如何在 Polly 的异步 WaitAndRetryPolicy 上取消挂起的重试

Question

我正在使用 polly 对 HTTP POST 请求进行简单的 n 次重试。它应该处理任何异常并重试将我的有效负载发布到 api 端点 n 次。因此，我使用 WaitAndRetryPolicy 包装 TimoutPolicy 和悲观策略，每次尝试超时。两者都是异步策略。

当重试案例发生时，每一次重试尝试都会在重新建立连接后发布到端点。

包装这两个策略的方法：

    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 api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);
    }

当代码遇到重试情况并在多次重试后成功时，每次重试尝试都会发布到端点，尽管我将取消令牌传递给 ExecuteAsync-Task 和 HttpClient。

据我了解，第一个成功的请求应该取消所有挂起的重试。谁能指出我做错了什么？

Answer 1

问题看起来是这一行：

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

正在使用一个名为 cancellationToken 的变量，而不是 Polly 传递给 async token => 处已执行委托的变量 token。

使用以下应该可以解决它：

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

说明

Polly 超时策略 combine a timing-out CancellationToken into any cancellationToken the caller passes into the execution，但要使该超时令牌产生任何效果，在执行的委托中，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，我们看不出有任何迹象表明取消了cancellationToken；如果有，请评论或编辑问题以澄清。）

使用代码 client.PostAsync(uri, content, cancellationToken)，如果没有取消 cancellationToken，则每个 POST 都不会被取消，这可能解释了为什么您会看到多个 POST 运行完成。

演示

我在您发布的代码附近创建了一个可运行的reproducible example，以进行演示。

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run this in DotNetFiddle here 并看到它通常会提供如下输出：

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!

(代码示例随机化以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。)

多个调用清楚地被放置 (Placing call)，并且多次运行完成 (Now Checked in!)，因为没有任何东西可以取消它们。

将指示的行改成token，可见即使多次调用，前面的尝试也被取消，只有一次成功。

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CancellationTokens，所以您可以使用效率稍高的TimeoutStrategy.Optimistic。