如何取消对Polly的异步WaitAndRetryPolicy的挂起重试

Question

我使用polly对HTTP请求进行n次简单重试。它应该处理任何异常并重新尝试将我的有效负载发送到api端点n次。因此，我使用了一个WaitAndRetryPolicy，它用悲观策略包装了一个TimoutPolicy，每次尝试都会超时。都是异步策略。

当重试案例发生时，进行的每一次重试尝试都会在连接重新配置后发布到端点。

将这两个策略封装在一起的方法：

    public static PolicyWrap WaitAndRetryNTimesWithTimeoutPerTry(int n, TimeSpan sleepDuration, TimeSpan retryTimeout)
    {
        var waitAndRetryPolicy = Policy.Handle<Exception>().WaitAndRetryAsync(
        retryCount: n,
            sleepDurationProvider: attempt => sleepDuration,
            onRetry: (exception, waitDuration, ctx) =>
            {
                Debug.WriteLine($"[Polly.OnRetry due '{exception.Message}'; waiting for {waitDuration.TotalMilliseconds} ms before retrying.");
            }
        );

        var timeoutPerTryPolicy = Policy.TimeoutAsync(
            retryTimeout, TimeoutStrategy.Pessimistic);

        return waitAndRetryPolicy.WrapAsync(timeoutPerTryPolicy);
    }

调用web的代码：

    var waitAndRetry5TimesWithShortTimeout = ResiliencePolicyFactory.WaitAndRetryNTimesWithTimeoutPerTry(
        n: 5,
        sleepDuration: TimeSpan.FromMilliseconds(700),
        retryTimeout: TimeSpan.FromMilliseconds(2300));
        }

    try
    {
        await waitAndRetry5TimesWithShortTimeout.ExecuteAndCaptureAsync(async token =>
        {
            if (!cancellationToken.IsCancellationRequested)
            {
                response = await client.PostAsync(uri, content, cancellationToken);
                if (response.IsSuccessStatusCode)
                {
                    Debug.WriteLine($"[{nameof(CheckinService)}] ===>> Now Checked in!");
                }
            }
        }, cancellationToken);
    }
    catch(Exception ex)
    {
        throw new ApplicationException("NoCheckInPossible", ex);
    }

当代码命中重试案例并在几次重试后成功时，所做的每次重试尝试都会被发送到端点，尽管我将取消令牌传递给ExecuteAsync-Task和HttpClient。

据我理解，第一个成功请求应该取消所有挂起的重试。有人能指出我做错了什么吗？

Answer 1

问题似乎是这句话：

response = await client.PostAsync(uri, content, cancellationToken);

使用一个名为cancellationToken的变量，而不是Polly在async token =>上传递给已执行的委托的变量token。

使用下面的方法应该可以修复它：

response = await client.PostAsync(uri, content, token);

解释

Polly超时策略在任何cancellationToken中调用者传递给执行。，但要想使该超时令牌产生任何效果，在执行委托中，您必须使用Polly提供给执行的令牌(在本例中为变量token )。

(从问题中发布的代码中，我们看不到任何取消cancellationToken的信号；如果存在，请评论或编辑该问题以澄清。)

使用代码client.PostAsync(uri, content, cancellationToken)，如果没有取消cancellationToken，那么每个帖子都不会被取消，这很可能解释了为什么会看到多个帖子运行到完成。

游行示威

为了演示，我制作了一个接近您发布的代码的可运行的可复制示例。

public static Random rand = new Random();

public static async Task Main()
{

    var waitAndRetry5TimesWithShortTimeout = WaitAndRetryNTimesWithTimeoutPerTry(
        n: 5,
        sleepDuration: TimeSpan.FromMilliseconds(70),
        retryTimeout: TimeSpan.FromMilliseconds(230));

    CancellationToken cancellationToken = new CancellationTokenSource().Token;

    string response;
    try
    {
        await waitAndRetry5TimesWithShortTimeout.ExecuteAndCaptureAsync(async token =>
        {
            Console.WriteLine("Placing call");
            if (!cancellationToken.IsCancellationRequested)
            {
                response = await PretendPostAsync(cancellationToken); // Change 'cancellationToken' to 'token' here, and it will start to work as expected.
                if (response == "success")
                {
                    Console.WriteLine($"Now Checked in!");
                }
            }
        }, cancellationToken);
    }
    catch(Exception ex)
    {
        throw new ApplicationException("NoCheckInPossible", ex);
    }

}

public static async Task<string> PretendPostAsync(CancellationToken token)
{
    if (rand.Next(4) != 0)
    {
        await Task.Delay(TimeSpan.FromSeconds(0.5), token);
    }

    return "success";
}

public static AsyncPolicyWrap WaitAndRetryNTimesWithTimeoutPerTry(int n, TimeSpan sleepDuration, TimeSpan retryTimeout)
{
    var waitAndRetryPolicy = Policy.Handle<Exception>().WaitAndRetryAsync(
    retryCount: n,
        sleepDurationProvider: attempt => sleepDuration,
        onRetry: (exception, waitDuration, ctx) =>
        {
            Console.WriteLine($"[Polly.OnRetry due '{exception.Message}'; waiting for {waitDuration.TotalMilliseconds} ms before retrying.");
        }
    );

    var timeoutPerTryPolicy = Policy.TimeoutAsync(
        retryTimeout, TimeoutStrategy.Pessimistic);

    return waitAndRetryPolicy.WrapAsync(timeoutPerTryPolicy);
}

您可以在DotNetFiddle中运行这个并看到它通常提供的输出如下：

Placing call
[Polly.OnRetry due 'The delegate executed asynchronously through TimeoutPolicy did not complete within the timeout.'; waiting for 70 ms before retrying.
Placing call
Now Checked in!
[Polly.OnRetry due 'The delegate executed asynchronously through TimeoutPolicy did not complete within the timeout.'; waiting for 70 ms before retrying.
Placing call
Now Checked in!

(代码示例随机模拟不同程度的故障；您可能需要运行几次才能看到类似的结果。)

多个调用显然被放置(Placing call)，多次运行到完成(Now Checked in!)，因为没有什么可以取消它们。

更改指示使用token的行，可以看到，即使多个调用被放置，较早的尝试也会被取消，并且只有一个成功。

Placing call
[Polly.OnRetry due 'The delegate executed asynchronously through TimeoutPolicy did not complete within the timeout.'; waiting for 70 ms before retrying.
Placing call
[Polly.OnRetry due 'The delegate executed asynchronously through TimeoutPolicy did not complete within the timeout.'; waiting for 70 ms before retrying.
Placing call
Now Checked in!

整理

因为HttpClient.PostAsync(...)确实尊重CancellationToken，所以您可以使用稍微高效的TimeoutStrategy.Optimistic。