Business Process Definitions
The concept of a Business Process Execution
Language (BPEL) has gotten considerable press recently, particularly
since the OASIS publication of WS-BPEL (Web Services BPEL) in a mature
version 2. BPEL lets you embody the functions of Web services, which
have to cooperate to carry out a complex business task, in a
standardized XML format. Let's examine some of the trends which provide
the background for this idea.
Coordinating Complex Systems
Ask any expert to explain a complex system and, if there is a
whiteboard handy, you will soon be looking at a diagram in which named
entities are connected by lines representing processes, transactions,
or transitions. In order to talk about complex systems, people just
naturally prefer the spatial representation that a diagram offers. A
diagram is a big help when you are trying to communicate complex
relationships to people outside your immediate specialty, as I found
when doing environmental impact statements.
One important feature of a diagram is the hiding of details. Thats
why truly usable diagrams are relatively simple, too much detail just
overwhelms and prevents clarity. The whole history of computing has
been vitally concerned with hiding detail. In the early days before
object-oriented programming (OOP), we built flow charts that almost
tracked the code line for line. The progress that the computing
community has made in figuring out how to hide details of low level
processes so that more complex systems can be represented is the whole
reason for the success of OOP, Web services and now SOA.
Early History of Diagramming Computer Systems
The ideas of describing business processes by diagramming goes back
to the early days of object oriented software engineering. In the early
1990s there were three main methodologies, each with various
champions—mostly consultants pushing their competing unique approaches.
In what I thought at the time was a minor miracle, the Unified Modeling Language (UML)
uniting many competing approaches, was adopted in 1997. UML is now a
de-facto industry standard maintained by the Object Management Group
(OMG). In contrast with BPEL, UML does provide standardized process
diagramming methods.
OMG is also responsible for the Common Object Request Broker
Architecture (CORBA) standard. I think it is fair to call CORBA an
important precursor to SOAP and Web Services. OMG also manages the
Business Process Modeling Notation (BPMN) standard for diagramming
business processes at a higher level of abstraction than UML. A recent
OMG development is the new Service-oriented architecture Markup
Language (SoaML) standard based on UML.
Evolution of BPEL
Some early developments leading to BPEL include Microsoft's XLANG
(December 2000)and IBM's Web Services Flow Language (March 2001). The
intent of both was to provide process definition at a level of
abstraction above that of a single Web service port. IBM then merged
the concepts as BPEL4WS in 2002. Together with a number of industry
heavyweights, IBM submitted version 1.1 of BPEL4WS to OASIS
(Organization for the Advancement of Structured Information Standards).
The 2002 version of BPEL4WS and the principles behind the specification
are summarized in this IBM library publication.
The Pivotal Role of WSDL
The evolving low-level support toolkits for web services such as Apache AXIS2
have quickly zeroed in on the essential role of WSDL and XML Schema for
defining to the outside world what a service does and how to talk to
it. WSDL hides the details of service implementation with an abstract
representation. Many tools for many computer languages can take a WSDL
description and automatically create client side code. The WSDL version 1.1 (2001)
as published by the W3C tries for a level of abstraction suitable for
general use and explicitly allows for extensions. WSDL version 2.0
offers better support for REST-style Web services and is a formal
W3C-recommended standard (2007), however it is not yet widely supported
in tools.
OASIS BPEL 2.0
The OASIS non-profit industry group took over standardization and
has released version 2.0. OASIS describes BPEL succinctly as "a
language for specifying business process behavior based on Web
services." You might say that BPEL lets you "orchestrate" interactions
between Web services by defining the roles each service performs and
the communication between services. This OASIS tutorial gives a history of develoment and examples of WS-BPEL 2.0 XML.
WS-BPEL version 2.0 relies heavily on an extended version of WSDL
1.1 using XML schema 1.0 to define data types. OASIS uses the acronym
WS-BPEL to emphasize the interrelated nature of all of the WS-*
standards. In the typical picture of a services "stack," BPEL sits on
top where it works through the WSDL descriptions of services which are,
at lower levels, conducted by messaging and communication protocols
such as SOAP.
The WSDL extensions are needed to support long-running asynchronous
processes. Recall that SOAP by itself is essentially a stateless
protocol, so any process that requires continuity between messages and
extended conversations between components requires extra definitions.
Rather than define a replacement for WSDL, the architects have chosen
to extend it. Ease of extension is of course one of the great
advantages of XML...
Read the complete article in SearchSOA.
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