Creating Structure

The process engine has the task of allocating a success message to a process instance long after the corresponding request has been issued, and in some cases, many instances are active at the same time in the engine. For example, some instances of "Approve trip" could be running simultaneously in the process engine because several members of staff are planning to visit a trade fair. If it is impossible to add a transaction ID to the request because the interface is defined externally, the WFMS attempts to identify the process instance by reference to the parameters. In the case of a business trip, the name and departure date would probably suffice. Attributes comparable to primary keys are referred to as correlation sets in BPEL.

Scopes in modeling are similar to blocks in C++ or Java. A scope is an area with local variables, process sequences, and fault handlers that allow a developer to keep a process definition tidy, in that local structures and local variables are available just in the local scope.

A delivery example modeled with the use of scopes will help to illustrate: A company charges an amount to a credit card (subscope invoice), but can't deliver the goods because the address is wrong (subscope delivery). The corresponding fault handler passes the error on to the surrounding scope, which also has a fault handler. The fault handler notifies the order processing clerk and compensates the activities after having done so by crediting the amount to the credit card account. The advantage is that the process designer can assign error handling and countermeasures locally for each step.

Example: Booking a Business Trip

Using the travel example described at the beginning of this article, I can now model and later start a simple process. Once the application server and the BPEL are downloaded and installed, the designer models part of the business trip reservation. This simple example does without compensation and asynchronous calls. The result is a collection of XML files that the designer can bundle into an archive and transfer to the engine.

The input for the process comprises four records: the username of the person going on the trip, the starting and finishing points of the trip, and the date and time. The process starts by selecting a method of transport. To allow this to happen, a web service refers to the duration of the trip and the identity of the person traveling to decide whether to book a train or a plane ticket. CEOs and service engineers could have a higher flight priority, for example. After making this decision, the process engine books the tickets. The schedule for the trip is then sent to the traveler (see Figure 3).

Point and Click

The graphical ActiveBPEL Designer [5] helps convert the individual modeling steps to XML files. BPEL strictly separates interface information from web services. The languages uses operations, parameter descriptions, and partner links (abstract representations of the actual service addresses). The model itself defines the structure, sequence, and exception handling. The files are available at the Linux Magazine website [6].

To follow this example, BPEL newcomers can download the ActiveBPEL Designer, unpack it, then launch it as follows:

./ActiveVOS_Designer_unix_6_0_2.sh

Once the license file has been mailed to you, there is nothing to stop you from installing. If you prefer not to use the test version, you can use any other BPEL designer. A wizard installs a full version of the engine, allowing users to test the engine directly from the editor.

After installing, users can fix the encoding problem with version 6.0.2 for Linux and recode the file:

cd ./Designer/designer
recode ibmpc..latin1 jre/lib/i386/jvm.cfg

Then type ./designer to launch the designer and accept the path suggested for the workspace. Closing the tab closes the welcome page. Eclipse users will soon feel at home. In the designer's default workspace, you can select File | New | Orchestration Project to create a new project.

The WSDL file, services.wsdl describes all the interfaces. First store the file in the project tree below wsdl; the same applies to the BPEL process description, travelbooking.bpel, below bpel. The deployment descriptor contains the address details for the services. The important thing is that all the definitions find each other; if the designer makes a mistake, you need to check the WSDL references in the BPEL and PDD files.

export CATALINA_HOME=$(pwd)/apachetomcat
cd activebpel5.0.2
./install.sh

cd ../apachetomcat/bin
./startup.sh