c# - 用于工作流的 C# 中的 Polly 模式？

Question

我有一个这样的工作流程:

调用 API 方法

如果抛出任何异常但超时，程序会记录该异常并抛出它。

如果抛出超时，程序必须调用另一个 API 方法。

调用另一个 API 方法后，如果一切正常，程序将返回结果。

否则程序会抛出异常并记录下来。

两种 API 方法具有相同类型的结果。
我想用波利政策来实现这一点。
这是我的示例代码:

var retryPolicy = Policy
    .Handle<HttpRequestException>(ex => ex.StatusCode == HttpStatusCode.RequestTimeout)
    .RetryAsync(1, async (exception, retryCount) =>
        await CallAnotherAPI());

var fallbackPolicy = Policy<HttpResponseMessage>
    .Handle<Exception>()
    .FallbackAsync((r, c, ct) => throw r.Exception,
    async (r, c) =>
    {
        Log(r.Message);
    });

var result = await fallbackPolicy
    .WrapAsync(retryPolicy)
    .ExecuteAsync(async () =>
    {
        await CallAPI();
    });

但它不起作用，并且一直在执行 fallbackPolicy。我如何编写代码，如果 retryPolicy 为真，则不会执行 fallbackPolicy？

Answer 1

如果我正确理解您的工作流程，那么您根本不需要重试策略。回退策略就足够了。
所以，假设 CallApi和 CallAnotherApi是这样实现的:

private static HttpClient client = new HttpClient();
public static async Task<HttpResponseMessage> CallAPI()
{
    return await client.GetAsync("http://httpstat.us//408");
}

public static async Task<HttpResponseMessage> CallAnotherAPI()
{
    var response = await client.GetAsync("http://httpstat.us//500");
    response.EnsureSuccessStatusCode();
    return response;
}

我用过 httpstatus.us模拟某些 http 状态码

CallApi总是会因请求超时而失败

CallAnotherApi总是会抛出 HttpRequestException因为Ensure方法调用

现在让我们看看策略定义和用法:

public static async Task Main(string[] args)
{
    var fallbackPolicy = Policy<HttpResponseMessage>
    .HandleResult(msg => msg.StatusCode == HttpStatusCode.RequestTimeout)
    .FallbackAsync(async (_) => await CallAnotherAPI());

    HttpResponseMessage result;
    try
    {
        result = await fallbackPolicy
            .ExecuteAsync(async () =>
            {
                return await CallAPI();
            });
    }
    catch (Exception ex) {
        Console.WriteLine(ex.Message); //TODO: replace with logging
        throw;
    }

    Console.WriteLine(result.StatusCode);
}

仅当响应的状态代码为 408

时才应触发回退策略

ExecuteAsync如果 CallApi 将抛出异常或 CallAnotherApi抛出

让我们一一看看不同的场景 CallApi成功

public static async Task<HttpResponseMessage> CallAPI()
{
    return await client.GetAsync("http://httpstat.us//200");
}

输出

OK

CallApi失败

public static async Task<HttpResponseMessage> CallAPI()
{
    var response = await client.GetAsync("http://httpstat.us//500");
    response.EnsureSuccessStatusCode();
    return response;
}

输出

Response status code does not indicate success: 500 (Internal Server Error).

然后应用程序因为 throw; 而崩溃 CallApi超时和 CallAnotherApi成功

public static async Task<HttpResponseMessage> CallAPI()
{
    return await client.GetAsync("http://httpstat.us//408");
}

public static async Task<HttpResponseMessage> CallAnotherAPI()
{
    var response = await client.GetAsync("http://httpstat.us//200");
    response.EnsureSuccessStatusCode();
    return response;
}

输出

OK

CallApi超时和 CallAnotherApi失败

public static async Task<HttpResponseMessage> CallAPI()
{
    return await client.GetAsync("http://httpstat.us//408");
}

public static async Task<HttpResponseMessage> CallAnotherAPI()
{
    var response = await client.GetAsync("http://httpstat.us//500");
    response.EnsureSuccessStatusCode();
    return response;
}

输出

Response status code does not indicate success: 500 (Internal Server Error).

然后应用程序因为 throw; 而崩溃