WS-BPEL language basics
Before we can design an orchestration layer, we need to acquire a good understanding of how the operational characteristics of the process can be formally expressed. This book uses the WS-BPEL language to demonstrate how process logic can be described as part of a concrete definition (Figure 16.1) that can be implemented and executed via a compliant orchestration engine.
Figure 16.1. A common WS-BPEL process definition structure.
Although you likely will be using a process modeling tool and will therefore not be required to author your process definition from scratch, a knowledge of WS-BPEL elements still is useful and often required. WS-BPEL modeling tools frequently make reference to these elements and constructs, and you may be required to dig into the source code they produce to make further refinements.
Note
If you are already comfortable with the WS-BPEL language, feel free to skip ahead to the Service-oriented business process design (a step-by-step process) section.
16.1.1. A brief history of BPEL4WS and WS-BPEL
Before we get into the details of the WS-BPEL language, let's briefly discuss how this specification came to be. The Business Process Execution Language for Web Services (BPEL4WS) was first conceived in July, 2002, with the release of the BPEL4WS 1.0 specification, a joint effort by IBM, Microsoft, and BEA. This document proposed an orchestration language inspired by previous variations, such as IBM's Web Services Flow Language (WSFL) and Microsoft's XLANG specification.
Joined by other contributors from SAP and Siebel Systems, version 1.1 of the BPEL4WS specification was released less than a year later, in May of 2003. This version received more attention and vendor support, leading to a number of commercially available BPEL4WS-compliant orchestration engines. Just prior to this release, the BPEL4WS specification was submitted to an OASIS technical committee so that the specification could be developed into an official, open standard.
The technical committee is in the process of finalizing the release of the next version of BPEL4WS. It has been announced that the language itself has been renamed to the Web Services Business Process Execution Language, or WS-BPEL (and assigned the 2.0 version number). The changes planned for WS-BPEL have been made publicly available on the OASIS Web site at www.oasis-open.org.
Notes have been added to the element descriptions in this section where appropriate to indicate changes in syntax between BPEL4WS and WS-BPEL. For simplicity's sake, we refer to the Business Process Execution Language as WS-BPEL in this book.
16.1.2. Prerequisites
It's time now to learn about the WS-BPEL language. If you haven't already done so, it is recommended that you read Chapter 6 prior to proceeding with this section. Concepts relating to orchestration, coordination, atomic transactions, and business activities are covered in Chapter 6, and are therefore not repeated here. This chapter also assumes you have read through the WSDL tutorial provided in Chapter 13.
16.1.3. The process element
Let's begin with the root element of a WS-BPEL process definition. It is assigned a name value using the name attribute and is used to establish the process definition-related namespaces.
Example 16.1. A skeleton process definition.
<process name="TimesheetSubmissionProcess" targetNamespace="http://www.xmltc.com/tls/process/" xmlns= "http://schemas.xmlsoap.org/ws/2003/03/ business-process/" xmlns:bpl="http://www.xmltc.com/tls/process/" xmlns:emp="http://www.xmltc.com/tls/employee/" xmlns:inv="http://www.xmltc.com/tls/invoice/" xmlns:tst="http://www.xmltc.com/tls/timesheet/" xmlns:not="http://www.xmltc.com/tls/notification/"> ... ... ... ...