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
    }
  );