给HttpClient添加请求头(HttpClientFactory)

前言

在微服务的大环境下，会出现这个服务调用这个接口，那个接口的情况。假设出了问题，需要排查的时候，我们要怎么关联不同服务之间的调用情况呢？换句话就是说，这个请求的结果不对，看看是那里出了问题。

最简单的思路应该就是请求头加一个标识，从头贯穿到尾，这样我们就可以知道，对于这一个请求，在不同的服务都经历了什么样的过程。

在.NET Core时代，相信大部分都是在用HttpClientFactory来创建HttpClient，然后在发起请求。

这篇短文就简单介绍一下如何实现。

示例

我们先定义一个自己的DelegatingHandler，这里取名为HeadersPropagationDelegatingHandler

代码如下：

public class HeadersPropagationDelegatingHandler : DelegatingHandler
{
    private readonly IHttpContextAccessor _accessor;

    public HeadersPropagationDelegatingHandler(IHttpContextAccessor accessor)
    {
        _accessor = accessor;
    }

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {            
        var traceId = string.Empty;

        if (_accessor.HttpContext.Request.Headers.TryGetValue("traceId", out var tId))
        {
            traceId = tId.ToString();
            Console.WriteLine($"{traceId} from request {DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()}.");
        }
        else
        {
            traceId = System.Guid.NewGuid().ToString("N");
            _accessor.HttpContext.Request.Headers.Add("traceId", new Microsoft.Extensions.Primitives.StringValues(traceId));
            Console.WriteLine($"{traceId} from generated {DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()}.");
        }

        if (!request.Headers.Contains("trace-id"))
        {
            request.Headers.TryAddWithoutValidation("traceId", traceId);
        }

        return await base.SendAsync(request, cancellationToken);
    }
}

应该不用太多解释，就是在HttpClient发起请求之前，给它加多一个请求头，这个请求头的值要么是从上一个请求的请求头中取，要么就是重新生成一个。

下面就是主角IHttpMessageHandlerBuilderFilter出场了，它只是一个接口，我们需要自己去实现里面的Configure。

简单的示例如下：

public class HeadersPropagationMessageHandlerBuilderFilter : IHttpMessageHandlerBuilderFilter
{
    private readonly IHttpContextAccessor httpContextAccessor;        
    
    public HeadersPropagationMessageHandlerBuilderFilter(IHttpContextAccessor httpContextAccessor)
    {
        this.httpContextAccessor = httpContextAccessor;
    }

    public Action<HttpMessageHandlerBuilder> Configure(Action<HttpMessageHandlerBuilder> next)
    {
        if (next == null)
        {
            throw new ArgumentNullException(nameof(next));
        }

        return (builder) =>
        {
            next(builder);

            builder.AdditionalHandlers.Add(new HeadersPropagationDelegatingHandler(httpContextAccessor));
        };
    }
}

万事具备，下面我们只需要在Startup中进行注入即可。

public void ConfigureServices(IServiceCollection services)
{
    services.AddHttpContextAccessor();

    services.AddTransient<Ext.HeadersPropagationDelegatingHandler>();
    services.AddSingleton<IHttpMessageHandlerBuilderFilter, Ext.HeadersPropagationMessageHandlerBuilderFilter>();
    services.AddHttpClient();

    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
}

最后就是调用看看效果，这里为了简单，选择创建多个路由，用路由间发起HTTP请求来模拟。当然，最好的还是多个项目模拟。

示例如下：

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    private readonly IHttpClientFactory _clientFactory;

    public ValuesController(IHttpClientFactory clientFactory)
    {
        this._clientFactory = clientFactory;
    }

    // GET api/values
    [HttpGet]
    public async Task<string> GetAsync()
    {
        var traceId = string.Empty;

        if (Request.Headers.TryGetValue("traceId", out var tId))
        {
            traceId = tId.ToString();
            Console.WriteLine($"{traceId} from request {DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()}.");
        }
        else
        {
            traceId = System.Guid.NewGuid().ToString("N");
            Request.Headers.Add("traceId", new Microsoft.Extensions.Primitives.StringValues(traceId));
            Console.WriteLine($"{traceId} from generated {DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()}.");
        }

        using (HttpClient client = new HttpClient())
        {
            client.DefaultRequestHeaders.Clear();
            client.DefaultRequestHeaders.TryAddWithoutValidation("traceId", traceId);

            var res = await client.GetAsync("http://localhost:9898/api/values/demo1");
            var str = await res.Content.ReadAsStringAsync();
            Console.WriteLine($"{traceId} demo1 return {str} at {DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()}");
            return str;
        }
    }

    // GET api/values/demo1
    [HttpGet("demo1")]
    public async Task<string> GetDemo1()
    {
        var client = _clientFactory.CreateClient("demo2");
        var res = await client.GetAsync("http://localhost:9898/api/values/demo2");
        var str = await res.Content.ReadAsStringAsync();            
        return str;
    }

    // GET api/values/demo2
    [HttpGet("demo2")]
    public async Task<string> GetDemo2()
    {
        var client = _clientFactory.CreateClient("demo3");
        var res = await client.GetAsync("http://localhost:9898/api/values/demo3");
        var str = await res.Content.ReadAsStringAsync();
        return str;
    }

    // GET api/values/demo3
    [HttpGet("demo3")]
    public ActionResult<string> GetDemo3()
    {
        return "demo3";
    }
    
    // GET api/values/demo4
    [HttpGet("demo4")]
    public async Task<string> GetDemo4()
    {
        var client = _clientFactory.CreateClient("demo1");
        var res = await client.GetAsync("http://localhost:9898/api/values/demo3");
        var str = await res.Content.ReadAsStringAsync();

        var traceId = string.Empty;
        if (Request.Headers.TryGetValue("traceId", out var tId)) traceId = tId.ToString();
        Console.WriteLine($"{traceId} demo3 return {str} at {DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()}");

        return str;
    }
}