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Semantic Web Services 189
and ActiveBPEL 9 . These two provide means to execute composite web services. BPEL4WS is
not so complex solution as WSMX, but it is already implemented and also used in practice.
Business processes can be described by BPEL4WS in two ways:
– Executable business processes: model actual behavior of a participant in a business
interaction.
– Abstract business processes: describes the mutually visible message exchange behavior
of each of the parties involved, without revealing their interior behavior.
BPEL4WS is meant to describe both kinds of these processes [80]. It provides an XML
language for formal specification and extends the web service interaction model and enables
it to support business transactions. It is itself a flow-chart where each element is a primitive
or structured activity. The primitive activities include the following:
– invoke: invocation of web service operations,
– receive: waiting for an message from external source,
– reply: replying to an external source,
– waiting: waiting for a certain time,
– assign: copying data,
– throws: indication of errors in execution,
– terminate: terminate the process,
– empty: does nothing.
The structured activities contain the control constructs like sequence, switch, while. BPEL4WS
contain also elements for fault and exception handling. Using the primitive and structured
activities complex, executable workflows can be described.
Before execution of composite web services it is suitable to analyze it. The aim is to
examine the correctness of web services. This include checking the pre and postconditions,
input and output data compatibility between successive services, infinite loops or deadlocks
detection. The compatibility of successive services is checked by a simple algorithm. It
verifies if for each service the preconditions are satisfied and input data are available as
a result of the previous services or as initial data. More complicated is the infinite loop of
deadlock detection in the created composition. OWL-S and BPEL4WS do not offer ways to
verify this. We need other means to do this [58]. Approaches based on Petri nets, Π-calculus
exist to perform this.
Petri nets are used for controlling the execution of the workflow [79]. Petri net is
a mathematical representation of discrete systems. It can be seen as a graphical notation
9 http://www.active-endpoints.com/active-bpel-engine-overview.htm