Introduction - Business and Scientific Workflows - page 16

BPEL defines a meta-model and an XML-based grammar to
describe the behavior of a business process that is composed of Web
services as well as exposed as a Web service. A BPEL process defines
the execution structure on how multiple Web service invocations are
coordinated to achieve a business goal. The main constructs and their
functions are listed below while a reader may refer to [8] for a
comprehensive description.
< partnerLinks > : It refers to the communication channels
between a process and partners. The process can invoke a partner
and/or be invoked by a partner.
< variables > : It defines the variables used by the process;
< assign > activity is used to manipulate these variables and,
in turn, change the state of the process.
< receive > : It is used to receive messages from an external
partner.
< reply > : It is used to respond to an external partner with
messages. Typically, a < reply > activity is associated with a
< receive > one to implement a WSDL request-response opera-
tion on a particular partner link. By this means, a BPEL process
exposes a WSDL operation to be invoked by others.
< invoke > : It is used to call a WSDL operation of a Web service.
Structural constructs such as < sequence > , < if-else > , < while > ,
< repeatUntil > , < forEach > ,and < flow > define the sequential
( < sequence > ), conditional ( < if-else > ), repetitive
( < repeatUntil > and < forEach > ), and parallel ( < flow > )execu-
tion of the activities contained in them.
< faultHandlers > : It is used to handle exceptions and faults in a
process.
As seen in the description above, another feature that makes BPEL
favorable is that it is tightly integrated with many XML specifications
such as XML Schema, WSDL, XPath (XML Path language), and XSLT
(Extensible Stylesheet Language Transformation). XML Schemas are
used as data type definitions; XPath and XSLT provide the support for
data assignment; and WSDL is used to model partner services as well as
the interface exposed by a BPEL process.