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SOA with .NET and Windows Azure : Orchestration Patterns with WF - Compensating Service Transaction

4/7/2011 4:04:12 PM
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With WF, compensation logic can be defined via the graphic design by dragging-and-dropping activities, just as you would define regular workflow logic. WF compensations not only support common transaction semantics, they can be also used to clean up the programming model. All code necessary to run compensations is grouped in compensation handlers (similar to catch() blocks in C#). The main difference with compensation handlers is that they run when an error occurs after a block of operations has already executed and completed successfully.

Creating Compensations

Other than transactable resources in the context of ACID transactions, compensatable resources must provide explicit access to the compensation logic. WF 3.0 supports the application of Compensating Service Transaction  for one or more workflow activities through the CompensatableSequence and the CompensatableTransactionScope activities. Each has an associated compensation handler that runs when:

  • the compensatable scope completed successfully (for example, without throwing an exception), but the orchestration failed with an unhandled exception

  • the compensatable scope completed successfully, but a Compensate activity triggered the compensation

By default, the WF 3.0 process designer does not show compensation blocks to avoid any distraction from the main process flow. You can switch the designer view from the process flow to the compensation flow by clicking on the compensation handler icon.

WF 4.0 introduces some changes to give you more granular control over the execution of compensation logic. The CompensatableActivity allows compensation of the embedded activities when certain conditions are met. The activity’s compensation handler runs when:

  • the compensatable scope completed successfully (for example, without throwing an exception), but an unhandled exception occurred afterwards and the executing WorkflowApplication’s OnUnhandledException handler returned UnhandledExceptionAction.Cancel

  • the compensatable scope completed successfully, but a Compensate activity explicitly triggered the compensation (this is true unless the CompensatableActivity has been confirmed with the Confirm activity)

The Confirm activity in WF 4.0 can prevent a completed CompensatableActivity from running the compensation logic even when compensation is triggered explicitly. This lets workflows mark certain tasks as irreversible, even if a Compensate activity triggers compensation at higher levels.

Triggering Compensations

Note also that you need to trigger compensations explicitly when exceptions are handled in any part of the workflow. Error handlers (such as Fault Handlers in WF 3.0 and Catch activities in WF 4.0) need to invoke the compensation handlers with the Compensate activity.

In WF 3.0, the Compensate activity can only be placed inside a Fault handler or a CompensationHandler. In WF 4.0, you can place Compensate activities in the regular flow of a workflow, but the activity expects a valid CompensationToken. These tokens are only set once a CompensatableActivity completes successfully. Calling Compensate, without a valid token, results in an exception.

Compensate also raises an exception if you invoke it for a CompensatableActivity that has been explicitly confirmed with a Confirm activity. Therefore, it’s important to wrap Compensate activities in a TryCatch activity.

Note

WF also supports Atomic Service Transaction via two-phase ACID transactions inside workflows. A TransactionScope or a CompensatableTransactionScope can define the scope of a .NET transaction, as provided by the System.Transactions.Transaction namespace.

Other -----------------
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