HttpClientStudy/Docs/1.2.使用准则.ipynb

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   "source": [
    "# HttpClient 使用准则\n",
    "System.Net.Http.HttpClient 类用于发送 HTTP 请求以及从 URI 所标识的资源接收 HTTP 响应。 HttpClient 实例是应用于该实例执行的所有请求的设置集合，每个实例使用自身的连接池，该池将其请求与其他请求隔离开来。 \n",
    "\n",
    "从 .NET Core 2.1 开始，SocketsHttpHandler 类提供实现，使行为在所有平台上保持一致。"
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    "## 1、DNS 行为\n",
    "HttpClient 仅在创建连接时解析 DNS。它不跟踪 DNS 服务器指定的任何生存时间 (TTL)。 \n",
    "\n",
    "如果 DNS 条目定期更改（这可能在某些方案中发生），客户端将不会遵循这些更新。 要解决此问题，可以通过设置 PooledConnectionLifetime 属性来限制连接的生存期，以便在替换连接时重复执行 DNS 查找。"
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   "source": [
    "using System.Net.Http;\n",
    "\n",
    "var handler = new SocketsHttpHandler\n",
    "{\n",
    "    // Recreate every 15 minutes\n",
    "    PooledConnectionLifetime = TimeSpan.FromMinutes(15) \n",
    "};\n",
    "var sharedClient = new HttpClient(handler);"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "上述 HttpClient 配置为重复使用连接 15 分钟。 PooledConnectionLifetime 指定的时间范围过后，系统会关闭连接，然后创建一个新连接。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 2、共用连接(底层自动管理连接池)"
   ]
  },
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   "source": [
    "HttpClient 的连接池链接到其基础 SocketsHttpHandler。 \n",
    "释放 HttpClient 实例时，它会释放池中的所有现有连接。 如果稍后向同一服务器发送请求，则必须重新创建一个新连接。 \n",
    "因此，创建不必要的连接会导致性能损失。 \n",
    "此外，TCP 端口不会在连接关闭后立即释放。 （有关这一点的详细信息，请参阅 RFC 9293 中的 TCP TIME-WAIT。）如果请求速率较高，则可用端口的操作系统限制可能会耗尽。 \n",
    "\n",
    "为了避免端口耗尽问题，建议将 HttpClient 实例重用于尽可能多的 HTTP 请求。"
   ]
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   "source": [
    "### 什么是连接池\n",
    "SocketsHttpHandler为每个唯一端点建立连接池，您的应用程序通过HttpClient向该唯一端点发出出站HTTP请求。在对端点的第一个请求上，当不存在现有连接时，将建立一个新的HTTP连接并将其用于该请求。该请求完成后，连接将保持打开状态并返回到池中。\n",
    "\n",
    "对同一端点的后续请求将尝试从池中找到可用的连接。如果没有可用的连接，并且尚未达到该端点的连接限制，则将建立新的连接。达到连接限制后，请求将保留在队列中，直到连接可以自由发送它们为止。"
   ]
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  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 如何控制连接池"
   ]
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   "source": [
    "有三个主要设置可用于控制连接池的行为。\n",
    "\n",
    "+ PooledConnectionLifetime，定义连接在池中保持活动状态的时间。此生存期到期后，将不再为将来的请求而合并或发出连接。\n",
    "\n",
    "+ PooledConnectionIdleTimeout，定义闲置连接在未使用时在池中保留的时间。一旦此生存期到期，空闲连接将被清除并从池中删除。\n",
    "\n",
    "+ MaxConnectionsPerServer，定义每个端点将建立的最大出站连接数。每个端点的连接分别池化。例如，如果最大连接数为2，则您的应用程序将请求发送到两个www.github.com和www.google.com，总共可能最多有4个打开的连接。\n",
    "\n",
    "默认情况下，从.NET Core 2.1开始，更高级别的HttpClientHandler将SocketsHttpHandler用作内部处理程序。没有任何自定义配置，将应用连接池的默认设置。\n",
    "\n",
    "该**PooledConnectionLifetime默认是无限的，因此，虽然经常使用的请求，连接可能会无限期地保持打开状态。该PooledConnectionIdleTimeout默认为2分钟，如果在连接池中长时间未使用将被清理。MaxConnectionsPerServer**默认为int.MaxValue，因此连接基本上不受限制。\n",
    "\n",
    "如果希望控制这些值中的任何一个，则可以手动创建SocketsHttpHandler实例，并根据需要进行配置。"
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   "source": [
    "using System.Net.Http;\n",
    "var socketsHandler = new SocketsHttpHandler\n",
    "{\n",
    "\tPooledConnectionLifetime = TimeSpan.FromMinutes(10),\n",
    "\tPooledConnectionIdleTimeout = TimeSpan.FromMinutes(5),\n",
    "\tMaxConnectionsPerServer = 10\n",
    "};\n",
    "\t\n",
    "var client = new HttpClient(socketsHandler);"
   ]
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   "cell_type": "markdown",
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   "source": [
    "在前面的示例中，对SocketsHttpHandler进行了配置，以使连接将最多在10分钟后停止重新发出并关闭。如果闲置5分钟，则连接将在池的清理过程中被更早地删除。我们还将最大连接数（每个端点）限制为十个。如果我们需要并行发出更多出站请求，则某些请求可能会排队等待，直到10个池中的连接可用为止。\n",
    "要应用处理程序，它将被传递到HttpClient的构造函数中。"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 测试连接寿命"
   ]
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   "source": [
    "using System.Net;\n",
    "using System.Net.Http;\n",
    "\n",
    "var ips = await Dns.GetHostAddressesAsync(\"www.hao123.com\");\n",
    "string firstIp = ips.FirstOrDefault().ToString();\n",
    "\t\n",
    "foreach (var ipAddress in ips)\n",
    "{\n",
    "    Console.WriteLine(ipAddress.MapToIPv4().ToString());\n",
    "}\n",
    "\n",
    "//自定义行为\n",
    "var socketsHandler = new SocketsHttpHandler\n",
    "{\n",
    "    //连接池生命周期为10分钟：连接在池中保持活动时间为10分钟\n",
    "    PooledConnectionLifetime = TimeSpan.FromMinutes(10),\n",
    "\n",
    "    //池化链接的空闲超时时间为5分钟: 5分钟内连接不被重用，则被释放后销毁\n",
    "    PooledConnectionIdleTimeout = TimeSpan.FromMinutes(5),\n",
    "    \n",
    "    //每端点的最大连接数设置为10个\n",
    "    MaxConnectionsPerServer = 10\n",
    "};\n",
    "\n",
    "var client = new HttpClient(socketsHandler);\n",
    "\n",
    "for (var i = 0; i < 5; i++)\n",
    "{\n",
    "    _ = await client.GetAsync(\"https://www.hao123.com\");\n",
    "    await Task.Delay(TimeSpan.FromSeconds(2));\n",
    "}\n",
    "\n",
    "Console.WriteLine(\"请在程序退出后，执行下面命令行查看网络情况\");\n"
   ]
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   "metadata": {},
   "source": [
    "使用我们刚刚讨论的设置，此代码依次向同一端点发出5个请求。在每个请求之间，它会暂停两秒钟。该代码还输出从DNS检索到的Google服务器的IPv4地址。我们可以使用此IP地址来查看通过PowerShell中发出的netstat命令对其打开的连接："
   ]
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   "source": [
    "#!set --value @csharp:firstIp --name queryIp\n",
    "Write-Host \"请先执行上面的单元，再执行本单元\"\n",
    "Write-Host \"异常话，很可能是：未查找IP为 $queryIp 的网络状\"\n",
    "\n",
    "netstat -ano | findstr $queryIp"
   ]
  },
  {
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   "metadata": {},
   "source": [
    "## 3、推荐使用方式"
   ]
  },
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   "source": [
    "#!set --value @csharp:ips --name ips\n"
   ]
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   "source": [
    "#!set --value @csharp:ips --name ips\n"
   ]
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   "source": [
    "#!set --value @csharp:ips --name ips\n"
   ]
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   "outputs": [],
   "source": [
    "#!set --value @csharp:firstIp --name firstIp\n"
   ]
  },
  {
   "cell_type": "code",
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   "outputs": [],
   "source": [
    "#!set --value @csharp:firstIp --name firstIp\n"
   ]
  },
  {
   "cell_type": "code",
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    "dotnet_interactive": {
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     "kernelName": "javascript"
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    "vscode": {
     "languageId": "polyglot-notebook"
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   "outputs": [],
   "source": [
    "#!set --value @csharp:firstIp --name firstIp\n"
   ]
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   "cell_type": "code",
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   "outputs": [],
   "source": [
    "#!set --value @pwsh:$sss --name $sss\n"
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