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182 Selected Studies on Software and Information Systems
Another approach to web service discovery is presented in [81]. The authors state that
the existing web service definition languages have lack of the particular specification to
the function of web services. From this reasons they define a functional semantic of web
services. The functional description is expressed in a functional ontology. It helps the user to
understand the function of the web service unambiguously. For semantic annotation of web
service a mechanism presented by authors is used.
The results of the web service discovery is a set of services, which are assumed to be
helpful. From this set we need to select the most suitable one. Most approaches for web
service discovery offer as a result an ordered list of services based on their relevance. To
select the most relevant web service we exploit any additional information describing also
non-functional properties of web services. In [64] a method for evaluation of the semantic
composability of Semantic web services is presented. This approach tries to completely utilize
the semantic aspect and use ontological description of the services and their properties to
measure the relevance.
The effective and successful web service discovery is one of the basic aspects of the
Semantic web services vision. If we do not have methods to efficiently find required web
services, the whole vision falls down. However, there are approaches to find relevant web
services, the discovery problem is still not solved. The main problems are:
– Insufficient description of services: The web service descriptions are often not precise
and complete. Hence, it is hard to determine the usefulness of the service.
– Credibility of descriptions: The web service descriptions offers information about the
functionality, required input data, pre and post conditions, output data. Despite that
these information may be relevant, there is no guarantee that the description absolutely
corresponds with the reality. The descriptions may contain incorrect information.
This causes problems when the execution of the service does not achieve the expected
results. The detection of this presents a serious problem if no checking is available.
– Computational strenuosity: The sophisticated web service discovery approaches may
require a lot of computation to find a set of candidate services and determine their
relevance. The required resources do not have to be available in each situation.
In [5] the problems of web service discovery are discussed. The paper deals with the
reasons why the brokerage aspect of the web service vision has proven so difficult to realize
in practice. It also describes a pragmatic approach to web service brokerage based on
automated indexing and discuss the required technological foundations. Ideas for improving
the existing standards are presented too.
6.3.2 Semantic Web Service Composition
Although, a variety of different web services are available, there often occur situations when
no service can satisfy our needs. This can be because there is no service which can fully
achieve our goal, but only its part. Between the available services may exist such one, which
is able to solve the rest of the problem and bring a solution.
One of the most appealing attribute of the web service is the ability to aggregate with other
services and thus create a composite one, called also Web process. Taking this advantage, we