前言
官方文档对 HttpClientHandler.MaxConnectionsPerServer 属性有如下说明：
获取或设置使用 HttpClient 对象发出请求时允许的最大并发连接数（每个服务器终结点）。请注意，该限制针对每个服务器终结点，例如，值为 256 表示允许 256 个到 http://www.adatum.com/ 的并发连接，以及另外 256 个到 http://www.adventure-works.com/ 的并发连接。
有网友在交流群中询问，终结点到底指的啥，怎么判断是同一终结点？.
让我们通过源码来看看吧！
源码分析
HttpClientHandler.MaxConnectionsPerServer
HttpClientHandler.MaxConnectionsPerServer 的实现代码如下：
public int MaxConnectionsPerServer
{
get => _underlyingHandler.MaxConnectionsPerServer;
set => _underlyingHandler.MaxConnectionsPerServer = value;
}
实际使用的是_underlyingHandler.MaxConnectionsPerServer。而_underlyingHandler是SocketsHttpHandler类的实例:
using HttpHandlerType = System.Net.Http.SocketsHttpHandler;
private readonly HttpHandlerType _underlyingHandler;
SocketsHttpHandler.MaxConnectionsPerServer
SocketsHttpHandler.MaxConnectionsPerServer 的实现代码如下：
public int MaxConnectionsPerServer
{
get => _settings._maxConnectionsPerServer;
set
{
...
_settings._maxConnectionsPerServer = value;
}
}
实际使用的是_settings._maxConnectionsPerServer。那么，谁在使用这个设置值呢？
HttpConnectionPool
我们定位到了HttpConnectionPool的构造函数：
public HttpConnectionPool(HttpConnectionPoolManager poolManager, HttpConnectionKind kind, string? host, int port, string? sslHostName, Uri? proxyUri)
{
...
_maxHttp11Connections = Settings._maxConnectionsPerServer;
...
}
而HttpConnectionPool的作用就是，提供到同一终结点的连接池。看来我们离真相越来越近了。
/// <summary>Provides a pool of connections to the same endpoint.</summary>
internal sealed class HttpConnectionPool : IDisposable
HttpConnectionPoolManager
HttpConnectionPool的构造函数只在HttpConnectionPoolManager类中被调用：
public ValueTask<HttpResponseMessage> SendAsyncCore(HttpRequestMessage request, Uri? proxyUri, bool async, bool doRequestAuth, bool isProxyConnect, CancellationToken cancellationToken)
{
HttpConnectionKey key = GetConnectionKey(request, proxyUri, isProxyConnect);
HttpConnectionPool? pool;
while (!_pools.TryGetValue(key, out pool))
{
pool = new HttpConnectionPool(this, key.Kind, key.Host, key.Port, key.SslHostName, key.ProxyUri);
可以看到，HttpConnectionPool是从ConcurrentDictionary _pools 中获取的，而key的值是HttpConnectionKey类型。
HttpConnectionKey
而 HttpConnectionKey 是这样判断是否相等的：
public readonly HttpConnectionKind Kind;
public readonly string? Host;
public readonly int Port;
public readonly string? SslHostName; // null if not SSL
public readonly Uri? ProxyUri;
public readonly string Identity;
public bool Equals(HttpConnectionKey other) =>
Kind == other.Kind &&
Host == other.Host &&
Port == other.Port &&
ProxyUri == other.ProxyUri &&
SslHostName == other.SslHostName &&
Identity == other.Identity;
结论
由上面的代码，我们可以得出如下结论：
如果连接类型、服务器地址、服务器端口、代理（如果有）全部相同，则认为是同一终结点。
