Polly 回退操作未引发指定的异常。发生了什么？答案

【问题标题】：Polly fallback action did not throw the specified exception. What happened?Polly 回退操作未引发指定的异常。发生了什么？
【发布时间】：2021-05-21 11:46:38
【问题描述】：

鉴于此代码尝试调用 AuthenticationManager.Authenticate() 10 次，然后在 10 次尝试失败后抛出 CustomException：

string res = Policy
    .Handle<Exception>() // Handles any exception
    .OrResult<string>(result => string.IsNullOrWhiteSpace(result)) // or if result is null
    .Fallback(() => throw new CustomException("Exception!")) // Not being thrown after 10 unsuccessful attempts
    .Wrap(
        Policy
            .Handle<Exception>()
            .OrResult<string>(result => string.IsNullOrWhiteSpace(result))
            .WaitAndRetry(
                10,
                retryAttempt => TimeSpan.FromSeconds(60),
                onRetry: (response, delay, retryCount, context) => Trace.WriteLine($"[{DateTime.UtcNow}] Authentication failed. Retrying after 60 seconds...(Attempt {retryCount} of 10)")))
    .ExecuteAndCapture(() => AuthenticationManager.Authenticate())
    .Result;

为什么CustomException 没有从回退操作中抛出？正确的做法应该是什么？

【问题讨论】：

您是否对您的策略进行单元测试？ "请注意，.ExecuteAndCapture/Async(...) 会捕获整个执行结果是否是被策略视为失败的结果。因此，如果您的 FallbackPolicy 将失败结果替换为将被视为失败的结果成功（正如许多后备策略所做的那样 - “优雅降级”模式），然后 .ExecuteAndCapture/Async(...) 自然会报告 PolicyResult.Outcome == OutcomeType.Success。” - FallbackPolicy with ExecuteAndCapture()
^^这可能是个问题吗？
也发现了这个问题：github.com/App-vNext/Polly/issues/477
我只是将您的代码放入 xUnit-Test 并发现它“有点”工作：Outcome 是 Failure，Result 是 null 和 FinalException 是 @987654333 @.
ExecuteAndCapture 不会抛出异常。它捕获委托的结果，即使是Tx 或任何类型的Exception。 Outcome 会告诉你它是成功还是失败。如果失败，您可以通过FinalException、Reference访问异常

标签： c# exception polly resiliency transient-failure

【解决方案1】：

这些单元测试通过：(xUnit)

[Fact]
public void Test1()
{
    var policy = Policy<string>
        .Handle<Exception>()
        .OrResult(result => string.IsNullOrWhiteSpace(result))
        .Fallback(() => throw new CustomException())
        .Wrap(
              Policy<string>
                 .Handle<Exception>()
                 .OrResult(result => string.IsNullOrWhiteSpace(result))
                 .WaitAndRetry(
                  10,
                  _ => TimeSpan.FromSeconds(1),
                  onRetry: (response, delay, retryCount, context) => Trace.WriteLine($"[{DateTime.UtcNow}] Authentication failed. Retrying after 1 second...(Attempt {retryCount} of 10)")
                  )
        );

    var result = policy.ExecuteAndCapture(() => throw new Exception("Muh"));
    Assert.IsType<CustomException>(result.FinalException);
    Assert.Null(result.Result);
    Assert.Equal(OutcomeType.Failure, result.Outcome);
}

[Fact]
public void Test2()
{
    var policy = Policy<string>
        .Handle<Exception>()
        .OrResult(result => string.IsNullOrWhiteSpace(result))
        .Fallback(() => throw new CustomException())
        .Wrap(
              Policy<string>
                 .Handle<Exception>()
                 .OrResult(result => string.IsNullOrWhiteSpace(result))
                 .WaitAndRetry(
                  10,
                  _ => TimeSpan.FromSeconds(1),
                  onRetry: (response, delay, retryCount, context) => Trace.WriteLine($"[{DateTime.UtcNow}] Authentication failed. Retrying after 1 second...(Attempt {retryCount} of 10)")
                  )
        );

    Assert.Throws<CustomException>(() => policy.Execute(() => throw new Exception("Muh")));
}

所以，我想，您可以使用Execute，或者检查结果并重新抛出异常，如下所示：

var result = policy.ExecuteAndCapture(() => AuthenticationManager.Authenticate());
if( result.Outcome == OutcomeType.Failure && result.FinalException is not null)
{
    throw result.FinalException;
}
return result.Result;

【讨论】：

【解决方案2】：

发现仅使用 WaitAndRetry 并仅检查 Outcome 对我有用（以防有人遇到此问题）：

var policyResult = Policy
    .Handle<Exception>()
    .OrResult<AuthenticationResult>(result => result is null)
        .WaitAndRetry(
            10,
            retryAttempt => TimeSpan.FromSeconds(60),
            onRetry: (response, delay, retryCount, context) => Trace.WriteLine($"[{DateTime.UtcNow}] Call failed. Retrying after 60 seconds...(Attempt {retryCount} of 10)"))
    .ExecuteAndCapture(() => AuthenticationManager.Authenticate());

if (policyResult.Outcome == OutcomeType.Failure)
{
    throw new CustomException("FAILED", policyResult.FinalException);
}

else
{
    string value = policyResult.FinalHandledResult;
}

【讨论】：