NuGet Package of the Week: Polly wanna fluently express transient exception handling policies in .NET?

LOL at my own post title. Pardon me.

Michael Wolfenden has a very clever open source library called Polly. Polly is a .NET 3.5 / 4.0 / 4.5 / PCL library that allows developers to express transient exception handling policies such as Retry, Retry Forever, Wait and Retry or Circuit Breaker in a fluent manner.

Handling exceptions can be a hassle sometimes. Not just setting the try/catches up, but deciding on the policy for the catch can make the exception management code more complex than the method itself!

Polly has a fluent interface to make expressing rules like that much easier. For example:

// 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 => x.ParamName == "example")

Then you can add Retry() logic, which is fantastic.

// 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 

    });

Even do retries with multiplicative back off!

// 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

    }

  );

Once you have set up a policy, you execute on it.

Policy

  .Handle<SqlException>(ex => ex.Number == 1205)

  .Or<ArgumentException>(ex => ex.ParamName == "example")

  .Retry()

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

Polly also supports the more sophisticated "Circuit Breaker" policy. For more information on the Circuit Breaker pattern see:

• Making the Netflix API More Resilient
• The Circuit Breaker
• Circuit Breaking with Polly

Circuit breaker tries and then "Trips the circuit breaker" so you'll get a BrokenCircuitException for some amount of time. This is a great way to give an external system to chill for a minute if it's down. It also externalizes the concept so that you could theoretically handle a down database the same as you handle a down external web API.

// Break the circuit after the specified number of exceptions

// and keep circuit broken for the specified duration

Policy

  .Handle<DivideByZeroException>()

  .CircuitBreaker(2, TimeSpan.FromMinutes(1))

You can explore the code, of course, up on Github in the Polly repository. Go give the Polly project a star. They've recently added async support for .NET 4.5 as well!

Sponsor: Welcome this week's feed sponsor, Stackify! - The only developer-friendly solution that fully integrates .NET error & log management with application monitoring. Easily isolate issues and focus your efforts – Support less, Code more. Free trial