Modeling Semantic Web Services using UML 2

Florian Lautenbacher and Bernhard Bauer

Programming Distributed Systems Lab

Institute of Computer Science, University of Augsburg, Germany

Abstract: The development of web services and especially semantic web ser-

vices is a tremendous task for people who are not familiar with all language

constructs. Especially with the growing mass of several semantic web service

approaches a language-independent service development is needed. This gap

can be filled by using model-driven software development. Therefore, in this

paper we introduce a meta-model for semantic web services and show the bene-

fits of automatic code generation on the basis of a small example.

1 Introduction and Motivation

Globalization and the new possibilities the internet provides have resulted in the need

to define more loosely coupled components in distributed heterogeneous environ-

ments. Service Oriented Architecture (SOA) represents an approach that facilitates

this loose coupling while at the same time providing sufficient qualities of services

which are necessary for acceptable solutions. Therefore, many companies and re-

search institutes have standardization efforts for web services (which are one way to

realize an SOA). On the other side this results in a non-manageable amount of stan-

dards where each standard covers different parts than the others (see e.g. [1] for a

short overview). But the large quantity of different ‘standards’ and the heterogeneity

of platforms intensify the problem of interoperability. Annotating web services with

semantic information, e.g. from an ontology, can help to resolve these shortcomings.

Future protection of investment makes companies unwilling to change from one

standard to another from time to time. They need the possibility to easily adapt their

companies to upcoming standards. This already begins with the design of company

goals and business processes. Companies more and more demand a stable IT envi-

ronment from business processes to executable services in order to improve the inter-

operability with other enterprises.

The Object Management Group (OMG) therefore promotes the model-driven ar-

chitecture (MDA, [2]) approach towards the analysis, design and implementation of

systems. The primary goals of MDA are portability, interoperability and reusability

through an architectural separation of concerns between the specification and imple-

mentation of software. One of the key aspects is the usage of the Unified Modeling

Language (UML, [3]) to model all kind of aspects. UML2 supports with its well-

defined meta-model (building on the Meta Object Facility – MOF, [4]) model trans-

formation and code generation. Using the MDA one can e.g. model (semantically

Lautenbacher F. and Bauer B. (2007).

Modeling Semantic Web Services using UML 2.

In Proceedings of the 3rd International Workshop on Model-Driven Enterprise Information Systems, pages 9-18

DOI: 10.5220/0002421800090018

 SciTePress

enhanced) business processes and transform them to an SOA-profile for semantic

web services which enables the code generation of executable web services.

Therefore we recognized the need to develop a UML-profile and meta-model for

semantic web services (called MM4SWS in the future) and to define transformation

rules to automatically generate code in languages like OWL-S, WSMO, WSDL-S,

SWSF or SAWSDL. The advantage of using a UML profile is clear: a service can be

modeled and code be generated by using one of the existing standards and if it is

foreseeable that another standard becomes accepted the code can simply be generated

by using the other language, too. Thus, the increasing investment risk can be pre-

vented and a persistent IT environment from (semantic) business process models over

semantic web services models to executable code is ensured.

This paper is structured as follows: In the next section we introduce our meta-

model MM4SWS and describe the concepts based on the existing semantic web ser-

vice approaches. In section 3 we present an example how our meta-model and UML-

profile can be used. An overview about existing approaches for modeling semantic

web services will be shown in section 4, before we conclude.

2 A Meta-Model for Semantic Web Services

Recognizing the need for an independent way of modeling semantic web services, we

designed a meta-model for semantic web services (MM4SWS) which can be trans-

formed into the current W3C submissions and working drafts. Therefore, we analysed

the current approaches, evaluated their similarities and differences and developed a

meta-model which covers all details needed in the different standards. Thus, we con-

sidered ODM [5] for the modeling of the underlying ontologies and built on current

W3C semantic web service submissions named OWL-S [6], WSDL-S [7], WSMO [8]

and SWSF [9] and the candidate recommendation SAWSDL [10]. A detailed descrip-

tion of these standards is out of scope of this paper and can for example be found on

[11]. MM4SWS integrates all parts of these approaches (many concepts are similar or

equal in different standards besides their names) and one can generate code for all of

them using our meta-model.

In [12] five key workflow aspects have been identified, namely the functional, be-

havioural, informational, organisational and operational perspective. Since semantic

web services are often used to execute a specific workflow we categorized MM4SWS

similarly: our profile consists of five packages which interact with each other. The

Ontology-package contains all concepts (organisational aspects) that are needed to

model an ontology. 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 be-

havioural elements and composite services can be modeled. Every action can be an-

notated with functional descriptions as described in the Functional-package.

Fig. 1. SWS meta-model for the ontology.

At the bottom of MM4SWS are elements to model the constructs in an ontology

. To

model all kind of syntactic and semantic data, the top-level element of our ontology-

package (compare Fig. 1) is a DataElement. A DataElement can be a SyntacticEle-

ment (e.g. an element specified in an XSD-file: XSDType) or it can be a Semanti-

cElement. This might be (referring to ODM) an RDFSResource which is 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 classes 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 concepts in current ontologies.

The DataElement and XSDType are necessities from OWL-S where it should be

possible to reference not only semantic elements as input or output, but also elements

which are not semantically described. The other classes are directly adapted from

ODM, where some of the classes in ODM have been neglected for the sake of sim-

plicity.

Each web service (as defined in WSDL) has an Interface (cf. Fig. 2) which in-

cludes a number of operations. An Operation is callable from other web services.

Every operation can have one input and output Message and zero or more fault mes-

sages. These messages are exchanged through channels between at least two services.

Each operation might be described with a semantic element of the ontology. A mes-

sage 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.). Interfaces, Opera-

tions, Messages, Documents and Attributes are needed to generate WSDL code, the

dependencies to the SemanticElements have their origin in the WSDL-S and

SAWSDL approach.

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.

whereby it doesn’t matter whether this ontology should be in OWL or in WSML since both can be gener-

ated

Fig. 2. SWS meta-model for interfaces, operations and messages.

A web service is a Process with a specified behavior. A ServiceProvider (e.g. an

institution, person or organization) offers a number of processes which can be exe-

cuted. 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 processs can be additionally described with a Ser-

viceDescriptor which includes a serviceName (name), textDescription (description)

and contactInformation (contact_info) as needed in OWL-S and additional informa-

tion which is needed in SWSF such as the author of the service, the version, release-

date, etc.

Fig. 3. SWS meta-model for a service provider.

Some more non-functional descriptions are defined in the WSMO-approach which

are covered in the properties of the class NonFunctionals. The whole class and prop-

erties of NonFunctionals can be neglected, if the user models a semantic web service

and is sure that he does not want to generate WSMO code in the future. Each process

can communicate with other processes via Channels (a requirement of SWSF) where

the above introduced messages are exchanged.

Each process has an internal behavior which can be described using a Process-

Flow. A process flow consists of Nodes and Connections between these nodes. A

node might either be a control node like the ones known in UML activity diagrams

(InitialNode, ForkNode, JoinNode, DecisionNode, MergeNode, etc.) or an Action or a

CompositeProcess which consists of several other nodes. A CompositeProcess de-

scribes a composite service whereas a ProcessFlow, with only one node, specifies an

atomic process. An Action can be either an AtomicProcess which calls an operation or

a CallCompositeProcess which can start a new behavior or a CompositeProcess.

Fig. 4. SWS meta-model for the process flow.

Each action might require Inputs and produces Outputs (see Fig. 5) which can be

described more abstract as DataElements as introduced above. Every action can also

be described with its Preconditions and Effects, which themselves can be described

with a class or concept of the ontology. The Preconditions describe the necessary

information state and state of the world before an execution of the action is possible,

the Effects show how the state of the world and the information state have changed

after executing the action.

A more detailed description of MM4SWS, the developed UML-profile including

transformation rules and examples can be found in [11].

It is difficult to integrate MM4SWS into the layers of MDA: in principal it is plat-

form independent, but also includes constructs for each specific platform and SWS

language and code can directly be generated from MM4SWS. But, this matches to

our own experiences that business users prefer a single model, avoid model transfor-

mations where possible and use different views on a model instead.

3 Overall Example

As an overall example we shortly introduce a well-known example that has been

created within the OWL-S specification.

Fig. 5. SWS meta-model for the functional description of an action.

The example is called CongoBuy and is a B2C bookbuying example showing the

OWL-S usage, illustrating a simple use of the process model. The service described is

a fictional book-buying-service from www.congo.com. The example is divided in two

parts: ExpressCongoBuy as a very small example with only one web service invoca-

tion (atomic action) and FullCongoBuy as a composite web service. Our UML-profile

has been implemented in a UML tool-suite and we specified informal transformation

rules to generate code from the meta-model and implemented these rules using the

openArchitectureWare-language XPand [13].

Fig. 6. Example: Documents.

After the modeling of the ontology (similar to ODM) all documents which are needed

to exchange data between processes need to be created. In the CongoBuy-example no

documents have been specified, therefore we only show the usage of two simple

documents which have been nested.

Fig. 7. Example: Messages.

The messages which are exchanged between two services need to be modeled next.

The ExpressCongoBuy-example has one input and one output message with two

inputs and three outputs (creditCardNumber, creditCardType and creditCardExpira-

tionDate).

The above modeled messages can then be used to model the interfaces and their

operations. The ExpressCongoBuy-example has only one interface, the FullCon-

goBuy-example has much more operations which were not modeled in fully detail

here.

Fig. 8. Example: Interfaces.

Additional data types which are needed to model the inputs and outputs of the process

are modeled in an own diagram. These elements can be imported from existing XSD-

files or modeled from scratch.

Fig. 9. Example: XSD-Types.

The next diagram specifies the details of each web service (Process). A service pro-

vider (here: Congo) can have multiple processes (ExpressCongoBuy and FullCon-

goBuy) which themselves can be described with additional information.

Fig. 10. Example: ServiceProvider with processes.

They can be categorized and described with functional and non-functional informa-

tion (e.g. cost or trust). Similar to the attribute “isAbstract” which has a concrete type

and a standard value, each of the attributes can be given a value which has been sup-

pressed here for the sake of simplicity.

Each process has a behavior and in the example of ExpressCongoBuy this behav-

ior consists of one simple action. This action has several inputs and one output and

can be described more detailed with preconditions and effects. These can be specified

in OCL, KIF or SWRL, where the former is a language used only in UML diagrams

which needs to be transformed into a semantic web language and the latter two are

used in several semantic web service languages.

Fig. 11. Example: ExpressCongoBuy atomic process.

The FullCongoBuy-example does not only consist of one action, but of multiple ac-

tions (CompositeProcess) which are coordinated using the control nodes introduced

above. These actions can be combined to composite processes again. To keep it sim-

ple we removed the more detailed functional description of each action (meaning

inputs, outputs, preconditions and effects) in Fig. 12.

4 Related Work

There are several efforts to create a UML profile for semantic web services. How-

ever, to our knowledge none of the existing approaches tries to consider every exist-

ing W3C submission of semantic web services (meaning OWL-S, WSMO, WSDL-S,

SWSF and SAWSDL).

In [14, 15] Grønmo et al. 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 et al. for UML 1.5 class diagrams) to model

the concepts of the ontology. They use a UML activity to describe a web service and

attaching 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, but is not designed to generate SWSF, WSDL-S or

SAWSDL-code.

In [16] a model-driven approach for specifying semantic web services has been de-

veloped. 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 other approaches. [17] describes how semantic

web services and policies can be modeled using the REWERSE Rule Markup Lan-

guage (R2ML). [18] develops an MDD annotation methodology for semantic en-

hanced SOAs, but does not develop a UML profile for semantic web services in

greater detail. [19] describes a case study with a methodological framework for the

development of semantic web information systems (MIDAS-S) building on WSMO.

In [20] (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 the other approaches of semantic web services, too.

Fig. 12. Example: FullCongoBuy composite process flow.

5 Conclusions and Future Investigations

We presented a meta-model and UML-profile which can be used to simply model a

semantic web service and then generate code in one of the currently proposed SWS-

languages. MM4SWS provides independence from the languages of all SWS ap-

proaches and provides the possibility to generate code for all of them. We integrated

our profile into a UML CASE-tool and modeled a well-known example.

In order to make modeling of preconditions and effects easier we will develop a

way to include rules in upcoming versions of our profile to support users in this still

difficult task. The modeling of preconditions and effects currently needs to be done

manually in UML2 constraints, so the user needs to know the expression from SWRL

or KIF (or applies an automatic mapping from OCL to SWRL as described in [21]).

Another issue is the difference between first-order and description logic that needs to

be considered during the transformation process.

In the future we will also focus on a continuous approach from enterprise models

to executable semantic web service code using this semantic web service model and

work on an automatic discovery and integration of existing services into the model.

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