You can synchronize the execution of activities within a flow activity to ensure that certain activities only execute after other activities have completed. For example, assume you have an invoke activity, verifyFlight, that is executed in parallel with other invoke activities (verifyHotel, verifyCarRental, and scheduleFlight) when the flow activity begins. However, scheduling a flight is necessary only after verifying that a flight is available. Therefore, you can add a link between the verifyFlight and scheduleFlight invoke activities. Links provide a level of dependency indicating that the activity that is the target of the link (scheduleFlight) is only executed if the activity that is the source of the link (verifyFlight) has completed.

The following example provides details. The link name verifyFlight-To-scheduleFlight is assigned to the source verifyFlight and target scheduleFlight invoke activities. If the source verifyFlight completes execution, the target scheduleFlight is then executed.

<flow ...>
   <links>
      <link name="verifyFlight-To-scheduleFlight" />
   </links>
   <documentation>
      Verify the availability of a flight, hotel, and rental car in parallel
   </documentation>
   <invoke name="verifyFlight" ...>
      <sources>
         <source linkName="verifyFlight-To-scheduleFlight" />
      </sources>
   </invoke>
   <invoke name="verifyHotel" ... />
   <invoke name="verifyCarRental" ... />
   <invoke name="scheduleFlight" ...>
      <targets>
         <target linkName="verifyFlight-To-scheduleFlight" />
      </targets>
   </invoke>
</flow>

The preceding code provides an example of link syntax in BPEL version 2.0. The link syntax between BPEL version 1.1 and BPEL version 2.0 is slightly different.

• BPEL version 1.1 uses <target> and <source>.

• BPEL version 2.0 uses <targets> and <sources>.

Table 10-1 provides details.

<flow>
   <links>
     <link name="XtoY"/>
     <link name="CtoD"/>
   </links>
   <sequence name="X">
      <source linkName="XtoY"/>
      <invoke name="A" .../>
      <invoke name="B" .../>
   </sequence>
   <sequence name"Y">
      <target linkName="XtoY"/>
      <receive name="C" ...>
         <source linkName="CtoD"/>
      </receive>
      <invoke name="E" .../>
   </sequence>
   <invoke partnerLink="D" ...>
     <target linkName="CtoD"/>
   </invoke>
 </flow>

<flow>
      <links>
        <link name="AtoB"/>
      </links>
      <assign name="B">
        <targets>
          <target linkName="AtoB"/>
        </targets>
        <copy>
          <from>concat($output.payload,
            'B')</from>
          <to>$output.payload</to>
        </copy>
      </assign>
      <assign name="A">
        <sources>
          <source linkName="AtoB"/>
        </sources>
        <copy>
          <from>concat($output.payload,
           'A')</from>
          <to>$output.payload</to>
        </copy>
      </assign>
    </flow>