通过HttpClientFactory实现动态代理

Question

我有Asp.Net Core WebApi。我正在根据HttpClientFactory pattern进行Http请求。下面是我的示例代码：

public void ConfigureServices(IServiceCollection services)
{
    ...
    services.AddHttpClient<IMyInterface, MyService>();
    ...
}

public class MyService: IMyInterface
{
    private readonly HttpClient _client;

    public MyService(HttpClient client)
    {
        _client = client;
    }

    public async Task CallHttpEndpoint()
    {
        var request = new HttpRequestMessage(HttpMethod.Get, "www.customUrl.com");
        var response = await _client.SendAsync(request);
        ...
    }

}

我想实现通过动态代理发送请求。这基本上意味着我可能需要在每次请求时更改代理。到目前为止，我发现了两种方法，没有一种对我来说是好的：

1.有一个像这样的静态代理：

public void ConfigureServices(IServiceCollection services)
{
    ...
    services.AddHttpClient<IMyInterface, MyService>().ConfigurePrimaryHttpMessageHandler(() =>
        {
            return new HttpClientHandler
            {
                Proxy = new WebProxy("http://127.0.0.1:8888"),
                UseProxy = true
            };
        });
    ...
}

但在这种方法中，每个服务只能有一个代理。

2.对每个请求处理HttpClient：

    HttpClientHandler handler = new HttpClientHandler()
    {
        Proxy = new WebProxy("http://127.0.0.1:8888"),
        UseProxy = true,
    };

    using(var client = new HttpClient(handler))
    {
        var request = new HttpRequestMessage(HttpMethod.Get, "www.customUrl.com");
        var response = await client.SendAsync(request);
        ...
    }

但在这种情况下，我违反了HttpClientFactory模式，并且可能会导致应用程序性能出现问题，如下面的article所述

有没有第三种方法可以动态地更改代理，而不需要重新创建HttpClient

Answer 1

实例化后，无法更改HttpClientHandler的任何属性，也无法将新版本的HttpClientHandler分配给现有的HttpClient。因此，不可能为特定的HttpClient指定动态代理:您只能指定一个代理。

实现这一点的正确方法是使用命名客户端，并为每个代理端点定义一个客户端。然后，您将需要注入IHttpClientFactory并选择要使用的代理之一，请求实现该代理的指定客户端。

services.AddHttpClient("MyServiceProxy1").ConfigurePrimaryHttpMessageHandler(() =>
{
    return new HttpClientHandler
    {
        Proxy = new WebProxy("http://127.0.0.1:8888"),
        UseProxy = true
    };
});

services.AddHttpClient("MyServiceProxy2").ConfigurePrimaryHttpMessageHandler(() =>
{
    return new HttpClientHandler
    {
        Proxy = new WebProxy("http://127.0.0.1:8889"),
        UseProxy = true
    };
});

...

然后：

public class MyService : IMyInterface
{
    private readonly HttpClient _client;

    public MyService(IHttpClientFactory httpClientFactory)
    {
        _client = httpClientFactory.CreateClient("MyServiceProxy1");
    }

    public async Task CallHttpEndpoint()
    {
        var request = new HttpRequestMessage(HttpMethod.Get, "www.customUrl.com");
        var response = await _client.SendAsync(request);
        ...
    }
}

Answer 2

我可以通过继承HttpClientHandler来实现：

public class ProxyHttpHandler : HttpClientHandler
{
    private int currentProxyIndex = 0;

private ProxyOptions proxyOptions;

public ProxyHttpHandler(IOptions<ProxyOptions> options)
{
    proxyOptions = options != null ? options.Value : throw new ArgumentNullException(nameof(options));
    UseProxy = true;
}

protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
    var proxy = proxyOptions.Proxies[currentProxyIndex];
    var proxyResolver = new WebProxy(proxy.Host, proxy.Port)
    {
        Credentials = proxy.Credentials
    };

    Proxy = proxyResolver;

    currentProxyIndex++;
    if(currentProxyIndex >= proxyOptions.Proxies.Count)
        currentProxyIndex = 0;

    return base.SendAsync(request, cancellationToken);
}

}

然后我在IoC中注册我的ProxyHttpHandler和ProxyOptions：

public IForksCoreConfigurationBuilder ConfigureProxy(Action<ProxyOptions> options)
{
    Services.AddOptions<ProxyOptions>().Configure(options);
    Services.AddTransient<ProxyHttpHandler>();
    Services.AddHttpClient<IService, MyService>()
            .ConfigurePrimaryHttpMessageHandler<ProxyHttpHandler>();

    return this;
}