Polly

Polly is a .NET 3.5 / 4.0 / 4.5 / PCL (Profile 259) library that allows developers to express transient exception- and fault-handling policies such as Retry, Retry Forever, Wait and Retry or Circuit Breaker in a fluent and thread-safe manner.

We are now a member of the .NET Foundation!

Keep up to date with new feature announcements, tips & tricks, and other news through www.thepollyproject.org

Installing via NuGet

Install-Package Polly

You can install the Strongly Named version via:

Install-Package Polly-Signed

There are now .NET 4.0 Async versions (via Microsoft.Bcl.Async) of the signed and unsigned NuGet packages, which can be installed via:

Install-Package Polly.Net40Async
Install-Package Polly.Net40Async-Signed

Please note: The Polly.Net40Async package is only needed if you are targeting .NET 4.0 and need async capabilities. If you are targeting .NET 4.5 or greater, please use the standard Polly package.

Usage

Step 1 : Specify the type of exceptions you want the policy to handle

// Single exception type
Policy
  .Handle<DivideByZeroException>()

// Single exception type with condition
Policy
  .Handle<SqlException>(ex => ex.Number == 1205)

// Multiple exception types
Policy
  .Handle<DivideByZeroException>()
  .Or<ArgumentException>()

// Multiple exception types with condition
Policy
  .Handle<SqlException>(ex => ex.Number == 1205)
  .Or<ArgumentException>(ex => ex.ParamName == "example")

Step 1b: (optionally) Specify return results you want to handle

From Polly v4.3.0 onwards, policies wrapping calls returning a TResult can also handle TResult return values:

// Handle return value with condition 
Policy
  .HandleResult<HttpResponse>(r => r.StatusCode == 404)

// Handle multiple return values 
Policy
  .HandleResult<HttpResponse>(r => r.StatusCode == 500)
  .OrResult<HttpResponse>(r => r.StatusCode == 502)

// Handle primitive return values (implied use of .Equals())
Policy
  .HandleResult<HttpStatusCode>(HttpStatusCode.InternalServerError)
  .OrResult<HttpStatusCode>(HttpStatusCode.BadGateway)
 
// Handle both exceptions and return values in one policy
int[] httpStatusCodesWorthRetrying = {408, 500, 502, 503, 504}; 
HttpResponse result = Policy
  .Handle<HttpException>()
  .OrResult<HttpResponse>(r => httpStatusCodesWorthRetrying.Contains(r.StatusCode))

For more information, see Handling Return Values at foot of this readme.

Step 2 : Specify how the policy should handle those faults

Retry

// Retry once
Policy
  .Handle<DivideByZeroException>()
  .Retry()

// Retry multiple times
Policy
  .Handle<DivideByZeroException>()
  .Retry(3)

// Retry multiple times, calling an action on each retry 
// with the current exception and retry count
Policy
    .Handle<DivideByZeroException>()
    .Retry(3, (exception, retryCount) =>
    {
        // do something 
    });

// Retry multiple times, calling an action on each retry 
// with the current exception, retry count and context 
// provided to Execute()
Policy
    .Handle<DivideByZeroException>()
    .Retry(3, (exception, retryCount, context) =>
    {
        // do something 
    });

Retry forever

// Retry forever
Policy
  .Handle<DivideByZeroException>()
  .RetryForever()

// Retry forever, calling an action on each retry with the 
// current exception
Policy
  .Handle<DivideByZeroException>()
  .RetryForever(exception =>
  {
        // do something       
  });

// Retry forever, calling an action on each retry with the
// current exception and context provided to Execute()
Policy
  .Handle<DivideByZeroException>()
  .RetryForever((exception, context) =>
  {
        // do something       
  });

Retry and Wait

// Retry, waiting a specified duration between each retry
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetry(new[]
  {
    TimeSpan.FromSeconds(1),
    TimeSpan.FromSeconds(2),
    TimeSpan.FromSeconds(3)
  });

// Retry, waiting a specified duration between each retry, 
// calling an action on each retry with the current exception
// and duration
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetry(new[]
  {
    TimeSpan.FromSeconds(1),
    TimeSpan.FromSeconds(2),
    TimeSpan.FromSeconds(3)
  }, (exception, timeSpan) => {
    // do something    
  }); 

// Retry, waiting a specified duration between each retry, 
// calling an action on each retry with the current exception, 
// duration and context provided to Execute()
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetry(new[]
  {
    TimeSpan.FromSeconds(1),
    TimeSpan.FromSeconds(2),
    TimeSpan.FromSeconds(3)
  }, (exception, timeSpan, context) => {
    // do something    
  });

// Retry, waiting a specified duration between each retry, 
// calling an action on each retry with the current exception, 
// duration, retry count, and context provided to Execute()
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetry(new[]
  {
    TimeSpan.FromSeconds(1),
    TimeSpan.FromSeconds(2),
    TimeSpan.FromSeconds(3)
  }, (exception, timeSpan, retryCount, context) => {
    // do something    
  });

// Retry a specified number of times, using a function to 
// calculate the duration to wait between retries based on 
// the current retry attempt (allows for exponential backoff)
// In this case will wait for
//  2 ^ 1 = 2 seconds then
//  2 ^ 2 = 4 seconds then
//  2 ^ 3 = 8 seconds then
//  2 ^ 4 = 16 seconds then
//  2 ^ 5 = 32 seconds
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetry(5, retryAttempt => 
	TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)) 
  );

// Retry a specified number of times, using a function to 
// calculate the duration to wait between retries based on 
// the current retry attempt, calling an action on each retry 
// with the current exception, duration and context provided 
// to Execute()
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetry(
    5, 
    retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)), 
    (exception, timeSpan, context) => {
      // do something
    }
  );

// Retry a specified number of times, using a function to 
// calculate the duration to wait between retries based on 
// the current retry attempt, calling an action on each retry 
// with the current exception, duration, retry count, and context 
// provided to Execute()
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetry(
    5, 
    retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)), 
    (exception, timeSpan, retryCount, context) => {
      // do something
    }
  );

Wait and retry forever

// Wait and retry forever
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetryForever(retryAttempt => 
	TimeSpan.FromSeconds(Math.Pow(2, retryAttempt))
    );

// Wait and retry forever, calling an action on each retry with the 
// current exception and the time to wait
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetryForever(
    retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)),    
    (exception, timespan) =>
    {
        // do something       
    });

// Wait and retry forever, calling an action on each retry with the
// current exception, time to wait, and context provided to Execute()
Policy
  .Handle<DivideByZeroException>()
  .WaitAndRetryForever(
    retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)),    
    (exception, timespan, context) =>
    {
        // do something       
    });

For further information on the operation of retry policies, see also the wiki.

Circuit Breaker

// Break the circuit after the specified number of consecutive exceptions
// and keep circuit broken for the specified duration.
Policy
    .Handle<DivideByZeroException>()
    .CircuitBreaker(2, TimeSpan.FromMinutes(1));

// Break the circuit after the specified number of consecutive exceptions
// and keep circuit broken for the specified duration,
// calling an action on change of circuit state.
Action<Exception, TimeSpan> onBreak = (exception, timespan) => { ... };
Action onReset = () => { ... };
CircuitBreakerPolicy breaker = Policy
    .Handle<DivideByZeroException>()
    .CircuitBreaker(2, TimeSpan.FromMinutes(1), onBreak, onReset);

// Break the circuit after the specified number of consecutive exceptions
// and keep circuit broken for the specified duration,
// calling an action on change of circuit state,
// passing a context provided to Execute().
Action<Exception, TimeSpan, Context> onBreak = (exception, timespan, context) => { ... };
Action<Context> onReset = context => { ... };
CircuitBreakerPolicy breaker = Policy
    .Handle<DivideByZeroException>()
    .CircuitBreaker(2, TimeSpan.FromMinutes(1), onBreak, onReset);

// Monitor the circuit state, for example for health reporting.
CircuitState state = breaker.CircuitState;

/*
CircuitState.Closed - Normal operation. Execution of actions allowed.
CircuitState.Open - The automated controller has opened the circuit. Execution of actions blocked.
CircuitState.HalfOpen - Recovering from open state, after the automated break duration has expired. Execution of actions permitted. Success of subsequent action/s controls onward transition to Open or Closed state.
CircuitState.Isolated - Circuit held manually in an open state. Execution of actions blocked.
*/

// Manually open (and hold open) a circuit breaker - for example to manually isolate a downstream service.
breaker.Isolate(); 
// Reset the breaker to closed state, to start accepting actions again.
breaker.Reset(); 

For further information on the operation of circuit breaker, see also the wiki.

Advanced Circuit Breaker

// Break the circuit if, within any period of duration samplingDuration, 
// the proportion of actions resulting in a handled exception exceeds failureThreshold, 
// provided also that the number of actions through the circuit in the period
// is at least minimumThroughput.

Policy
    .Handle<DivideByZeroException>()
    .AdvancedCircuitBreaker(
        failureThreshold: 0.5, // Break on >=50% actions result in handled exceptions...
        samplingDuration: TimeSpan.FromSeconds(10), // ... over any 10 second period
        minimumThroughput: 8, // ... provided at least 8 actions in the 10 second period.
        durationOfBreak: TimeSpan.FromSeconds(30) // Break for 30 seconds.
                );

// Configuration overloads taking state-change delegates are
// available as described for CircuitBreaker above.

// Circuit state monitoring and manual controls are
// available as described for CircuitBreaker above.

For further information on the operation of Advanced Circuit Breaker, see the Wiki

For more information on the Circuit Breaker pattern in general see:

• Making the Netflix API More Resilient
• Circuit Breaker (Martin Fowler)
• Circuit Breaker Pattern (Microsoft)
• Circuit breaking with Polly
• Original Circuit Breaking Link

Step 3 : Execute the policy

// Execute an action
var policy = Policy
              .Handle<DivideByZeroException>()
              .Retry();

policy.Execute(() => DoSomething());

// Execute an action passing arbitrary context data
var policy = Policy
    .Handle<DivideByZeroException>()
    .Retry(3, (exception, retryCount, context) =>
    {
        var methodThatRaisedException = context["methodName"];
		Log(exception, methodThatRaisedException);
    });

policy.Execute(
	() => DoSomething(),
	new Dictionary<string, object>() {{ "methodName", "some method" }}
);

// Execute a function returning a result
var policy = Policy
              .Handle<DivideByZeroException>()
              .Retry();

var result = policy.Execute(() => DoSomething());

// Execute a function returning a result passing arbitrary context data
var policy = Policy
    .Handle<DivideByZeroException>()
    .Retry(3, (exception, retryCount, context) =>
    {
        object methodThatRaisedException = context["methodName"];
        Log(exception, methodThatRaisedException)
    });

var result = policy.Execute(
    () => DoSomething(),
    new Dictionary<string, object>() {{ "methodName", "some method" }}
);

// You can of course chain it all together
Policy
  .Handle<SqlException>(ex => ex.Number == 1205)
  .Or<ArgumentException>(ex => ex.ParamName == "example")
  .Retry()
  .Execute(() => DoSomething());

Post Execution Steps

Using the ExecuteAndCapture method you can capture the result of executing a policy.

var policyResult = Policy
              .Handle<DivideByZeroException>()
              .Retry()
              .ExecuteAndCapture(() => DoSomething());
/*              
policyResult.Outcome - whether the call succeeded or failed         
policyResult.FinalException - the final exception captured, will be null if the call succeeded
policyResult.ExceptionType - was the final exception an exception the policy was defined to handle (like DivideByZeroException above) or an unhandled one (say Exception). Will be null if the call succeeded.
policyResult.Result - if executing a func, the result if the call succeeded or the type's default value
*/

Thread safety

The existing policies (retry and circuit-breaker) are fully thread-safe, and the intention is that the forthcoming wider resilience policies will be thread-safe, too.

While the internal operation of the policy is thread-safe, this does not magically make delegates you execute through the policy thread-safe: if delegates you execute through the policy are not thread-safe, they remain not thread-safe.

Asynchronous Support (.NET 4.5, PCL and .NET4.0)

You can use Polly with asynchronous functions by using the asynchronous methods

• RetryAsync
• RetryForeverAsync
• WaitAndRetryAsync
• WaitAndRetryForeverAsync
• CircuitBreakerAsync
• AdvancedCircuitBreakerAsync
• ExecuteAsync
• ExecuteAndCaptureAsync

In place of their synchronous counterparts

• Retry
• RetryForever
• WaitAndRetry
• WaitAndRetryForever
• CircuitBreaker
• AdvancedCircuitBreaker
• Execute
• ExecuteAndCapture

For example

await Policy
  .Handle<SqlException>(ex => ex.Number == 1205)
  .Or<ArgumentException>(ex => ex.ParamName == "example")
  .RetryAsync()
  .ExecuteAsync(() => DoSomethingAsync());

SynchronizationContext

Async continuations and retries by default do not run on a captured synchronization context. To change this, use .ExecuteAsync(...) overloads taking a boolean continueOnCapturedContext parameter.

Cancellation support

Async policy execution supports cancellation via .ExecuteAsync(...) overloads taking a CancellationToken.

Cancellation cancels Policy actions such as further retries and waits between retries. The delegate taken by the relevant .ExecuteAsync(...) overloads also takes a cancellation token input parameter, to support cancellation during delegate execution.

// Try several times to retrieve from a uri, but support cancellation at any time.
CancellationToken cancellationToken = // ...
var policy = Policy
    .Handle<WebException>()
    .Or<HttpRequestException>()
    .WaitAndRetryAsync(new[] { 
        TimeSpan.FromSeconds(1), 
        TimeSpan.FromSeconds(2), 
        TimeSpan.FromSeconds(4) 
    });
var response = await policy.ExecuteAsync(ct => httpClient.GetAsync(uri, ct), cancellationToken);

.NET4.0 Async support

The .NET4.0 Async support uses Microsoft.Bcl.Async to add async support to a .NET4.0 package. To minimise extra dependencies on the main Polly nuget package, the .NET4.0 async version is available as separate Nuget packages Polly.Net40Async and Polly.Net40Async-signed.

Handing return values, and Policy<TResult>

As described at step 1b, from Polly v4.3.0 onwards, policies can handle return values and exceptions in combination:

// Handle both exceptions and return values in one policy
int[] httpStatusCodesWorthRetrying = {408, 500, 502, 503, 504}; 
HttpResponse result = Policy
  .Handle<HttpException>()
  .OrResult<HttpResponse>(r => httpStatusCodesWorthRetrying.Contains(r.StatusCode))
  .Retry(...)
  .Execute( /* some Func<HttpResponse> */ )

The exceptions and return results to handle can be expressed fluently in any order.

Strongly-typed Policy<TResult>

Configuring a policy with .HandleResult<TResult>(...) or .OrResult<TResult>(...) generates a strongly-typed Policy<TResult> of the specific policy type, eg Retry<TResult>, AdvancedCircuitBreaker<TResult>.

These policies must be used to execute delegates returning TResult, ie:

• Execute(Func<TResult>) (and related overloads)
• ExecuteAsync(Func<CancellationToken, Task<TResult>>) (and related overloads)

ExecuteAndCapture<TResult>()

.ExecuteAndCapture(...) on non-generic policies returns a PolicyResult with properties:

policyResult.Outcome - whether the call succeeded or failed         
policyResult.FinalException - the final exception captured; will be null if the call succeeded
policyResult.ExceptionType - was the final exception an exception the policy was defined to handle (like DivideByZeroException above) or an unhandled one (say Exception)? Will be null if the call succeeded.
policyResult.Result - if executing a func, the result if the call succeeded or the type's default value

.ExecuteAndCapture<TResult>(Func<TResult>) on strongly-typed policies adds two properties:

policyResult.FaultType - was the final fault handled an exception or a result handled by the policy? Will be null if the delegate execution succeeded. 
policyResult.FinalHandledResult - the final result handled; will be null if the call succeeded or the type's default value

State-change delegates on Policy<TResult> policies

In non-generic policies handling only exceptions, state-change delegates such as onRetry and onBreak take an Exception parameter.

In generic-policies handling TResult return values, state-change delegates are identical except they take a DelegateResult<TResult> parameter in place of Exception. DelegateResult<TResult> has two properties:

• Exception // The exception just thrown if policy is in process of handling an exception (otherwise null)
• Result // The TResult just raised, if policy is in process of handling a result (otherwise default(TResult))

BrokenCircuitException<TResult>

Non-generic CircuitBreaker policies throw a BrokenCircuitException when the circuit is broken. This BrokenCircuitException contains the last exception (the one which caused the circuit to break) as the InnerException.

For CircuitBreakerPolicy<TResult> policies:

• A circuit broken due to an exception throws a BrokenCircuitException with InnerException set to the exception which triggered the break (as previously).
• A circuit broken due to handling a result throws a BrokenCircuitException<TResult> with the Result property set to the result which caused the circuit to break.

3rd Party Libraries

• Fluent Assertions - A set of .NET extension methods that allow you to more naturally specify the expected outcome of a TDD or BDD-style test | Apache License 2.0 (Apache)
• xUnit.net - Free, open source, community-focused unit testing tool for the .NET Framework | Apache License 2.0 (Apache)
• [Ian Griffith's TimedLock] (http://www.interact-sw.co.uk/iangblog/2004/04/26/yetmoretimedlocking)
• [Steven van Deursen's ReadOnlyDictionary] (http://www.cuttingedge.it/blogs/steven/pivot/entry.php?id=29)

Acknowledgements

• lokad-shared-libraries - Helper assemblies for .NET 3.5 and Silverlight 2.0 that are being developed as part of the Open Source effort by Lokad.com (discontinued) | New BSD License
• @michael-wolfenden - The creator and mastermind of Polly!
• @ghuntley - Portable Class Library implementation.
• @mauricedb - Async implementation.
• @robgibbens - Added existing async files to PCL project
• Hacko - Added extra NotOnCapturedContext call to prevent potential deadlocks when blocking on asynchronous calls
• @ThomasMentzel - Added ability to capture the results of executing a policy via ExecuteAndCapture
• @yevhen - Added full control of whether to continue on captured synchronization context or not
• @reisenberger - Added full async cancellation support
• @reisenberger - Added async support for ContextualPolicy
• @reisenberger - Added ContextualPolicy support for circuit-breaker
• @reisenberger - Extended circuit-breaker for public monitoring and control
• @reisenberger - Added ExecuteAndCapture support with arbitrary context data
• @kristianhald and @reisenberger - Added AdvancedCircuitBreaker
• @reisenberger - Allowed async onRetry delegates to async retry policies
• @Lumirris - Add new Polly.Net40Async project/package supporting async for .NET40 via Microsoft.Bcl.Async
• @SteveCote - Added overloads to WaitAndRetry and WaitAndRetryAsync methods that accept an onRetry delegate which includes the attempt count.
• @reisenberger - Allowed policies to handle returned results; added strongly-typed policies Policy<TResult>;.
• @christopherbahr - Added optimisation for circuit-breaker hot path.
• @Finity - Fixed circuit-breaker threshold bug.

Sample Projects

Polly-Samples contains practical examples for using various implementations of Polly. Please feel free to contribute to the Polly-Samples repository in order to assist others who are either learning Polly for the first time, or are seeking advanced examples and novel approaches provided by our generous community.

Instructions for Contributing

Please check out our Wiki for contributing guidelines. We are following the excellent GitHub Flow process, and would like to make sure you have all of the information needed to be a world-class contributor!

Since Polly is part of the .NET Foundation, we ask our contributors to abide by their Code of Conduct.

Also, we've stood up a Slack channel for easier real-time discussion of ideas and the general direction of Polly as a whole. Be sure to join the conversation today!

License

Licensed under the terms of the New BSD License

Laptop Stickers!

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