在使用Flurl作为HttpClient向Server请求时，由于网络或者其它一些原因导致请求会有失败的情况，比如
HttpStatusCode.NotFound、HttpStatusCode.ServiceUnavailable、HttpStatusCode.RequestTimeout等；网络上有比较多的HttpClientFactory使用Polly来实现重试的内容，奈何已经习惯使用Flurl的人，要全部换回到IHttpClient的确有不方便的地方，因为本文使用Flurl使用Polly来实现重试机制做一个整理；
不使用Polly来测试
-
提供一个接口以便做请求测试
[Route("api/[controller]")] [ApiController] public class PollyController : ControllerBase { private readonly ILogger<PollyController> _logger; public PollyController(ILogger<PollyController> logger) { _logger = logger; } // GET: api/<PollyController> [HttpGet] public IActionResult Get() { var random = new Random().Next(0, 8); switch (random) { case 0: _logger.LogInformation("About to serve a 404"); return StatusCode(StatusCodes.Status404NotFound); case 1: _logger.LogInformation("About to serve a 503"); return StatusCode(StatusCodes.Status503ServiceUnavailable); case 2: _logger.LogInformation("Sleeping for 10 seconds then serving a 504"); Thread.Sleep(10000); _logger.LogInformation("About to serve a 504"); return StatusCode(StatusCodes.Status504GatewayTimeout); default: _logger.LogInformation("About to correctly serve a 200 response"); return Ok(new {time = DateTime.Now.ToLocalTime()}); } } } -
创建一个请求客户端
public class HomeController : Controller { private readonly ILogger<HomeController> _logger; public HomeController(ILogger<HomeController> logger) { _logger = logger; } public async Task<IActionResult> Index() { try { var time = await "http://127.0.0.1:5000/api/polly" .GetJsonAsync(); _logger.LogInformation($"App: success - {time.time}"); return View(time.time); } catch (Exception e) { _logger.LogWarning($"App: failed - {e.Message}"); throw; } } } -
尝试请求，可以发现有很多请求失败的地方，这个情况很不理想，服务器有较大的机率不能正常的响应
info: SuppertRcsInterfaceTest.Controllers.PollyController[0] About to serve a 404 info: SuppertRcsInterfaceTest.Controllers.PollyController[0] About to correctly serve a 200 response info: SuppertRcsInterfaceTest.Controllers.PollyController[0] About to correctly serve a 200 response info: SuppertRcsInterfaceTest.Controllers.PollyController[0] About to correctly serve a 200 response info: SuppertRcsInterfaceTest.Controllers.PollyController[0] About to serve a 503 info: SuppertRcsInterfaceTest.Controllers.PollyController[0] About to serve a 503 info: SuppertRcsInterfaceTest.Controllers.PollyController[0] About to correctly serve a 200 response info: SuppertRcsInterfaceTest.Controllers.PollyController[0] About to serve a 404针对这个情况有没有什么解决的办法呢，答案是肯定的，粗暴的想法就是失败了再重新做请求，直接在Flurl的返回结果中过这个逻辑处理会比较麻烦也不方便统一的管理，如此就找到了Polly
使用Polly来测试
-
首先安装Polly,
Install-Package Polly -
下面先给出Polly的简单介绍后接着给出
Policy的代码片段Polly的七种策略：重试、断路、超时、隔离、回退和缓存策略，本文使用到了重试、超时策略
重试（Retry）：出现故障自动重试，这个是常见的场景
断路（Circuit-breaker）：当系统遇到严重的问题时，快速回馈失败比让用户/调用者等待要好，限制系统出错的消耗，有助于系统恢复，比如，当我们去调用一个第三方的API，有很长一段时间API都没有响应，可能对方服务器瘫痪了，如果我们的系统还不停地重试，不公会加重系统的负担，还有可能导致系统其它任务受影响，因此，当系统出错的次数超过了指定的阈值，就是中断当前线程，等待一段时间后再继续；比如:
Policy.Handle<SomeException>().CircuitBreaker(2, TimeSpan.FromMinutes(1));表示当系统出现两次某个异常时就停下来，等待1分钟后再继续，还可以在断路时定义中断的回调和重启的回调超时（Timeout）：当系统超时一定时间的等待，就位就可以判断不可能会有成功的结果；比如平时一个网络请求瞬间就完成了，如果有一次网络请求超过了30秒还没有完成，我们就可以判定不可能会返回成功的结果了，因此，我们需要设置系统的超时时间，避免系统长时间就无谓的等待；比如：
Policy.Timeout(30, (context, span, task) => {// do something});表示设置了超时时间不能超过30秒，否则就认为是错误的结果，并执行回调隔离（Bulkhead Isolation）：当系统的一处出现故障时，可能触发多个失败的调用，对资源有较大的消耗，下游系统出现故障可能导致上游的故障的调用，甚至可能蔓延到导致系统崩溃，所以要将可控的操作限制在一个固定大小的资源池中，以隔离有潜在可能相互影响的操作；比如：
Policy.Bulkhead(12, context => {// do something});表示最多允许12个线程并发执行，如果执行被拒绝，则执行回调回退（Fallback）：有些错误无法避免，就要有备用的方案，当无法避免的错误发生时，我们要有一个合理的返回来代替失败；比如：
Policy.Handle<Whatever>().Fallback<UserAvatar>(() => UserAvatar.GetRandomAvatar());表示当用户没有上传头像时，我们就给他一个默认头像缓存（Cache）：一般我们会把频繁使用且不会怎么变化 的资源缓存起来，以提高系统的响应速度，如果不对缓存资源的调用进行封装，那么我们调用的时候就要先判断缓存中有没有这个资源，有的话就从缓存返回，否则就从资源存储的地方获取后缓存起来再返回，而且有时还要考虑缓存过期和如何更新缓存的问题；Polly提供了缓存策略的支持，使得问题变得简单
策略包（Policy Wrap）：一种操作会有多种不同的故障，而不同的故障处理需要不同的策略，这些不同的策略必须包在一起，作为一个策略包，才能应用在同一种操作上，这就是Polly的弹性特性，即各种不同的策略能够灵活地组合起来
using System; using System.Linq; using System.Net; using System.Net.Http; using System.Threading; using System.Threading.Tasks; using Flurl.Http.Configuration; using Microsoft.Extensions.Logging; using Polly; using Polly.Retry; using Polly.Timeout; using Polly.Wrap; namespace WithPollyClient.Services { public class Policies { private readonly ILogger<Policies> _logger; public Policies(ILogger<Policies> logger) { _logger = logger; } private AsyncTimeoutPolicy<HttpResponseMessage> TimeoutPolicy { get { return Policy.TimeoutAsync<HttpResponseMessage>(3, (context, span, task) => { _logger.LogInformation($"Policy: Timeout delegate fired after {span.Seconds} seconds"); return Task.CompletedTask; }); } } private AsyncRetryPolicy<HttpResponseMessage> RetryPolicy { get { HttpStatusCode[] retryStatus = { HttpStatusCode.NotFound, HttpStatusCode.ServiceUnavailable, HttpStatusCode.RequestTimeout }; return Policy .HandleResult<HttpResponseMessage>(r => retryStatus.Contains(r.StatusCode)) .Or<TimeoutRejectedException>() .WaitAndRetryAsync(new[] { // 表示重试3次，第一次1秒后重试，第二次2秒后重试，第三次4秒后重试 TimeSpan.FromSeconds(1), TimeSpan.FromSeconds(2), TimeSpan.FromSeconds(4) }, (result, span, count, context) => { _logger.LogInformation($"Policy: Retry delegate fired, attempt {count}"); }); } } public AsyncPolicyWrap<HttpResponseMessage> PolicyStrategy => Policy.WrapAsync(RetryPolicy, TimeoutPolicy); } public class PolicyHandler : DelegatingHandler { private readonly Policies _policies; public PolicyHandler(Policies policies) { _policies = policies; } protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { return _policies.PolicyStrategy.ExecuteAsync(ct => base.SendAsync(request, ct), cancellationToken); } } public class PollyHttpClientFactory : DefaultHttpClientFactory { private readonly Policies _policies; public PollyHttpClientFactory(Policies policies) { _policies = policies; } public override HttpMessageHandler CreateMessageHandler() { return new PolicyHandler(_policies) { InnerHandler = base.CreateMessageHandler() }; } } } -
接下来在
Starup中对Flurl进行配置public void ConfigureServices(IServiceCollection services) { services.AddControllersWithViews(); services.AddSingleton<Policies>(); } // This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IApplicationBuilder app, IWebHostEnvironment env) { var policies = app.ApplicationServices.GetService<Policies>(); FlurlHttp.Configure(setting => setting.HttpClientFactory = new PollyHttpClientFactory(policies)); ...... -
再次尝试请求，可以看到结果非常之理想
WithPollyClient.Services.Policies: Information: Policy: Retry delegate fired, attempt 1 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:14 WithPollyClient.Services.Policies: Information: Policy: Retry delegate fired, attempt 1 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:17 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:22 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:23 WithPollyClient.Services.Policies: Information: Policy: Retry delegate fired, attempt 1 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:25 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:31 WithPollyClient.Services.Policies: Information: Policy: Retry delegate fired, attempt 1 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:34 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:39 WithPollyClient.Services.Policies: Information: Policy: Retry delegate fired, attempt 1 WithPollyClient.Services.Policies: Information: Policy: Timeout delegate fired after 3 seconds WithPollyClient.Services.Policies: Information: Policy: Retry delegate fired, attempt 2 WithPollyClient.Controllers.HomeController: Information: App: success - 2021/3/14 16:50:46
富客户端中使用的情况
有时候呢，例如在
WPF或者是其它的富客户端上面也会经常使用到Flurl的情况，如下
var time = await Policy
.Handle<FlurlHttpException>()
.OrResult<IFlurlResponse>(r => !r.ResponseMessage.IsSuccessStatusCode)
.WaitAndRetryAsync(new[]
{
TimeSpan.FromSeconds(1),
TimeSpan.FromSeconds(2),
TimeSpan.FromSeconds(4)
}, (result, span, count, context) =>
{
_logger.LogInformation(count.ToString());
})
.ExecuteAsync(() => "http://127.0.0.1:5000/api/polly".WithTimeout(3).GetAsync())
.ReceiveJson();
_logger.LogInformation($"App: success - {time.time}");
return View(time.time);