写在前面
前面两篇文章透过源码角度，理解了HttpClientFactory的内部实现，当我们在项目中使用时，总会涉及以下几个问题：
- HttpClient超时处理以及重试机制
- HttpClient熔断器模式的实现
- HttpClient日志记录与追踪链
接下来我们将从使用角度对上述问题作出说明。
详细介绍
以下代码参考了MSDN，因为代码里展示的GitHub接口确实可以调通，省的我再写一个接口出来测试了。
HttpClient超时处理和重试机制
在此之前，我们需要了解一下Polly这个库，Polly是一款基于.NET的弹性及瞬间错误处理库, 它允许开发人员以顺畅及线程安全的方式执行重试(Retry)，断路器(Circuit)，超时(Timeout)，隔板隔离(Bulkhead Isolation)及后背策略(Fallback)。
以下代码描述了在.NET Core 3.0中如何使用超时机制。
1: Policy.TimeoutAsync(TimeSpan.FromSeconds(10))
那么如何将其注册到对应的HttpClient实例呢，有很多种方式：
- 通过AddPolicyHandler注册
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(TimeSpan.FromSeconds(10)))；
- 声明Policy注册对象，并将超时策略对象添加进去
1: var registry = services.AddPolicyRegistry();
2: var timeout = Policy.TimeoutAsync(TimeSpan.FromSeconds(10));
3: registry.Add("regular", timeout);
调用方式
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重试也很简单
1: var policyRegistry = services.AddPolicyRegistry();
2:
3: policyRegistry.Add("MyHttpRetry",HttpPolicyExtensions.HandleTransientHttpError().WaitAndRetryAsync(
3
,retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)
)));
这里的重试设置是在第一次调用失败后，还会有三次机会继续重试，每个请求的时间间隔是指数级延迟。
重试功能除了可以使用Polly实现外，还可以使用DelegatingHandler，DelegatingHandler继承自HttpMessageHandler，用于”处理请求、响应回复“，本质上就是一组HttpMessageHandler的有序组合，可以视为是一个“双向管道”。
此处主要展示DelegatingHandler的使用方式，在实际使用中，仍然建议使用Polly重试。
1: private class RetryHandler : DelegatingHandler
2: {
3: public int RetryCount { get; set; } = 5;
4:
5: protected override async TaskSendAsync(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: // 五十毫秒后重试
20: await Task.Delay(TimeSpan.FromMilliseconds(50));
21: }
22: }
23: }
24: }
注册方式如下：
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熔断器模式的实现
如果非常了解Polly库的使用，那么熔断器模式的实现也会非常简单，
1: var policyRegistry = services.AddPolicyRegistry();
2:
3: policyRegistry.Add("MyCircuitBreaker",HttpPolicyExtensions.HandleTransientHttpError().CircuitBreakerAsync(handledEventsAllowedBeforeBreaking: 10,durationOfBreak: TimeSpan.FromSeconds(30)));
这里的熔断器设置规则是在连续10次请求失败后，会暂停30秒。这个地方可以写个扩展方法注册到IServiceCollection中。
HttpClient日志记录与追踪链
日志记录这块与追踪链，我们一般会通过request.Header实现，而在微服务中，十分关注相关调用方的信息及其获取，一般的做法是通过增加请求Id的方式来确定请求及其相关日志信息。
实现思路是增加一个DelegatingHandler实例，用以记录相关的日志以及请求链路
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 TaskSendAsync(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: }
我在查找相关资料的时候，发现有个老外使用CorrelationId组件实现，作为一种实现方式，我决定要展示一下，供大家选择：
1: public class CorrelationIdDelegatingHandler : DelegatingHandler
2: {
3: private readonly ICorrelationContextAccessor correlationContextAccessor;
4: private readonly IOptionsoptions;
5:
6: public CorrelationIdDelegatingHandler(
7: ICorrelationContextAccessor correlationContextAccessor,
8: IOptionsoptions)
9: {
10: this.correlationContextAccessor = correlationContextAccessor;
11: this.options = options;
12: }
13:
14: protected override TaskSendAsync(
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: }
参考链接：
https://docs.microsoft.com/zh-cn/aspnet/core/fundamentals/http-requests?view=aspnetcore-3.0
https://rehansaeed.com/optimally-configuring-asp-net-core-httpclientfactory/