BPEL transactions, part 2 - Understanding compensation

In yesterdays note, I talked a little bit about transactions, and how they are handled within the boundaries of a process instance. Today in part 2, we will dive a little bit more into the concepts of <compensation> activities.

As BPEL itself is considered to be stateless, Alex, one of my colleques and the fellows from the WSBPEL spec team, came up with the concepts of compensation.

Compensations consist of 2 parts,
the compensation handler, and the compensate activity

Defintion:

• Each scope can have one compensation handler


• A compensation handler defines an undo sequence for the scope it is bound to, for example "cancel flight"


• A compensation handler can be called directly (by specifying a scope) or indirectly by a compensate activity

The usecase:
Consider a flow, where a user can pass a date and a location, and the flow will search for / book a flight - and afterwards try to book a hotel room.
Given the hotel is not available, the flight should be cancelled.

The solution:
As per above definitions the ReserveAirline scope will have a compensation handler attached, that contains the logic for canceling the previously booked flight.

  <scope name="ReserveAirline">
    <!-- define a compensation handler -->
    <compensationHandler>
      <!-- call cancel ticked -->
        <invoke name="Invoke_CancelTicket" 
                partnerLink="AirlineBookingService"
                portType="airline:airlineBooking"
                operation="cancelSeat"/>
    </compensationHandler>
    <sequence name="Sequence_1">
        <invoke name="Invoke_BookTicket" 
                partnerLink="AirlineBookingService"
                portType="airline:airlineBooking"
                operation="reserveSeat"/>
    </sequence>
  </scope>

The next step in this little sample happens within the scope, responsible for booking a hotel room ()

The operation to reserve a hotel room (reserveHotel) throws a fault if no room is avaiable for the given dates hotel:NoRoomAvailfault

  <scope name="ReserveHotel">
    <faultHandlers>
      <catch faultName="hotel:NoRoomAvailfault"
        <throw name="RollBack" 
               faultName="client:rollBackFault"/>
      </catch>
    </faultHandlers>
    <sequence name="Sequence_2"
      <invoke name="Invoke_ReserveHotel" 
              partnerLink="hotel" 
              portType="ReservationSystem" 
              operation="bookRoom"/>
    </sequence>
  </scope>

In the above code a catch block is defined to catch the specific fault we
expect and throws itself a rollBackFault to the parent scope, that will trigger the compensation, we have defined earlier.

The last part is is to define a catch block on the parent scope, that triggers in case of a rollbackFault and fires the compensation defined on the ReserveAirline scope.

  <faultHandlers>
    <!-- catch the rollback fault and call -->
    <catch faultName="client:rollbackFault">
      <compensate name="CompensateBoookings" 
                  scope="ReserveAirline"/>
    </catch>
  </faultHandlers>

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