Structured Activities

Structured activities can be thought of as containers that can hold one or more activities. The Controls section of the Palette contains all of the structured activities. When you drag and drop one of these onto the drawing canvas, the BPEL Designer will create a basic skeleton of the activity, and assign default properties.

All structured activities will require some additional configuration before they are considered valid. For example, BPEL does not allow an empty Sequence activity. Invalid structured activities will be decorated with an error icon similar to basic activities.

Structured activities can be expanded and collapsed on the drawing canvas by clicking the plus and minus buttons at the bottom of the figure. Illustrated below is a collapsed and expanded Sequence:

Collapsed Sequence

Collapsed Sequence

Expanded Sequence

Expanded Sequence

The following sections describe the structured activities and how each must be configured to be considered valid for BPEL.

If

If, ElseIf and Else

If, ElseIf and Else

The If activity allows conditional execution of one or more sequences of activities. It consists of a sequence of one or more conditional branches defined by If and optional ElseIf elements. The elements are evaluated in left-to-right order (or top-to-bottom if you have selected horizontal layout). An optional Else branch will be executed if none of the other conditions are true.

An If activity must define a condition (expressed as an XPath) and an activity which is executed if the condition evaluates true. To insert additional ElseIf and Else elements, right-click the If figure and select the desired element from the context menu. The figure above shows a complete If activity with optional ElseIf and Else elements.

Pick

Pick

Pick

The Pick activity will cause the process to wait for one of any number of messages to be received. An optional timer can be set to limit the time to wait for receipt of these messages. Activities to handle receipt of messages and timer expiration are defined in the Pick. Message receipts are handled by OnMessage activities ( OnMessage), and timer expiration is handled by the OnAlaram activity (OnAlarm).

While

While

While

The While activity repeatedly executes the contained activity as long as a condition evaluates true at the beginning of each iteration. A While activity must define a condition and must contain an activity.

ForEach

ForEach

ForEach

ForEach is a looping activity that executes the activities contained in its Scope a specified number of times. A counter variable, defined in the ForEach property detail tab, is used to keep track of the iterations. The ForEach properties must be configured with starting and ending value expressions for this counter variable. The counter is initially set to the starting value and activities in the Scope are executed until the counter exceeds the ending value.

This activity can also be configured to execute all iterations in parallel, meaning the enclosed Scope activity behaves as if multiple Scopes are enclosed in a Flow activity.

An optional early termination value can be defined, which will cause the loop to complete before the counter has reached its ending value. The ForEach will complete when the counter is equal to this early termination value for the sequential execution case. For the parallel execution case, the early termination value is the number of completed iterations. For example, the ForEach completes when at least some number of some action have finished.

RepeatUntil

RepeatUntil

RepeatUntil

The RepeatUntil activity repeatedly executes the contained activity as long as a condition evaluates true at the end of each iteration. A condition must be defined for a RepeatUntil, and it must contain an activity.

Sequence

A Sequence is a container for one or more other activities, which are executed in sequential order and (unlike Scope and Flow activities), has no other special characteristics. Because the conditional activities ( If, While, RepeatUntil and ForEach) can have only one activity as the target of their execution, a Sequence is typically used to execute multiple activities.

For example, the If shown below contains only a single Assign activity:

Sequence

Sequence

If it were necessary to perform an assignment and then invoke another web service, the Assign and Invoke could be contained within a Sequence. The Sequence would then become the target of the If:

Sequence

Sequence

Note

The BPEL Designer will automatically create a Sequence if you drag-drop a second activity into any of the conditional activities.

Scope

Scope

Scope

A Scope provides a context for its enclosed activity. This context includes variables, partner links, message exchanges, correlation sets, event handlers, fault handlers, a compensation handler and a termination handler. These Scope contexts can be nested hierarchically where the root context is provided by the process itself.

A Scope can be thought of as a compartmentalized sub-process. If the Scope is declared as being isolated , then the variables and partner links shared with the process are locked to prevent other concurrent Scopes from altering them while a Scope is executing. Scope may also be nested to any depth and all variables, partner links and others defined in a Scope, are inherited by its children. Refer to Flow for a discussion of concurrent execution.

To be valid, a Scope must have a single activity. The typical use of a Scope activity is to invoke a service and wait for a response message or timeout. In the above figure, the Scope has defined a message variable and a partner link used to interact with the invoked service.

Flow

The Flow activity allows multiple activities to be executed in parallel. All activities or Sequences of activities that are contained in a Flow, are executed (or begun) at the same time by the BPEL engine. A Flow completes when all of its enclosed activities have completed.

Parallel processing is typically used to save time by doing more than one thing at a time and as a result, speed up the process. However, in many situations several tasks can be started at the same time, but one or more tasks may depend on the successful completion of other tasks. This task dependency sequencing is called synchronization and is achieved using Links.

For example, a process that handles purchase orders for material goods needs to:

  1. Calculate the total order price

  2. Calculate shipping costs for the order

  3. Send a customer invoice

All of these activities can be started at the same time, however the shipping cost must be finalized before the total order price can be determined, and the invoice can be sent.

To create a Link, right-click on the activity that must be completed first (the activity that is the source of the dependency) and select Add Link from the context menu. Next, move the mouse to the activity in the Flow that depends on this one (the target) and click the left mouse button to create the link.

A Link is identified by a name that must be unique within the Flow. The BPEL Designer generates a reasonable default name, but you can change this in its properties. You can also add a test to the Link that defines the conditions for considering an activity to be complete. For example an activity in a Flow may require two pieces of information, provided by other services, in order to continue. Consider the process Flow shown below:

Flow example

Flow example

In this example, the Billing department can begin preparation of a customer invoice, but it needs to know if sufficient stock is on hand to fulfill the order and if an outside vendor needs to provide the additional quantities ordered. The following condition would enable the Link so that execution can continue for the price calculation and customer invoicing:

Link example

Link example

This process is only partially complete though as it does not consider the number of outside vendors, or if the total quantity being ordered can ever be filled.