You can use a forEach activity to process multiple sets of activities sequentially or in parallel. The forEach activity executes a contained (child) scope activity exactly N+1 times, where N equals a final counter value minus a starting counter value that you specify in the Counter Values tab of the For Each dialog. While other structured activities such as a flow activity can have any type of activity as its contained activity, the forEach activity can only include a scope activity.

When the forEach activity is started, the expressions you specify for the starting counter and final counter values are evaluated. Once the two values are returned, they remain constant for the lifecycle of the activity. Both expressions must return a value containing at least one character. If these expressions do not return valid values, a fault is thrown. If the starting counter value is greater than the final counter value, the contained scope activity is not performed and the forEach activity is considered complete.

During each iteration, the variable specified in the Counter Name field on the General tab is implicitly declared in the forEach activity's contained scope. During the first iteration of the scope, the counter variable is initialized with the starting counter value. The next iteration causes the counter variable to be initialized with the starting counter value, plus one. Each subsequent iteration increments the previously initialized counter variable value by one until the final iteration, where the counter is set to the final counter value. The counter variable is local to the enclosed scope activity. Although its value can be changed during an iteration, that value is lost after each iteration. Therefore, the counter variable value does not impact the value of the next iteration's counter.

The forEach activity supports the following looping iterations:

• Sequential (default)

  The forEach activity performs looping iterations sequentially N times over a given set of activities defined within a scope activity. As an example, the forEach activity iterates over an incoming purchase order message where the purchase order message consists of N order items. The enclosed scope activity must be executed N+1 times, with each instance starting only after the previous iteration has completed.

• Parallel

  All looping iterations are started at the same time and processed in parallel. Parallel iterations are useful in environments in which sets of independent data are processed or independent interaction with different partners is performed in parallel. To enable parallel looping, you select the Parallel Execution check box on the General tab. In these scenarios, execution of the N+1 instances of the contained scope activity occurs in parallel. Each copy of the scope activity has the same counter variable that you specify in the Counter Name field of the General tab declared in the same way as specified for a sequential forEach activity. Each instance's counter variable must be uniquely initialized in parallel with one of the integer values beginning with the starting counter value and proceeding up to and including the final counter value.

  Unlike a flow activity, the number of parallel branches is not known at design time with the forEach activity. The specified counter variable iterates through the number of parallel branches, controlled by the starting counter value and final counter value.

You can also specify a completion condition on the Completion tab. This condition enables the forEach activity to execute the condition and complete without executing or finishing all the branches specified. As an example, you send out parallel requests and a sufficient subset of the recipients have responded. A completion condition is optionally specified to prevent the following:

• Some children from executing (in the sequential case)

• To force early termination of some of the children (in the parallel case)

If you do not specify a completion condition, the forEach activity completes when the contained scope has completed.

If a premature termination occurs (due to a fault or the completion condition evaluating to true), then the N+1 requirement does not apply.

The following example shows the forEach activity syntax.

<forEach counterName="MyVariableName" parallel="yes|no"
   standard-attributes>
   standard-elements
   <startCounterValue expressionLanguage="anyURI"?>
      unsigned-integer-expression
   </startCounterValue>
   <finalCounterValue expressionLanguage="anyURI"?>
      unsigned-integer-expression
   </finalCounterValue>
   <completionCondition>?
      <branches expressionLanguage="anyURI"?
         successfulBranchesOnly="yes|no"?>?
         unsigned-integer-expression
      </branches>
   </completionCondition>
   <scope ..>...</scope>
</forEach>