Business Process Execution Language

As defined in the abstract of the Web Services Business Process Execution Language OASIS Standard WS-BPEL 2.0, WS-BPEL (or BPEL for short) is a language for specifying business process behavior based on Web Services. Processes in WS-BPEL export and import functionality by using Web Service interfaces exclusively.

Business processes can be described in two ways. Executable business processes model actual behavior of a participant in a business interaction. Abstract business processes are partially specified processes that are not intended to be executed. An Abstract Process may hide some of the required concrete operational details. Abstract Processes serve a descriptive role, with more than one possible use case, including observable behavior and process template. WS-BPEL is meant to be used to model the behavior of both Executable and Abstract Processes.

WS-BPEL provides a language for the specification of Executable and Abstract business processes. By doing so, it extends the Web Services interaction model and enables it to support business transactions. WS-BPEL defines an interoperable integration model that should facilitate the expansion of automated process integration in both the intra-corporate and the business-to-business spaces.

The origins of BPEL can be traced to WSFL and XLANG. It is serialized in XML and aims to enable programming in the large. The concepts of programming in the large and programming in the small distinguish between two aspects of writing the type of long-running asynchronous processes that one typically sees in business processes.

Purpose

Programming in the large generally refers to the high-level state transition interactions of a process—BPEL refers to this concept as an Abstract Process. A BPEL Abstract Process represents a set of publicly observable behaviors in a standardized fashion. An Abstract Process includes information such as when to wait for messages, when to send messages, when to compensate for failed transactions, etc. Programming in the small, in contrast, deals with short-lived programmatic behavior, often executed as a single transaction and involving access to local logic and resources such as files, databases, etc. BPEL's development came out of the notion that programming in the large and programming in the small required different types of languages.

BPEL Design Goals

There were ten original design goals associated with BPEL:

Define business processes that interact with external entities through Web Service operations defined using WSDL 1.1, and that manifest themselves as Web services defined using WSDL 1.1. The interactions are “abstract” in the sense that the dependence is on portType definitions, not on port definitions.
Define business processes using an XML-based language. Do not define a graphical representation of processes or provide any particular design methodology for processes.
Define a set of Web service orchestration concepts that are meant to be used by both the external (abstract) and internal (executable) views of a business process. Such a business process defines the behavior of a single autonomous entity, typically operating in interaction with other similar peer entities. It is recognized that each usage pattern (i.e. abstract view and executable view) will require a few specialized extensions, but these extensions are to be kept to a minimum and tested against requirements such as import/export and conformance checking that link the two usage patterns.
Provide both hierarchical and graph-like control regimes, and allow their use to be blended as seamlessly as possible. This should reduce the fragmentation of the process modeling space.
Provide data manipulation functions for the simple manipulation of data needed to define process data and control flow.
Support an identification mechanism for process instances that allows the definition of instance identifiers at the application message level. Instance identifiers should be defined by partners and may change.
Support the implicit creation and termination of process instances as the basic lifecycle mechanism. Advanced lifecycle operations such as "suspend" and "resume" may be added in future releases for enhanced lifecycle management.
Define a long-running transaction model that is based on proven techniques like compensation actions and scoping to support failure recovery for parts of long-running business processes.
Use Web Services as the model for process decomposition and assembly.
Build on Web services standards (approved and proposed) as much as possible in a composable and modular manner.

The BPEL language

BPEL is an Orchestration language, not a choreography language (see Web Service Choreography).The primary difference between orchestration and choreography is executability and control. An orchestration specifies an executable process that involves message exchanges with other systems, such that the message exchange sequences are controlled by the orchestration designer. A choreography specifies a protocol for peer-to-peer interactions, defining, e.g., the legal sequences of messages exchanged with the purpose of guaranteeing interoperability. Such a protocol is not directly executable, as it allows many different realizations (processes that comply with it). A choreography can be realized by writing an orchestration (e.g. in the form of a BPEL process) for each peer involved in it. The orchestration and the choreography distinctions are based on analogies: orchestration refers to the central control (by the conductor) of the behavior of a distributed system (the orchestra consisting of many players), while choreography refers to a distributed system (the dancing team) without centralized control.

BPEL's focus on modern business processes, plus the histories of WSFL and XLANG, led BPEL to adopt web services as its external communication mechanism. Thus BPEL's messaging facilities depend on the use of the Web Services Description Language (WSDL) 1.1 to describe outgoing and incoming messages.

In addition to providing facilities to enable sending and receiving messages, the BPEL programming language also supports:

A property-based message correlation mechanism
XML and WSDL typed variables
An extensible language plug-in model to allow writing expressions and queries in multiple languages: BPEL supports XPath 1.0 by default
Structured-programming constructs including if-then-elseif-else, while, sequence (to enable executing commands in order) and flow (to enable executing commands in parallel)
A scoping system to allow the encapsulation of logic with local variables, fault-handlers, compensation-handlers and event-handlers
Serialized scopes to control concurrent access to variables

What's new in WS-BPEL 2.0?

New activity types: repeatUntil, validate, forEach (parallel and sequential), rethrow, extensionActivity, compensateScope
Renamed activities: switch/case renamed to if/else, terminate renamed to exit
Termination Handler added to scope activities to provide explicit behavior for termination
Variable initialization
XSLT for variable transformations (New XPath extension function bpws:doXslTransform)
XPath access to variable data (XPath variable syntax $variable[.part]/location)
XML schema variables in Web service activities (for WS-I doc/lit style service interactions)
Locally declared messageExchange (internal correlation of receive and reply activities)
Clarification of Abstract Processes (syntax and semantics)
Enable expression language overrides at each activity

Adding 'programming in the small' support to BPEL

BPEL's control structures such as 'if-then-elseif-else' and 'while' as well as its variable manipulation facilities depend on the use of 'programming in the small' languages to provide logic. All BPEL implementations must support XPath 1.0 as a default language. But the design of BPEL envisages extensibility so that systems builders can use other languages as well. BPELJ is an effort related to JSR 207 that may enable Java to function as a 'programming in the small' language within BPEL.

History

IBM and Microsoft had each defined their own, fairly similar, 'programming in the large' languages, WSFL and XLANG, respectively. With the popularity and advent of BPML, and the growing success of BPMI.org and the open BPMS movement led by JBoss and Intalio Inc., IBM and Microsoft decided to combine these languages into a new language, BPEL4WS. In April 2003, BEA Systems, IBM, Microsoft, SAP and Siebel Systems submitted BPEL4WS 1.1 to OASIS for standardization via the Web Services BPEL Technical Committee. Although BPEL4WS appeared as both a 1.0 and 1.1 version, the OASIS WS-BPEL technical committee voted on 14 September 2004 to name their spec WS-BPEL 2.0. This change in name was done to align BPEL with other Web Service standard naming conventions which start with WS- and accounts for the significant enhancements between BPEL4WS 1.1 and WS-BPEL 2.0. If you are not discussing a specific version, the moniker BPEL is commonly used^{[citation needed]}.

In June 2007, Active Endpoints, Adobe, BEA, IBM, Oracle and SAP published the BPEL4People and WS-HumanTask specifications, which describe how human interaction in BPEL processes can be implemented.

Relationship of BPEL to BPMN

There is no standard graphical notation for WS-BPEL, as the OASIS technical committee decided this was out of scope. Some vendors have invented their own notations. These notations take advantage of the fact that most constructs in BPEL are block-structured (e.g. sequence, while, pick, scope, etc.) This feature enables a direct visual representation of BPEL process descriptions in the form of structograms, in a style reminiscent of a Nassi-Shneiderman diagram.

Others have proposed to use a substantially different business process modeling language, namely Business Process Modeling Notation (BPMN), as a graphical front-end to capture BPEL process descriptions. As an illustration of the feasibility of this approach, the BPMN specification includes an informal and partial mapping from BPMN to BPEL 1.1. A more detailed mapping of BPMN to BPEL has been implemented in a number of tools, including an open-source tool known as BPMN2BPEL. However, the development of these tools has exposed fundamental differences between BPMN and BPEL, which make it very difficult, and in some cases impossible, to generate human-readable BPEL code from BPMN models. Even more difficult is the problem of BPMN-to-BPEL round-trip engineering: generating BPEL code from BPMN diagrams and maintaining the original BPMN model and the generated BPEL code synchronized, in the sense that any modification to one is propagated to the other.

External links

Standards

BPEL and business process sites

BPEL articles

Books on BPEL 2.0

SOA for the Business Developer: Concepts, BPEL, and SCA, ISBN 978-158347-065-7