httpClient 請求接口如何優雅的重試-魔扣目錄

httpClient 請求接口失敗要重試，一般人想到的可能是做try catch 然后循環請求，但是這種方法寫起來很麻煩，而且不優雅。今天就說下另外一種重試的方法，就是引入Microsoft.Extensions.Http.Polly，來進行請求的重試。

Polly 添加策略分為三種：

　　1. AddTransientHttpErrorPolicy：是處理Http請求的錯誤，如HTTP 5XX 的狀態碼，HTTP 408 的狀態碼以及System.Net.Http.HttpRequestException異常

　　2. AddPolicyHandler：添加自定義策列

　　3. AddPolicyHandlerFromRegistry：從Policy注冊表集合里面選擇添加

1. AddTransientHttpErrorPolicy

//錯誤重試上面是間隔都是500毫秒
services.AddHttpClient("client1")
.AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(3, t => TimeSpan.FromMilliseconds(500))));

或者

//定義每次重試的時間間隔
services.AddHttpClient("client1")
.AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(new[] {
TimeSpan.FromSeconds(5),
TimeSpan.FromSeconds(10),
TimeSpan.FromSeconds(30)
})

2. AddPolicyHandler

　　先定義策略,通過AddPolicyHandler方法添加，該方法接收一個泛型的IAsyncPolicy<HttpResponseMessage>。HttpResponseMessage我理解為出站請求上下文

var retryPolicy = Policy.Handle<HttpRequestException>()
.OrResult<HttpResponseMessage>(response => {
//異常處理的邏輯
return true;
}).WaitAndRetryAsync(3,t => TimeSpan.FromMilliseconds(50));
services.AddHttpClient("client1")
.AddPolicyHandler(retryPolicy);

3. AddPolicyHandlerFromRegistry

　　先注冊策略表服務,為策略表添加策略,最后通過AddPolicyHandlerFromRegistry方法選用某個或幾個策略

//注冊，策略表服務
var registry = services.AddPolicyRegistry();//創建策略var retryPolicy = Policy.Handle<HttpRequestException>().OrResult<HttpResponseMessage>(response => {//異常處理的邏輯return true;
}).WaitAndRetryAsync(3, t => TimeSpan.FromMilliseconds(500));registry.Add("registry1", retryPolicy);
services.AddHttpClient("client1")
.AddPolicyHandlerFromRegistry("registry1");

做一個例子看下，創建一個控制臺程序

using Microsoft.Extensions.DependencyInjection;
using Polly;
using System;
using System.Net.Http;
namespace ConsoleAppTest
{    class Program
    {        static void Main(string[] args)
        {            Console.WriteLine("Hello World!");
            var service = new ServiceCollection();
            service.AddHttpClient("client1").AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(3, t => TimeSpan.FromMilliseconds(500)));
            IHttpClientFactory httpClientFactory = service.BuildServiceProvider().GetService<IHttpClientFactory>();            var httpClient = httpClientFactory.CreateClient("client1");
            var response = httpClient.GetAsync("http://www.pikoudai.com/");
            Console.ReadKey();        }    }}

這里的http://www.pikoudai.com/ 其實是訪問不通的，就是要看看重試是否生效了。

抓包看下程序效果：