Integrating Web Services

Business processes are understood by developers as Web

services. To enable us to take advantage of pre-coded Web services, we have the

WSDL (Web Service Definition Language) documents that provide engines such as

BPEL with information about the where and how of using the exposed Web services.

Having understood the concepts of BPEL in our technology story last month (refer

to Interoperable Biz Processes with BPEL, page 46, April 2006, PCQuest) let us

move forward and implement an application where we integrate two Web services

using BPEL. This way, we shall get a better understanding of how BPEL works.

Sample Web services



Our sample Web services are meant to handle loan approval. The loan approval

process includes three tasks: computing the



associated risk level with the loan amount, processing the loan request if

approved and sending the accept/reject reply to the customer. The loan process

starts off with receiving a loan request. The next step includes risk assessment

based on the loan amount. The final step is to send an accept/reject message. As

you can see, each of theses tasks are handled by Web services and our BPEL

process will integrate them into a single business process. One of ou WSDL files

is as follows.

targetNamespace="http://loans.org/wsdl/loan-approval"



      xmlns="http://schemas.xmlsoap.org/wsdl/"


      xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"


      xmlns:xsd="http://www.w3.org/2001/XMLSchema"    


      mlns:plnk="http://schemas.xmlsoap.org/ws/2003/05/partner-link/"      


            ...

The message-type information is taken from the WSDL files

where each of these is defined in the '' tag. The other major

section of a BPEL process definition is the 'Fault handlers' that define the

proceedings in the 'not-going-right' case. That is, fault handlers define

activities that must be performed in response to faults that may occur either

during initialization or from the initialization of services. BPEL identifies

faults, both internal and ones resulting from invocation of service by a

qualified name. This is in contrast to WSDL 1.1 where all faults within a common

namespace may not have unique name-a serious limitation as it makes all faults

within a common namespace indistinguishable. However, it is overcome in BPEL due

to unique names. Thus, our sample process will also contain fault handlers as

follows.

     faultName="lns:unableToHandleRequest" 



operation="request" partnerLink="customer" 


portType="lns:loanServicePT" variable="error"/>

The diagram represents our sample loan application process, integrating the activities of request receipt, assessment and approval/rejection

Rest of the process defines normal behavior of the loan



request. This section ideally includes both compensation and correlation

descriptions. However our sample process does not require any such description.

We'll now define the process flow between activities. This definition contains

a description of all the links between activities, followed by description of

each of the activities executing to complete the business process. The sample

process will, therefore, have the following description.

operation="request" partnerLink="customer" portType="lns:loanServicePT"

variable="request">

























      name="ReplyAcceptMessage" operation="request" partnerLink="customer"

portType="lns:loanServicePT" variable="approval">

operation="check" outputVariable="risk" partnerLink="assessor"

portType="lns:riskAssessmentPT">

operation="approve" outputVariable="approval" partnerLink="approver"

portType="lns:loanApprovalPT">















































      name="AssignYesToAccept">

The link tag describes the links between activities, which



is followed by describing the activities themselves and their



operations. Once all this is done, you have a complete BPEL process definition

that can be deployed onto a BPEL engine. A more comprehensive process would also

have sections for describing event handlers apart from correlations and

compensations. We have omitted that for the sake of simplicity. BPEL gives a

complete definition for a process integrating Web services. You can also define

expressions for evaluating conditions and provide conditional and iterative

constructs. It also overcomes some of the serious limitations in other XML based

specifications as we have seen in the case of fault handlers in our sample

process. Not surprising that it has seen wide acceptance as a standard and is

here to stay.

Anadi Misra