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is, BPEL, as our input, and define service workflow net (SWF-net), a
kind of colored Petri nets, as a unified formalism to describe services,
composition, and mediator. We use a state-space-based method to check
the existence of mediation. We introduce the concept of communicating
reachability graph (CRG) whose function is to concurrently construct the
reachability graph of two services, using data mapping (that needs to
be provided as an input in our current solution) as the communication
mechanism. By using the concept of stubborn sets, we verify that the
CRG contains all needed properties in mediator existence checking.
Moreover, state-space exploration is sped up. We also propose the
guidance to generate a mediator to glue two services if through CRG
we can verify that mediation is possible. Finally, we validate the approach
through a real-life case.
4.1.1 AMotivating Scenario
The motivating scenario comes from the composition of eBay and
a third-party checkout (TPC) service [114]. It is an excerpt of a real
business scenario—simple yet illustrative enough to explain the pro-
posed concept, algorithm, and result.
As an online auction and shopping service provider, eBay allows
a third party to handle a seller's checkout processes. When buyers on
eBay finish shopping and want to check out, they are directed to a
checkout system provided by a third-party checkout service provider.
The eBay and third-party checkout services are to be composed to fulfill
the requirement of the online shopping and checkout business. They
provide complementary functionality, but because they are developed
by different entities, they do not fit each other as exactly as one desires.
Figure 4.2 shows the BPEL of both eBay and TPC services. When
a buyer wins an auction, the eBay service starts the checkout and then
invokes a TPC service by passing OrderID, UserID, and SecretID.
Next, eBay receives invocation from TPC (Receive FetchToken) with
UserID and SecretID, and replies with Token, which is a required
parameter in subsequent interactions. Finally, eBay receives invocation
from TPC with Token, OrderID, and UserID (Receive GetOrderData)
and replies with OrderData. On the TPC side, the TPC service initiates
its own process upon receiving Order and PartnerID. Then it invokes
GetOrderTrans with OrderID and UserID and waits for an asynchro-
nous reply with OrderData from the online merchant
(Receive
