Business Process Execution Language

There were ten original design goals associated with BPEL:

1. Define business processes that interact with external entities through web service operations defined using Web Services Description Language (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.
2. Define business processes using an XML-based language. Do not define a graphical representation of processes or provide any particular design methodology for processes.[9]
3. 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.
4. 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.
5. Provide data manipulation functions for the simple manipulation of data needed to define process data and control flow.
6. 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.
7. 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.
8. 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.
9. Use Web Services as the model for process decomposition and assembly.
10. Build on Web services standards (approved and proposed) as much as possible in a composable and modular manner.

BPEL is an orchestration language, and not a choreography language. 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) which operates according to rules (the choreography) but 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.

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, etcetera.) 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 Model and 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[10] 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.[11] 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.

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[12] is an effort related to JSR 207[13] that may enable Java to function as a 'programming in the small' language within BPEL.

Despite wide acceptance of Web services in distributed business applications, the absence of human interactions was a significant gap for many real-world business processes.

To fill this gap, BPEL4People extended BPEL from orchestration of Web services alone to orchestration of role-based human activities as well.

Within the context of a business process BPEL4People

• supports role-based interaction of people
• provides means of assigning users to generic human roles
• takes care to delegate ownership of a task to a person only
• supports scenario as
  • four-eyes scenario
  • nomination
  • escalation
  • chained execution

by extending BPEL with additional independent syntax and semantic.

The WS-HumanTask specification introduces the definition of human tasks and notifications, including their properties, behavior and a set of operations used to manipulate human tasks. A coordination protocol is introduced in order to control autonomy and life cycle of service-enabled human tasks in an interoperable manner.

The BPEL4People specification introduces a WS-BPEL extension to address human interactions in WS-BPEL as a first-class citizen. It defines a new type of basic activity which uses human tasks as an implementation, and allows specifying tasks local to a process or use tasks defined outside of the process definition. This extension is based on the WS-HumanTask specification.

Version 2.0 introduced some changes and new features:

• 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