A BPEL process service component must sometimes gather information from multiple asynchronous sources. Because each callback can take an undefined amount of time (hours or days), it may take too long to call each service one at a time. By breaking the calls into a parallel flow, a BPEL process service component can invoke multiple web services at the same time, and receive the responses as they come in. This method is much more time efficient.

Figure 10-1 shows a flow activity named Retrieve_QuotesFromSuppliers. The Retrieve_QuotesFromSuppliers flow activity sends order information to two suppliers in parallel:

• An internal warehouse (InternalWarehouseService)

• An external partner warehouse (PartnerSupplierMediator)

The two warehouses return their bids for the order to the flow activity. Here, two asynchronous callbacks execute in parallel. One callback does not have to wait for the other to complete first. Each response is stored in a different global variable.