.NET Core 3.0之深刻源碼理解HttpClientFactory之實戰

 

寫在前面

前面兩篇文章透過源碼角度，理解了HttpClientFactory的內部實現，當咱們在項目中使用時，總會涉及如下幾個問題：

• HttpClient超時處理以及重試機制
• HttpClient熔斷器模式的實現
• HttpClient日誌記錄與追蹤鏈

接下來咱們將從使用角度對上述問題做出說明。

詳細介紹

如下代碼參考了MSDN，由於代碼裏展現的GitHub接口確實能夠調通，省的我再寫一個接口出來測試了。

HttpClient超時處理和重試機制

在此以前，咱們須要瞭解一下Polly這個庫，Polly是一款基於.NET的彈性及瞬間錯誤處理庫, 它容許開發人員以順暢及線程安全的方式執行重試(Retry)，斷路器(Circuit)，超時(Timeout)，隔板隔離(Bulkhead Isolation)及後背策略(Fallback)。

如下代碼描述了在.NET Core 3.0中如何使用超時機制。

 1: Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(10))

那麼如何將其註冊到對應的HttpClient實例呢，有不少種方式：

• 經過AddPolicyHandler註冊

 1: services.AddHttpClient("github", c =>

 2:             {

 3:                 c.BaseAddress = new Uri("https://api.github.com/");

 4:  

 5:                 c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");

 6:                 c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample");

 7:             }).AddPolicyHandler(Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(10)))；

• 聲明Policy註冊對象，並將超時策略對象添加進去

 1: var registry = services.AddPolicyRegistry();

 2: var timeout = Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(10));

 3: registry.Add("regular", timeout);

調用方式

 1:  services.AddHttpClient("github", c =>

 2:             {

 3:                 c.BaseAddress = new Uri("https://api.github.com/");

 4:  

 5:                 c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json"); // GitHub API versioning

 6:                 c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample"); // GitHub requires a user-agent

 7:             }).AddPolicyHandlerFromRegistry("regular")

Polly重試也很簡單

 1: var policyRegistry = services.AddPolicyRegistry();

 2:  

 3: policyRegistry.Add("MyHttpRetry",HttpPolicyExtensions.HandleTransientHttpError().WaitAndRetryAsync(

3

,retryAttempt => TimeSpan.FromSeconds(

Math.Pow(2, retryAttempt)

)));

這裏的重試設置是在第一次調用失敗後，還會有三次機會繼續重試，每一個請求的時間間隔是指數級延遲。

重試功能除了可使用Polly實現外，還可使用DelegatingHandler，DelegatingHandler繼承自HttpMessageHandler，用於」處理請求、響應回覆「，本質上就是一組HttpMessageHandler的有序組合，能夠視爲是一個「雙向管道」。

此處主要展現DelegatingHandler的使用方式，在實際使用中，仍然建議使用Polly重試。

 1: private class RetryHandler : DelegatingHandler

 2: {

 3:     public int RetryCount { get; set; } = 5;

 4:  

 5:     protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)

 6:     {

 7:         for (var i = 0; i < RetryCount; i++)

 8:         {

 9:             try

 10:             {

 11:                 return await base.SendAsync(request, cancellationToken);

 12:             }

 13:             catch (HttpRequestException) when (i == RetryCount - 1)

 14:             {

 15:                 throw;

 16:             }

 17:             catch (HttpRequestException)

 18:             {

 19:                 // 五十毫秒後重試

 20:                 await Task.Delay(TimeSpan.FromMilliseconds(50));

 21:             }

 22:         }

 23:     }

 24: }

註冊方式以下：

 1: services.AddHttpClient("github", c =>

 2: {

 3:     c.BaseAddress = new Uri("https://api.github.com/");

 4:  

 5:     c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json"); // GitHub API versioning

 6:     c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample"); // GitHub requires a user-agent

 7: })

 8: .AddHttpMessageHandler(() => new RetryHandler());

HttpClient熔斷器模式的實現

若是很是瞭解Polly庫的使用，那麼熔斷器模式的實現也會很是簡單，

 1: var policyRegistry = services.AddPolicyRegistry();

 2:  

 3: policyRegistry.Add("MyCircuitBreaker",HttpPolicyExtensions.HandleTransientHttpError().CircuitBreakerAsync(handledEventsAllowedBeforeBreaking: 10,durationOfBreak: TimeSpan.FromSeconds(30)));

這裏的熔斷器設置規則是在連續10次請求失敗後，會暫停30秒。這個地方能夠寫個擴展方法註冊到IServiceCollection中。

HttpClient日誌記錄與追蹤鏈

日誌記錄這塊與追蹤鏈，咱們通常會經過request.Header實現，而在微服務中，十分關注相關調用方的信息及其獲取，通常的作法是經過增長請求Id的方式來肯定請求及其相關日誌信息。

實現思路是增長一個DelegatingHandler實例，用以記錄相關的日誌以及請求鏈路

 1:     public class TraceEntryHandler : DelegatingHandler

 2:     {

 3:         private TraceEntry TraceEntry { get; set; }

 4:  

 5:         public TraceEntryHandler(TraceEntry traceEntry)

 6:         {

 7:             this.TraceEntry = traceEntry;

 8:         }

 9:  

 10:        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)

 11:         {

 12:             request.Headers.TryAddWithoutValidation("X-TRACE-CID", this.TraceEntry.ClientId);

 13:             request.Headers.TryAddWithoutValidation("X-TRACE-RID", this.TraceEntry.RequestId);

 14:             request.Headers.TryAddWithoutValidation("X-TRACE-SID", this.TraceEntry.SessionId);

 15:             if (this.TraceEntry.SourceIP.IsNullOrEmpty())

 16:             {

 17:                 request.Headers.TryAddWithoutValidation("X-TRACE-IP", this.TraceEntry.SourceIP);

 18:             }

 19:  

 20:             if (this.TraceEntry.SourceUserAgent.IsNullOrEmpty())

 21:             {

 22:                 request.Headers.TryAddWithoutValidation("X-TRACE-UA", this.TraceEntry.SourceUserAgent);

 23:             }

 24:  

 25:             if (this.TraceEntry.UserId.IsNullOrEmpty())

 26:             {

 27:                 request.Headers.TryAddWithoutValidation("X-TRACE-UID", this.TraceEntry.UserId);

 28:             }

 29:  

 30:             return base.SendAsync(request, cancellationToken);

 31:         }

 32:     }

我在查找相關資料的時候，發現有個老外使用CorrelationId組件實現，做爲一種實現方式，我決定要展現一下，供你們選擇：

 1: public class CorrelationIdDelegatingHandler : DelegatingHandler

 2: {

 3:     private readonly ICorrelationContextAccessor correlationContextAccessor;

 4:     private readonly IOptions<CorrelationIdOptions> options;

 5:  

 6:     public CorrelationIdDelegatingHandler(

 7:         ICorrelationContextAccessor correlationContextAccessor,

 8:         IOptions<CorrelationIdOptions> options)

 9:     {

 10:         this.correlationContextAccessor = correlationContextAccessor;

 11:         this.options = options;

 12:     }

 13:  

 14:     protected override Task<HttpResponseMessage> SendAsync(

 15:         HttpRequestMessage request,

 16:         CancellationToken cancellationToken)

 17:     {

 18:         if (!request.Headers.Contains(this.options.Value.Header))

 19:         {

 20:             request.Headers.Add(this.options.Value.Header, correlationContextAccessor.CorrelationContext.CorrelationId);

 21:         }

 22:  

 23:         // Else the header has already been added due to a retry.

 24:  

 25:         return base.SendAsync(request, cancellationToken);

 26:     }

 27: }

參考連接：

https://docs.microsoft.com/zh-cn/aspnet/core/fundamentals/http-requests?view=aspnetcore-3.0

https://rehansaeed.com/optimally-configuring-asp-net-core-httpclientfactory/

原文出處：https://www.cnblogs.com/edison0621/p/11298882.html