Polly policy to log exception and rethrow

Question

I consider to use Polly to create policy to log exception and rethrow. I didn't find an existing method that allow it out of the box , but some options that I see are

Fallback

// Specify a substitute value or func, calling an action (eg for logging) if the fallback is invoked.
Policy.Handle<Whatever>()  
 .Fallback<UserAvatar>(UserAvatar.Blank, onFallback: (exception, context) =>  
   {   _logger.Log(exception, context);
       throw exception;
  });

Question: is it ok to throw exception from Fallback?

Timeout

Policy.Timeout(1, T30meoutStrategy.Pessimistic, (context, timespan, task) =>  
   {        task.ContinueWith(t =>
 { // ContinueWith important!: the abandoned task may very well still be executing, when the caller times out on waiting for it!    
   if (t.IsFaulted      )           
  {             
       logger.Error(context,t.Exception);         
       throw exception;
  }   );    

Or Retry

Policy.Handle<DivideByZeroException>().Retry(0, (exception, retryCount) =>   
  {                logger.Error(context,exception);         
throw exception;
  }   );  

Question: is 0 retries supported?

Or KISS and write simple try/catch with throw by myself.

Which of these methods is better? What are your recommendation?

Answer 1

If you do not already have Polly in the mix, try/catch would seem simplest.

If you already have Polly in the mix, FallbackPolicy can safely be re-purposed in the way you suggest. The onFallback delegate and fallback action or value are not governed by the .Handle<>() clauses of the Policy, so you can safely rethrow an exception from within the onFallback delegate.

 Policy<UserAvatar>.Handle<Whatever>()  
 .Fallback<UserAvatar>(UserAvatar.Blank, onFallback: (exception, context) =>  
   {   _logger.Log(exception, context);
       throw exception;
  });

---

The approach your question outlines with TimeoutPolicy would only capture exceptions thrown by delegates the caller had earlier walked away from due to timeout, and only in TimeoutMode.Pessimistic; not all exceptions.

---

The approach your question outlines with .Retry(0, ...) would not work. If no retries are specified, the onRetry delegate would not be invoked.

---

To avoid the untidiness of repurposing FallbackPolicy, you could also code your own LogThenRethrowPolicy, within Polly's structures. This commit (which added the simple NoOpPolicy) exemplifies the minimum necessary to add a new policy. You could add an implementation similar to NoOpPolicy but just try { } catch { /* log; rethrow */ }

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EDIT January 2019: Polly.Contrib now also contains a Polly.Contrib.LoggingPolicy which can help with this.

Answer 2

https://github.com/App-vNext/Polly-Samples/blob/master/PollyDemos/Async/AsyncDemo02_WaitAndRetryNTimes.cs shows that you can use the onRetry: option, at least for WaitAndRetryAsync. I haven't looked at the others yet.

HttpPolicyExtensions
.HandleTransientHttpError()
.WaitAndRetryAsync(3,
    retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt))  // exponential back-off: 2, 4, 8 etc
                    + TimeSpan.FromMilliseconds(Jitterer.Next(0, 1000)), // plus some jitter: up to 1 second
    onRetry: (response, calculatedWaitDuration) =>
    {
        logger.LogError($"Failed attempt. Waited for {calculatedWaitDuration}. Retrying. {response.Exception.Message} - {response.Exception.StackTrace}");
    }
);