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genetic algorithm [44] are used to solve the QoS constraints and yield
the optimal solution(s).
Automatic composition attracts the most attention in SOA research
because of its importance. However, most of the approaches are theory-
oriented and usually require a substantial effort to build a model for
each candidate service first. For example, planning-based approaches
require an IOPE (Input, Output, Precondition, and Effect) structure for
each service operation; optimization-based ones usually require the
QoS attributes of each operation as input to the optimization problem.
Such a prerequisite affects the applicability of these theoretically sound
but heavyweight approaches, and also calls for more lightweight ones.
1.3.2 Mediation-Aided Service Composition
The service orchestration and choreography specifications, and most of
the automatic service composition methods, assume the direct composi-
tion among services. This is possible under the following assumptions:
1. Services in a composition consent to the same vocabulary in
message exchange.
2. The incoming messages of one service are the exact ones
provided by its partner(s); the outgoing messages of one service
are the exact ones consumed by its partner(s).
3. Services in a composition consent
to message exchange
sequences.
Services may become partially compatible if any one of the above
assumptions no longer holds. Dealing with such cases leads to two
major methods, that is, configuration [46] and mediation [47,48], to
make partially compatible services work with each other. The former is
a heavyweight approach that equips a service with additional variable
points such that it can work smoothly with more partners.
Configuration-based approaches require the modification of ser-
vice implementation. On the other hand, mediation-based approaches
are lightweight and less intrusive than the former. The basic idea
behind it is that, if two services cannot be directly composed with each
other, an adaptor is developed to mediate the mismatch on message
formats and/or sequences. Benatallah et al. [49] provide a summary of
the mismatch patterns in service composition. Based on the work in
