Comparison of xpdl and WS-BPEL 2: a general overview of the content of the two

Strictly speaking, BPEL and xpdl are not a hierarchical specification.. For example, BPEL is mainly used to support automated business processes based on Web Services, without any content related to manual activities.

The xpdl standard developed by WfMC represents a process definition format. It is a model built around BPM modeling, simulation, operation, and management throughout the lifecycle. It can express the process definition. The node element of xpdl has x/y coordinates and sizes. xpdl has the concept of connecting an arc and can be used to set specific conditions for the transfer of connecting an arc. Nodes and connections specify the specific execution information, such as the color, activity description, implementation, and Web service call. Xpdl2.0 contains extensions that describe the use of BPMN. Xpdl ensures that the process definition is exchanged between two design tools and that the process execution semantics are consistent.

WS-BPEL is an execution language ". The goal of the WS-BPEL is to provide the definition of Web Service orchestration, such as sequential interaction, data flow from one point to another. It has a syntax structure, including variables and operations. The operation can send and receive soap messages, and has powerful support for XML and XML Conversion. You can conveniently call multiple web services at the same time and synchronize the results. It does not have any elements that support graphical representation. For example, an activity has no size or position, nor can it express "pointing" elements. The following describes the main content of BPEL:

1. Orchestrate various interactions of web services, such as invoke, receive, and reply.
2. Sequence, branch, and various process control methods.
3. Exception Handling, including error conditions, transactions, and compensation.
4. Data Flow and data operations.
5. Timeout processing.

Xpdl compared to WS-BPEL missing parts
Due to the difference of the two targets, the lack of xpdl compared with WS-BPEL mainly include:
1. the WS-BPEL uses structured XML elements (structured activities) such as sequence, if/else, repeatuntil, pick, links, and foreach to express the call sequence of the web service, similar to a language that uses XML to program Web Services. Xpdl does not have these elements, and xpdl uses the transition of the connected arc to express the sequence of execution of nodes. The xpdl node types include automatic nodes, manual nodes, nested subflows, block activities, and routing activities.
2. The WS-BPEL uses XML elements such as assign, query, from/to execute a value assignment operation on the process property data; xpdl does not have these elements.

Lack of WS-BPEL compared to xpdl
Similarly, due to the difference in the two goals, the lack of WS-BPEL compared to xpdl mainly include:
1. The WS-BPEL itself does not have the concept of a way of assigning tasks.
2. WS-BPEL lacks sub-flow semantics and IBM has a specification for extending subflow over BPEL4WS.
3. The WS-BPEL does not have the concept defined by the "Application application", for the WS-BPEL of invoke, receive, reply and other types of nodes and external call is the call of WebService. Xpdl defines a variety of applications through the "Application application", indicating the various programs that need to be executed manually involved in the process. These applications can be Java methods, C/S clients, electronic forms, and automatic web service calls.
4. xpdl defines elements such as flow simulation and flow version, and WS-BPEL does not have these concepts.