Service Composition - Guide to Cloud Computing for Business and Technology Managers - page 219

The next section describes briefly the problem of service composition before
moving on to an overview of BPEL.
10.4.1 Service Composition
The platform-neutral nature of services creates the opportunity for building
composite services by combining existing elementary or complex services
(the component services) from different enterprises and in turn offering them
as high-level services or processes. Composite services (and, thus, processes)
integrate multiple services—and put together new business functions—by
combining new and existing application assets in a logical flow.
The definition of composite services requires coordinating the flow of con-
trol and data between the constituent services. Business logic can be seen as
the ingredient that sequences, coordinates, and manages interactions among
Web Services. By programming a complex cross-enterprise workflow task or
business transaction, it is possible to logically chain discrete Web Services
activities into cross-enterprise business processes. This is enabled through
orchestration and choreography (because Web Services technologies sup-
port coordination and offer an asynchronous and message-oriented way to
communicate and interact with application logic).
10.4.1.1 Orchestration
Orchestration describes how Web Services can interact with each other at the
message level, including the business logic and execution order of the inter-
actions from the perspective and under control of a single endpoint. This is,
for instance, the case of the process flow where the business process flow is
seen from the vantage point of a single supplier. Orchestration refers to an
executable business process that may result in a long-lived, transactional,
multistep process model. With orchestration, business process interactions
are always controlled from the (private) perspective of one of the business
parties involved in the process.
10.4.1.2 Choreography
Choreography is typically associated with the public (globally visible) mes-
sage exchanges, rules of interaction, and agreements that occur between
multiple business process endpoints, rather than a specific business pro-
cess that is executed by a single party. Choreography tracks the sequence
of messages that may involve multiple parties and multiple sources, includ-
ing customers, suppliers, and partners, where each party involved in the
process describes the part it plays in the interaction and no party owns the
conversation. Choreography is more collaborative in nature than orchestra-
tion. It is described from the perspectives of all parties (common view) and,
in essence, defines the shared state of the interactions between business