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在使用Flurl作為HttpClient向Server請求時，由于網(wǎng)絡(luò)或者其它一些原因?qū)е抡埱髸惺〉那闆r，比如HttpStatusCode.NotFound、HttpStatusCode.ServiceUnavailable、HttpStatusCode.RequestTimeout等；網(wǎng)絡(luò)上有比較多的HttpClientFactory使用Polly來實現(xiàn)重試的內(nèi)容，奈何已經(jīng)習(xí)慣使用Flurl的人，要全部換回到IHttpClient的確有不方便的地方，因為本文使用Flurl的Polly來實現(xiàn)重試機制做一個整理；

不使用Polly來測試

提供一個接口以便做請求測試

[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()});
????????}
????}
}

創(chuàng)建一個請求客戶端

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;
????????}
????}
}

嘗試請求，可以發(fā)現(xiàn)有很多請求失敗的地方，這個情況很不理想，服務(wù)器有較大的機率不能正常的響應(yīng)

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

?
針對這個情況有沒有什么解決的辦法呢，答案是肯定的，粗暴的想法就是失敗了再重新做請求，直接在Flurl的返回結(jié)果中做這個邏輯處理會比較麻煩也不方便統(tǒng)一的管理，如此就找到了Polly

使用Polly來測試

首先安裝Polly, Install-Package Polly
下面先給出Polly的簡單介紹后接著給出Policy的代碼片段
?
Polly的七種策略：重試、斷路、超時、隔離、回退和緩存策略，本文使用到了重試、超時策略
重試（Retry）：出現(xiàn)故障自動重試，這個是常見的場景
斷路（Circuit-breaker）：當(dāng)系統(tǒng)遇到嚴(yán)重的問題時，快速回饋失敗比讓用戶/調(diào)用者等待要好，限制系統(tǒng)出錯的消耗，有助于系統(tǒng)恢復(fù)，比如，當(dāng)我們?nèi)フ{(diào)用一個第三方的API，有很長一段時間API都沒有響應(yīng)，可能對方服務(wù)器癱瘓了，如果我們的系統(tǒng)還不停地重試，不僅會加重系統(tǒng)的負(fù)擔(dān)，還有可能導(dǎo)致系統(tǒng)其它任務(wù)受影響，因此，當(dāng)系統(tǒng)出錯的次數(shù)超過了指定的閾值，就得中斷當(dāng)前線程，等待一段時間后再繼續(xù)；比如: Policy.Handle<SomeException>().CircuitBreaker(2, TimeSpan.FromMinutes(1));表示當(dāng)系統(tǒng)出現(xiàn)兩次某個異常時就停下來，等待1分鐘后再繼續(xù)，還可以在斷路時定義中斷的回調(diào)和重啟的回調(diào)。
超時（Timeout）：當(dāng)系統(tǒng)超過一定時間的等待，就可以判斷不可能會有成功的結(jié)果；比如平時一個網(wǎng)絡(luò)請求瞬間就完成了，如果有一次網(wǎng)絡(luò)請求超過了30秒還沒有完成，我們就可以判定不可能會返回成功的結(jié)果了，因此，我們需要設(shè)置系統(tǒng)的超時時間，避免系統(tǒng)長時間無謂的等待；比如：Policy.Timeout(30, (context, span, task) => {// do something});表示設(shè)置了超時時間不能超過30秒，否則就認(rèn)為是錯誤的結(jié)果，并執(zhí)行回調(diào)。
隔離（Bulkhead Isolation）：當(dāng)系統(tǒng)的一處出現(xiàn)故障時，可能觸發(fā)多個失敗的調(diào)用，對資源有較大的消耗，下游系統(tǒng)出現(xiàn)故障可能導(dǎo)致上游的故障的調(diào)用，甚至可能蔓延到導(dǎo)致系統(tǒng)崩潰，所以要將可控的操作限制在一個固定大小的資源池中，以隔離有潛在可能相互影響的操作；比如：Policy.Bulkhead(12, context => {// do something});表示最多允許12個線程并發(fā)執(zhí)行，如果執(zhí)行被拒絕，則執(zhí)行回調(diào)。
回退（Fallback）：有些錯誤無法避免，就要有備用的方案，當(dāng)無法避免的錯誤發(fā)生時，我們要有一個合理的返回來代替失敗；比如：Policy.Handle<Whatever>().Fallback<UserAvatar>(() => UserAvatar.GetRandomAvatar());表示當(dāng)用戶沒有上傳頭像時，我們就給他一個默認(rèn)頭像。
緩存（Cache）：一般我們會把頻繁使用且不會怎么變化的資源緩存起來，以提高系統(tǒng)的響應(yīng)速度，如果不對緩存資源的調(diào)用進行封裝，那么我們調(diào)用的時候就要先判斷緩存中有沒有這個資源，有的話就從緩存返回，否則就從資源存儲的地方獲取后緩存起來再返回，而且有時還要考慮緩存過期和如何更新緩存的問題；Polly提供了緩存策略的支持，使得問題變得簡單。
策略包（Policy Wrap）：一種操作會有多種不同的故障，而不同的故障處理需要不同的策略，這些不同的策略必須包在一起，作為一個策略包，才能應(yīng)用在同一種操作上，這就是Polly的彈性特性，即各種不同的策略能夠靈活地組合起來
更多...
```
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[]
????????????????????{
????????????????????????//?表示重試3次，第一次1秒后重試，第二次2秒后重試，第三次4秒后重試
????????????????????????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()
????????????};
????????}
????}
}
```

接下來在Starup中對Flurl進行配置

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));
????......

再次嘗試請求，可以看到結(jié)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

富客戶端中使用的情況

?
有時候呢，例如在WPF或者是其它的富客戶端上面也會經(jīng)常使用到Flurl的情況，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);

?
時間如流水，只能流去不流回。
公眾號：Dotnet9
號主微信號：dotnet9
作者：非法關(guān)鍵字
原文：Flurl使用Polly添加重試機制
編輯：沙漠之盡頭的狼
日期：2021-03-15
微信公眾號：Dotnet9

Flurl使用Polly實現(xiàn)重試Policy

不使用Polly來測試

使用Polly來測試

富客戶端中使用的情況