Information Technology Reference
In-Depth Information
2. Two services in a composition consent to the message format and
exchange sequence such that their composition process always
executes in a logically correct way (e.g., terminates properly).
The existing specifications and approaches in service composition
mostly concentrate on direct composition that requires the services to be
fully compatible. In practice, it may not be the case. Hence, we introduce
the concept of partial compatibility. It refers to the situation where two
(or more) Web services provide complementary functionality and could,
in principle, be linked together. However, in reality, their interfaces and
interaction patterns do not fit each other exactly. Hence, they cannot be
directly composed. The problem of partial compatibility arises mainly
because the services have to interact with one another in the ways not
necessarily foreseen when they are separately developed. Consequently,
the assumptions made for direct composition may no longer hold.
As presented later in this chapter, if two services have mismatches
in their interfaces and interaction patterns, they cannot be directly
composed. Moreover, even if they can be directly composed, we do
not have confidence whether their composition is correct. Note that
the meaning of correctness depends on specific requirements, but it
usually requires terminability, liveness, and boundedness by using the
terms from Petri nets [81]. However, current research in Web service
composition mainly focuses on automatic composition methods
[86,109-111] and formal verification of service composition [112].
Less attention is paid to the issue of partial compatibility.
Recently, the mediation approach is attracting more attention
[46,48-50,113] because it is considered as an economic and labor-
saving method to address the challenge of partial compatibility in real
world Web service composition. The basic idea of mediation-aided
composition is similar to the concept of an adapter to make two pieces
of hardware compatible. The mediator wraps the various services
such that they can appear homogeneous and be therefore integrated
and composed more easily.
Figure 4.1 illustrates direct and mediation-aided compositions.
Messages A and B are exchanged between Services 1 and 2. In
Figure 4.1a, Service 1 first sends A and then waits for B; Service 2
first waits for A and then sends B. Services 1 and 2 can be directly
composed by adding links between “Send A” and “Receive A”,
“Receive B” and “Send B”. This composition is direct because nothing
