Sommerville-Software-Engineering-10ed

18.4 ■ Service composition 545
Retry
Hotels.
GetRequirements
Hotels.
CheckAvailability
No rooms
Rooms OK
Hotels.
NoAvailability
Cancel
Hotels.
ReserveRooms
Hotels.
ConfirmReservation
Customer
Figure 18.15 A
fragment of a hotel
booking workflow
These key features are enough to describe most workflows. However, BPMN
includes many additional features that I don’t have space to describe here. These add
information to a business process description that allows it to be automatically translated
into an executable service.
Figure 18.15 shows a process that is enacted in a single organization, the company
that provides a booking service. However, the key benefit of a service-oriented
approach is that it supports interorganizational computing. This means that a computation
involves processes and services in different companies. This process is represented
in BPMN by developing separate workflows for each of the organizations
involved with interactions between them.
To illustrate multiple workflow processes, I use a different example, drawn
from high-performance computing, where hardware is offered as a service.
Services are created to provide access to high-performance computers to a geographically
distributed user community. In this example, a vector-processing computer
(a machine that can carry out parallel computations on arrays of values) is
offered as a service (VectorProcService) by a research laboratory. This is accessed
through another service called SetupComputation. These services and their
interactions are shown in Figure 18.16.
In this example, the workflow for the SetupComputation service asks for access
to a vector processor and, if a processor is available, establishes the computation
required and downloads data to the processing service. Once the computation is
complete, the results are stored on the local computer. The workflow for
VectorProcService includes the following steps:
Check if a processor is available
Allocate resources for the computation
Initialize the system