Polly 使用不同的 url 重试

我正在尝试使用 polly 创建一个解决方案，其中我请求其他 api。
我有同一服务的多个实例的 URL 列表。
我希望当第一个请求失败时，另一个请求应该自动从我的列表中的下一个网址开始。

这是一个示例，我使用两个静态地址尝试此行为
此解决方案的问题是，在我开始下一个请求之前，网址不会更改。 我希望每次重试时网址都会改变

 public static void ConfigureUserServiceClient(this IServiceCollection services)
    {

        _userServiceUri = new Uri("https://localhost:5001");

        services.AddHttpClient("someService", client =>
        {
            client.BaseAddress = _userServiceUri;
            client.DefaultRequestHeaders.Add("Accept", "application/json");
        }).AddPolicyHandler(retryPolicy());
    }

    private static IAsyncPolicy<HttpResponseMessage> retryPolicy()
    {
        return Policy.HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.RequestTimeout)
            .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(retryAttempt),
            onRetry: (result, span, ctx) =>
            {
                _userServiceUri = new Uri("https://localhost:5002");
            });
    }

您应该考虑使用Fallback政策代替。

像这样：

private static HttpClient client = new HttpClient();
static async Task Main(string[] args)
{
    var addressIterator = GetUrls().GetEnumerator();

    var retryLikePolicy = Policy<string>
        .Handle<HttpRequestException>()
        .FallbackAsync(fallbackAction: async (ct) =>
        {
            if (addressIterator.MoveNext())
               return await GetData(addressIterator.Current);
            return null;
        });

    addressIterator.MoveNext();
    var data = await retryLikePolicy.ExecuteAsync(
       async () => await GetData(addressIterator.Current));

    Console.WriteLine("End");
}

static async Task<string> GetData(string uri)
{
    Console.WriteLine(uri);
    var response = await client.GetAsync(uri);
    return await response.Content.ReadAsStringAsync();
}

static IEnumerable<string> GetUrls()
{
    yield return "http://localhost:5500/index.html";
    yield return "http://localhost:5600/index.html";
    yield return "http://localhost:5700/index.html";
}

请注意，此代码仅用于演示。

更新#1: 多重后备

如果您有多个后备网址，那么您可以像这样更改上述代码：

private static HttpClient client = new HttpClient();
static async Task Main(string[] args)
{
    var retryInCaseOfHRE = Policy
        .Handle<HttpRequestException>()
        .WaitAndRetryForeverAsync(_ => TimeSpan.FromSeconds(1));

    var response = await retryInCaseOfHRE.ExecuteAsync(
         async () => await GetNewAddressAndPerformRequest());
    
    if (response == null)
    {
        Console.WriteLine("All requests failed");
        Environment.Exit(1);
    }

    Console.WriteLine("End");
}

static IEnumerable<string> GetAddresses()
{
    yield return "http://localhost:5500/index.html";
    yield return "http://localhost:5600/index.html";
    yield return "http://localhost:5700/index.html";
    yield return "http://localhost:5800/index.html";
}

static readonly IEnumerator<string> AddressIterator = GetAddresses().GetEnumerator();

static async Task<string> GetNewAddressAndPerformRequest()
    => AddressIterator.MoveNext() ? await GetData(AddressIterator.Current) : null;

static async Task<string> GetData(string uri)
{
    Console.WriteLine(uri);
    var response = await client.GetAsync(uri);
    return await response.Content.ReadAsStringAsync();
}

• The trick: the retry policy wraps a method which is responsible to retrieve the next url and then call the GetData
  • 换句话说，我们需要将迭代过程移至要包装的方法中（GetNewAddressAndPerformRequest)
• 我已经更换了Fallback政策Retry因为我们需要（可能）执行超过 1 个后备操作
• 我用过null表明我们已经用完了后备网址，但为此使用自定义异常可能是更好的解决方案