Service Composition - Guide to Cloud Computing for Business and Technology Managers - page 223

operations on the interface of the composite BPEL service. BPEL provides a
mechanism for creating implementation- and platform-independent compo-
sitions of services woven strictly from the abstract interfaces provided in the
WSDL definitions. The definition of a BPEL business process also follows the
WSDL convention of strict separation between the abstract service interface
and service implementation. In particular, a BPEL process represents parties
and interactions between these parties in terms of abstract WSDL interfaces
(by means of <portType>s and <operation>s), while no references are made
to the actual services (binding and address information) used by a process
instance. Both the interacting process and its counterparts are modeled in
the form of WSDL services. Actual implementations of the services them-
selves may be dynamically bound to the partners of a BPEL composition,
without affecting the composition's definition. Business processes specified
in BPEL are fully executable portable scripts that can be interpreted by busi-
ness process engines in BPEL conformant environments.
BPEL distinguishes five main sections:
1. The message flow section of BPEL is handled by basic activities that
include invoking an operation on some Web Service, waiting for a
process operation to be invoked by some external client, and gener-
ating the response of an input/output operation.
2. The control flow section of BPEL is a hybrid model principally based
on block-structured definitions with the ability to define selective
state transition control flow definitions for synchronization purposes.
3. The dataflow section of BPEL comprises variables that provide the
means for holding messages that constitute the state of a business
process. The messages held are often those that have been received
from partners or are to be sent to partners. Variables can also hold
data that are needed for holding state related to the process and
never exchanged with partners. Variables are scoped, and the name
of a variable should be unique within its own scope.
4. The process orchestration section of BPEL uses partner links to
establish peer-to-peer partner relationships.
5. The fault and exception handling section of BPEL deals with errors
that might occur when services are being invoked with handling
compensations of units of work and dealing with exceptions during
the course of a BPEL computation.
BPEL consists of the following basic activities:
receive : The receive activity initiates a new process when used at its
start or does a blocking wait for a matching message to arrive when
used during a process.