如何取消 Polly 的异步 WaitAndRetryPolicy 上的待重试

Question

I\xe2\x80\x98m 使用 polly 对 HTTP POST 请求进行简单的重试 n 次 szenario。它应该处理任何异常并重试将我的负载发送到 api 端点 n 次。因此，我使用了 WaitAndRetryPolicy 来包装 TimoutPolicy，并采用悲观策略来实现每次尝试超时。两者都是异步策略。

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当重试情况发生时，所做的每次重试尝试都会在重新建立连接后发布到端点。

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封装这两个策略的方法：

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

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调用Web api的代码：

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\n    var waitAndRetry5TimesWithShortTimeout = ResiliencePolicyFactory.WaitAndRetryNTimesWithTimeoutPerTry(\n        n: 5,\n        sleepDuration: TimeSpan.FromMilliseconds(700),\n        retryTimeout: TimeSpan.FromMilliseconds(2300));\n        }\n\n    try\n    {\n        await waitAndRetry5TimesWithShortTimeout.ExecuteAndCaptureAsync(async token =>\n        {\n            if (!cancellationToken.IsCancellationRequested)\n            {\n                response = await client.PostAsync(uri, content, cancellationToken);\n                if (response.IsSuccessStatusCode)\n                {\n                    Debug.WriteLine($"[{nameof(CheckinService)}] ===>> Now Checked in!");\n                }\n            }\n        }, cancellationToken);\n    }\n    catch(Exception ex)\n    {\n        throw new ApplicationException("NoCheckInPossible", ex);\n    }\n\n

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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，即使 I\xe2\x80\x99m 将取消令牌传递给 ExecuteAsync-Task 和 HttpClient。

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根据我的理解，第一个成功的请求应该取消所有待处理的重试。有人能指出，我\xe2\x80\x99m 做错了什么吗？

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Answer 1

问题看起来是这一行：

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

正在使用名为 的变量，而不是Polly 传递给在 处执行的委托的cancellationToken变量。tokenasync token =>

使用以下内容应该可以修复它：

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

解释

Polly 超时策略将超时组合CancellationToken到调用者传递到执行中的任何取消令牌中，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（token在本例中为变量） ）。

（从问题中发布的代码中，我们看不到任何信号取消cancellationToken；如果有，请评论或编辑问题以澄清。）

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

示范

我制作了一个与您发布的代码接近的可运行的可重现示例，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);
}

您可以在此处的 DotNetFiddle 中运行它，并看到它通常给出如下输出：

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。）

显然会放置多个调用 ( Placing call)，并且多次运行直至完成 ( Now Checked in!)，因为没有任何东西可以取消它们。

将指示的线路改为use token，可以看到，即使多次调用，之前的尝试也会被取消，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 Honor CancellationTokens，所以您可以使用稍微更高效的TimeoutStrategy.Optimistic.