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190 Selected Studies on Software and Information Systems
based on directed bipartite graphs. There are two types of nodes: places and transitions.
Directed arcs exist only between different types of nodes (it is a bipartite graph). The
places from (to) which an arc runs to (from) a transition are called input (output) places.
Places contain tokens which are distributed over them. This distribution is called marking.
Transitions act in such a way that they take input tokens, process them and put the output
tokens into output places (the input tokens are consumed, i.e. will not be found far in the
input places). This is called firing. A transition is enabled if all input tokens are available. In
this situation it automatically fires.
Using the elements of Petri nets we describe compositions of web services [39, 60]. In
Table 6-2 the correspondence between Petri nets and web services’ elements is shown.
Table 6-2. Correspondance between Petri nets and web service elements.
Petri net
Tokens in input places
Tokens in output places
Transitions
Firing
Marking
Web service
Input data
Output data
Web services
Web service method execution
State of the execution
More detailed overview of how Petri nets formalize the elements of web services is available
in [60]. It also discusses how Petri nets are used for simulation, validation, verification,
composition, and performance analysis of web service compositions. From the web service
execution point of view the most important is the verification. It deals with the question if
it is possible to achieve a goal with available services. If a Petri net depicts the behavior of
available services, this problem is transformed to a reachability question. It is that if it is
possible to reach a marking which depicts the user’s goal. This question can be answered
for example by searching the state-space with breath-first search, however this approach is
in reality unpractical from computation reasons. Rather linear temporal logic with tableau
method is used to decide if the goal state can be reached. Petri nets are also used to check
the occurrence of deadlocks. A marking is a deadlock if it enables no transitions. We want
to avoid this situation. There exist algorithms which solve the reachability and deadlock
problem [60].
One of the most important topics within web services is the security [69]. In [35] a process
for web service security – PWSec is presented. This process specifies how to define security
requirements for web services-based systems describes logical security services-based reference
security architecture and explains how to instantiate it to obtain a concrete security
architecture based on the current web service security standards. In [50], they propose a uniform
format to describe communications and computer security through security objects
and coherent integrations of this format into WSDL as a proper extension and also into an
ontology framework for security knowledge representation and reasoning.
6.3.4 Semantic Web Service Composition Problems
The automatic composition of web services has shown to be a challenging task and it is not
clear which techniques serve the problem best [73]. There are several problems in different