and SWSF) with a categoryName, a path to

a taxonomy, a specific value in the taxonomy and the

code associated to a taxonomy. Each process can be

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379

additionally described with a ServiceDescriptor and

NonFunctionals. These contain attributes to describe

the non-functional properties of a web service. Each

process can communicate with other processes via

Channels.

Figure 7: SWS meta-model for the process flow.

Each process has an internal behaviour which

can be described using a ProcessFlow. A process

flow contains Nodes and Connections between these

nodes. A node might either be a control node like the

ones known in UML activity diagrams for XOR-

and AND-splits and joins and other control flow

directions (InitialNode, ForkNode, JoinNode,

DecisionNode, MergeNode, etc.) or an Action

(single-step) or a CompositeProcess (multiple steps)

which contains several other nodes. An action can be

either an AtomicProcess which calls an operation or

a CallCompositeProcess which can start a new

behavior or a CompositeProcess.

Figure 8: SWS meta-model for the functional description.

Each action has Inputs and Outputs which refer

to simple types or semantic elements as introduced

above. Every action can also be described with its

Preconditions and Effects, which itself can be

described in more detail with a class or concept of

the ontology.

We developed a UML-profile based on this

meta-model, integrated it into a UML tool suite and

modeled the CongoBuy-example which was first

introduced with OWL-S. For more details see our

technical report on (Lautenbacher, F., 2006).

4 RELATED WORK

There are several efforts to create a UML profile for

Semantic Web Services. However, to our knowledge

none of the existing approaches tries to consider

every existing W3C submission of semantic web

services (meaning OWL-S, WSMO, WSDL-S and

SWSF).

In (Skogan, D. et al, 2004) and (Gronmo, R. et

al, 2005) the authors define transformations between

UML and OWL-S and a web service composition

based on this information. The developed profile

uses the UML Ontology Profile (defined by Duric

for UML 1.5 class diagrams) to model the concepts

of the ontology. They use a UML activity to

describe a web service and to attach inputs and

outputs which makes it difficult to use control nodes

for the composition of several web services later.

Their profile supports the generation of OWL-S and

WSMO code (hence, there are no transformation

rules for the generation of WSMO), but doesn’t

consider SWSF and WSDL-S.

In (Timm, J. and Gannod, G., 2005) a model-

driven approach for specifying semantic web

services has been developed. However, the UML-

profile only considered AtomicProcesses in OWL-S,

not including the collaboration of several processes.

It is only applicable to OWL-S and misses

transformation rules for WSMO, SWSF and WSDL-

(Scicluna, J. et al., 2004) describes how OWL-S

services can be modeled, but in a proprietary format

not using the UML-profiling mechanism. (Pondrelli,

L., 2005) develops an MDD annotation

methodology for semantic enhanced SOAs, but does

not develop a UML profile for semantic web

services in greater detail. (Acuna, C. and Marcos, E.,

2006) describes a case study with a methodological

framework for the development of semantic web

information systems (MIDAS-S) building on

WSMO. In (Kendall, E., 2006) (and other talks) the

author promotes the integration of OWL-S and

SWSF within the ODM. We completely agree and

support this initiative, if the meta-model considers

other approaches of semantic web services like

WSMO and WSDL-S, too.

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5 CONCLUSIONS

Using our meta-model and UML-profile one can

simply model a semantic web service and then

generate code in one of the currently proposed SWS-

languages. Our profile provides independency from

each single SWS standard and can easily be adapted

in the future. We integrated our profile into a UML

CASE-tool and modeled a well-known example.

Our meta-model is compatible with all of the

current W3C SWS-submissions and builds on the

OMG specification draft, it enables a code

generation through a well-defined meta-model and it

includes the modeling of ontologies.

However, the integration of semantic web rules

is still missing. To make it easier to model

preconditions and effects we will include rules

(based on SWRL or WRL) in upcoming versions of

our profile. We are also interested on semantic

business process models and how to combine these

business processes with the developed meta-model

for semantic web services.

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