HttpStatusCode.Unauthorized 后使用 Polly 重试

Question

我正在调用外部 API 并且想要处理调用 returns 和 Unauthorized HttpResponseMessage 的事件。发生这种情况时，我想刷新访问令牌并再次拨打电话。

我正在尝试通过以下代码使用 Polly：

    public async Task<HttpResponseMessage> MakeGetRequestAsync()
    {
        var retryPolicy = Policy
            .HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.Unauthorized)
            .Retry(1, (exception, retryCount) =>
            {
                RefreshAccessToken();
            });

        var result = await retryPolicy.ExecuteAsync(() => CallApiAsync());

        return result;
    }


    private async Task<HttpResponseMessage> CallApiAsync()
    {
        var url = Options.ResourceSandboxUrl;

        var httpClient = new HttpClient();
        SetRequestHeaders(httpClient);
        var response = await httpClient.GetAsync(url);

        response.StatusCode = HttpStatusCode.Unauthorized;
        return response;
    }

I put breakpoints on the ExecuteAsync statement and in DoSomethingAsync - when I step over ExecuteAsync DoSomethingAsync is not called and control is returned to the function that called MakeGetRequestAsync

我不明白为什么 DoSomethingAsync 没有被调用 - 谁能帮助我实现我想要实现的目标？

我查看了 Stack Overflow 上的 Polly 文档和 Polly 问题，但我无法弄清楚发生了什么..

Answer 1

这就是异步等待在 .net 中的工作方式，当您的代码执行到 await 时，会发生两件事

您的代码的当前线程应该释放以使您的代码异步，这意味着您的方法应该 return
当您等待的任务完成时，您的方法应该从原来的地方继续

Answer 2

要使用 ExecuteAsync()，您必须将策略声明为 .RetryAsync(...)，而不是 .Retry(...)。

如果您的实际代码与上面的代码示例完全一致，.ExecuteAsync(...) 将因 .Retry(...) [同步策略] 和 .ExecuteAsync(...) [异步执行] 之间的不匹配而抛出].由于抛出此异常，因此 CallApiAsync() 确实从未被调用。调用 MakeGetRequestAsync()

时，您应该能够看到抛出的异常

总体代码方法看起来不错：这个 retry-refreshing-authentication 是 proven pattern with Polly!

Answer 3

我回答这个老问题只是为了指出官方记录此模式的 Polly wiki 页面：

retry-to-refresh-authorization

特别是建议的代码片段：

var authorisationEnsuringPolicy = Policy
    .HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.Unauthorized) 
    .RetryAsync(
       retryCount: 1, // Consider how many retries. If auth lapses and you have valid credentials, one should be enough; too many tries can cause some auth systems to blacklist. 
       onRetryAsync: async (outcome, retryNumber, context) => FooRefreshAuthorizationAsync(context), 
      /* more configuration */); 

var response = authorisationEnsuringPolicy.ExecuteAsync(context => DoSomethingThatRequiresAuthorization(context), cancellationToken);

The FooRefreshAuthorizationAsync(...) method can obtain a new authorization token and pass it to the delegate executed through the policy using Polly.Context.

HttpStatusCode.Unauthorized 后使用 Polly 重试

Using Polly to retry after HttpStatusCode.Unauthorized

c#

asp.net-mvc

polly