背景

Polly是一个.NET弹性和瞬态故障处理库，允许开发人员以流畅和线程安全的方式表达诸如重试，断路器，超时，隔离头和回退之类的策略。

Polly面向.NET Standard 1.1（覆盖范围：.NET Core 1.0，Mono，Xamarin，UWP，WP8.1 +）和.NET Standard 2.0+（覆盖范围：.NET Core 2.0 + 、. NET Core 3.0和更高版本的Mono，Xamarin和UWP目标）。nuget软件包还包括.NET Framework 4.6.1和4.7.2的直接目标。

使用方式

通过NuGet安装

Install-Package Polly

代码实现

故障处理，响应策略

故障处理策略处理您通过策略执行的委托引发的特定异常或          返回的结果。

步骤1：指定您希望策略处理的异常/错误

// Single exception type
Policy.Handle<HttpRequestException>()// Single exception type with condition
Policy.Handle<SqlException>(ex => ex.Number == 1205)// Multiple exception types
Policy.Handle<HttpRequestException>().Or<OperationCanceledException>()// Multiple exception types with condition
Policy.Handle<SqlException>(ex => ex.Number == 1205).Or<ArgumentException>(ex => ex.ParamName == "example")// Inner exceptions of ordinary exceptions or AggregateException, with or without conditions
// (HandleInner matches exceptions at both the top-level and inner exceptions)
Policy.HandleInner<HttpRequestException>().OrInner<OperationCanceledException>(ex => ex.CancellationToken != myToken)

步骤2：指定策略应如何处理这些故障

2.1、重试

‍

// Retry once
Policy.Handle<SomeExceptionType>().Retry()// Retry multiple times
Policy.Handle<SomeExceptionType>().Retry(3)// Retry multiple times, calling an action on each retry
// with the current exception and retry count
Policy.Handle<SomeExceptionType>().Retry(3, onRetry: (exception, retryCount) =>{// Add logic to be executed before each retry, such as logging});// Retry multiple times, calling an action on each retry
// with the current exception, retry count and context
// provided to Execute()
Policy.Handle<SomeExceptionType>().Retry(3, onRetry: (exception, retryCount, context) =>{// Add logic to be executed before each retry, such as logging });

2.2、永远重试（直到成功）

// Retry forever
Policy.Handle<SomeExceptionType>().RetryForever()// Retry forever, calling an action on each retry with the
// current exception
Policy.Handle<SomeExceptionType>().RetryForever(onRetry: exception =>{// Add logic to be executed before each retry, such as logging       });// Retry forever, calling an action on each retry with the
// current exception and context provided to Execute()
Policy.Handle<SomeExceptionType>().RetryForever(onRetry: (exception, context) =>{// Add logic to be executed before each retry, such as logging       });

2.3、等待并重试

// Retry, waiting a specified duration between each retry.
// (The wait is imposed on catching the failure, before making the next try.)
Policy.Handle<SomeExceptionType>().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<SomeExceptionType>().WaitAndRetry(new[]{TimeSpan.FromSeconds(1),TimeSpan.FromSeconds(2),TimeSpan.FromSeconds(3)}, (exception, timeSpan) => {// Add logic to be executed before each retry, such as logging    }); // 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<SomeExceptionType>().WaitAndRetry(new[]{TimeSpan.FromSeconds(1),TimeSpan.FromSeconds(2),TimeSpan.FromSeconds(3)}, (exception, timeSpan, context) => {// Add logic to be executed before each retry, such as logging    });// 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<SomeExceptionType>().WaitAndRetry(new[]{TimeSpan.FromSeconds(1),TimeSpan.FromSeconds(2),TimeSpan.FromSeconds(3)}, (exception, timeSpan, retryCount, context) => {// Add logic to be executed before each retry, such as logging    });// 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<SomeExceptionType>().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<SomeExceptionType>().WaitAndRetry(5, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)), (exception, timeSpan, context) => {// Add logic to be executed before each retry, such as logging});// 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<SomeExceptionType>().WaitAndRetry(5, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)), (exception, timeSpan, retryCount, context) => {// Add logic to be executed before each retry, such as logging});

开源地址

https://github.com/App-vNext/Polly