Information Technology Reference
In-Depth Information
Initiate Checkout
Invoke TPC
Receive Fetch Token
Initiate TPC
Reply Token
Invoke GetOrder Trans
Receive GetOrderData
Receive OrderData
Reply OrderData
Reply Ack
Figure 4.2
BPEL of eBay and TPC services.
OrderData). TPC returns an Ack after it receives OrderData (Reply
Ack). With the order data retrieved, TPC shows to the user the items
purchased and shipping address, and the buyer follows the TPC flow to
complete the checkout process. Note that this step is not relevant to the
problem we are going to address, and thus it is not shown in Figure 4.2.
Obviously these two services cannot be directly composed because
of the following reasons:
1. The messages exchanged between their operations do not match
exactly; and
2. The numbers of their operations and input/output messages are
not identical.
Besides, without formal analysis, the correctness of their composi-
tion cannot be guaranteed.
In the aforementioned scenario, there is no way to compose eBay
and TPC services directly. In other words, the current direct composi-
tion approaches fail to do so. To solve this problem, Section 4.2 givesthe
formalismforBPELservice,composition,mediation,andmediation-aided
