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model for a BPEL process, and this model covers the exceptional beha-
vior, for example, faults, events, and compensation. Lohmann et al. [120]
use Petri nets to decide the controllability of a service, that is, the
existence of a partner process, and compute its operating guideline.
Ouyang et al. [55] present a comprehensive and rigorously defined
mapping of BPEL constructs into Petri net structures. Other works focus
on the analysis of the direct composition of two or more BPEL services
[56,121,122]. Compared with the work in [56,121,122], we clearly move
one step forward to tackle the issue of indirect composition.
4.5.5 Component/Web Service Mediation
Compared with configuration, mediation is a relatively lightweight
approach. The idea of mediation-aided service composition is stimu-
lated by the related work in the software engineering area. For example,
generative programming [123] is a style of computer programming that
uses automated source code creation through generic classes, proto-
types, templates, aspects, and code generators to improve the produc-
tivity of programmers.
Yellin and Strom [113] propose a method to augment object-
oriented component interfaces with protocols that include sequencing
constraints. They define adaptors that can be used to bridge the dif-
ferences between components and present the method to automatically
generate adaptors. Our work is stimulated by [113], but significantly
different from it in the following aspects. First, the automata-based
approach has limitations in modeling the concurrent behavior of Web
services, for example, the flow construct in BPEL, while Petri nets are
good at modeling such behavior. Second, the automata-based approach
focuses on a message exchange sequence but ignores the different
exchange styles; for example, more subtle behaviors such as one-way
invoke, two-way invoke and receive with/without reply, cannot be
modeled by automata, while Petri nets can describe them well as
illustrated in this chapter. Third, in the automata model in [113], all
the state transitions are triggered by external messages. Thus, the
internal nondeterminism cannot be modeled, for example, the if con-
struct in BPEL; while our SWF-net can model internal as well as
external nondeterminism. Last, the automata-based approach can only
model the synchronous semantics of composition, while Petri net-based
one can model both synchronous and asynchronous semantics. This
