Section 9.5. BPEL, XLANG, and WSFL

9.5. BPEL, XLANG, and WSFL

The design of BPEL borrows elements from both XLANG and WSFL. Table 9-3 lists the major BPEL features and their corresponding features in XLANG and WSFL.

Table 9-3. XLANG and WSFL influences on BPEL

BPEL feature	XLANG	WSFL
Partner, partner links	Business party contracts	Global model, service providers, plug links
Process variables	No, implicit only	No, implicit only
Correlation	Correlation sets, correlation attributes of operation actions
Exception handling	Per-context exception handler	Weak, use control links to custom exception activities
Compensation	Per-context compensation handler
Scope	context element
Invoke	operation action	Activity
Receive	operation action	Activity
Reply	operation action	Activity
Wait	delay action
No op	empty action	Internal activity
Sequence	sequence action	Yes, as graph
Switch	switch action	Yes, as graph
While	while action	Yes, as graph, supports loops
Pick	pick action
Flow	All action provides parallelism but not with flow links	Flow model, control links
Assign
Global event handler
Dead path elimination		Inherent support, crucially important concept
Subprocess
Process initiation	An operation action with activation set to true	Lifecycle service has spawn operation

Two conclusions can be drawn from this survey:

• XLANG and WSFL are very different approaches, and, depending on your point of view, coexist either peacefully or tenuously in BPEL. XLANG is programmatic, where programming language-like elements such as while, sequence, and switch structure the control flow. WSFL prefers a directed graph approach, which control flow is the traversal of activities over links. The majority of BPEL resembles XLANG, but its flow activity, and its related concept of dead path elimination, is obviously derived from WSFL.

• BPEL is much more advanced than either of its ancestors, with support for process variables and global event handlers. In addition, in BPEL the direction of web service interaction (receive, invoke, and reply activities) is clearer than in WSFL and XLANG, which require you to refer back to the WSDL for the ordering of input and output message types on an operation.