Asynchronous action execution

Asynchronous action execution

For an overview of syntax and policies available, read first the readme: https://github.com/App-vNext/Polly#asynchronous-support-net-45-and-pcl-only. This article describes the asynchronous operation of policies in detail.

Asynchronous action execution is available in .NET4.5 and PCL.

Async execution of actions through policies

In general, asynchronous policies execute actions in the same manner as synchronous policies. The following are the differences:

Async / await

In asynchronous execution, all delegates are async and run with await.

Async synchronization context

As recommended for libraries offering an async API, by default, async continuations (when a method resumes from await) are not run on a captured synchronization context. However, if you require the whole .ExecuteAsync(…) call to continue on the captured synchronization context, use the overloads taking a bool continueOnCapturedContext, setting this to true.

Cancellation support

Using an .ExecuteAsync(…) (or similar) method taking a CancellationToken allows async action execution to be cancelled. Cancellation can occur:

• Before the delegate is actioned at all: In common with the Base Class Library implementation in Task.Run(…), if the cancellation token is cancelled before execution begins, the delegate is not executed at all.
• During delegate execution: The action delegates taken by the relevant .ExecuteAsync(…) overloads take a CancellationToken input parameter. Using this input parameter, the CancellationToken passed in to .ExecuteAsync(…) call is in turn passed in to (and can control cancellation within) the action delegate.
• During any wait between retries.

All cancellation throws the usual OperationCanceledException.