.NET Core 下的 API 网关
网关介绍
网关其实就是将我们写好的API全部放在一个统一的地址暴露在公网，提供访问的一个入口。在 .NET Core下可以使用Ocelot来帮助我们很方便的接入API 网关。与之类似的库还有ProxyKit，微软也发布了一个反向代理的库YARP。
关于网关的介绍不多说了，网上文章也挺多的，这些都是不错的选择，听说后期Ocelot将会使用YARP来重写。本篇主要实践一下在.NET Core环境下使用Ocelot。
- Ocelot官网：https://threemammals.com/ocelot
- Ocelot文档：https://ocelot.readthedocs.io
- GitHub：https://github.com/ThreeMammals/Ocelot
- Ocelot资源汇总：https://www.cnblogs.com/shanyou/p/10363360.html
接入使用
接口示例
先创建几个项目用于测试，创建两个默认的API项目，Api_A和Api_B，在创建一个网关项目Api_Gateway，网关项目可以选择空的模板。
现在分别在Api_A和Api_B中写几个api，将默认的WeatherForecastController中返回模型WeatherForecast添加一个字段Source，用于区分是哪个API返回的数据。
using System;namespace Api_A{ public class WeatherForecast { public string Source { get; set; } = "Api_A"; public DateTime Date { get; set; } public int TemperatureC { get; set; } public int TemperatureF => 32 + (int)(TemperatureC / 0.5556); public string Summary { get; set; } }}using System;namespace Api_B{ public class WeatherForecast { public string Source { get; set; } = "Api_B"; public DateTime Date { get; set; } public int TemperatureC { get; set; } public int TemperatureF => 32 + (int)(TemperatureC / 0.5556); public string Summary { get; set; } }}直接使用WeatherForecastController默认方法，在路由中添加api前缀。
using Microsoft.AspNetCore.Mvc;using System;using System.Collections.Generic;using System.Linq;namespace Api_A.Controllers{ [ApiController] [Route("api/[controller]")] public class WeatherForecastController : ControllerBase { private static readonly string[] Summaries = new[] { "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching" }; [HttpGet] public IEnumerable<WeatherForecast> Get() { var rng = new Random(); return Enumerable.Range(1, 5).Select(index => new WeatherForecast { Date = DateTime.Now.AddDays(index), TemperatureC = rng.Next(-20, 55), Summary = Summaries[rng.Next(Summaries.Length)] }).ToArray(); } }}using Microsoft.AspNetCore.Mvc;using System;using System.Collections.Generic;using System.Linq;namespace Api_B.Controllers{ [ApiController] [Route("api/[controller]")] public class WeatherForecastController : ControllerBase { private static readonly string[] Summaries = new[] { "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching" }; [HttpGet] public IEnumerable<WeatherForecast> Get() { var rng = new Random(); return Enumerable.Range(1, 5).Select(index => new WeatherForecast { Date = DateTime.Now.AddDays(index), TemperatureC = rng.Next(-20, 55), Summary = Summaries[rng.Next(Summaries.Length)] }).ToArray(); } }}再分别在Api_A和Api_B中添加两个控制器：ApiAController、ApiBController，然后加上几个简单的restful api。
using Microsoft.AspNetCore.Mvc;using System.Collections.Generic;namespace Api_A.Controllers{ [Route("api/[controller]")] [ApiController] public class ApiAController : ControllerBase { [HttpGet] public IEnumerable<string> Get() { return new string[] { "value1", "value2" }; } [HttpGet("{id}")] public string Get(int id) { return $"Get：{id}"; } [HttpPost] public string Post([FromForm] string value) { return $"Post:{value}"; } [HttpPut("{id}")] public string Put(int id, [FromForm] string value) { return $"Put:{id}:{value}"; } [HttpDelete("{id}")] public string Delete(int id) { return $"Delete:{id}"; } }}using Microsoft.AspNetCore.Mvc;using System.Collections.Generic;namespace Api_B.Controllers{ [Route("api/[controller]")] [ApiController] public class ApiBController : ControllerBase { [HttpGet] public IEnumerable<string> Get() { return new string[] { "value1", "value2" }; } [HttpGet("{id}")] public string Get(int id) { return $"Get：{id}"; } [HttpPost] public string Post([FromForm] string value) { return $"Post:{value}"; } [HttpPut("{id}")] public string Put(int id, [FromForm] string value) { return $"Put:{id}:{value}"; } [HttpDelete("{id}")] public string Delete(int id) { return $"Delete:{id}"; } }}方便查看接口，这里添加一下swagger组件，这样我们Api_A和Api_B项目分别就有了6个接口。
接着打包docker镜像，放在docker中运行这两个api项目。这一步可以用任何你熟悉的方式，run起来即可。
docker build -t api_a:dev -f ./Api_A/Dockerfile .docker build -t api_b:dev -f ./Api_B/Dockerfile .build成功后，指定两个端口运行api项目。
docker run -d -p 5050:80 --name api_a api_a:devdocker run -d -p 5051:80 --name api_b api_b:devApi_A指定了5050端口，通过 http://localhost:5050/swagger打开可以看到swagger文档界面，Api_B指定了5051端口，通过 http://localhost:5051/swagger打开可以看到swagger文档界面，这样就大功告成了，接下来才是重点将两个api项目配置到Api_Gateway网关项目中。
配置网关
在网关项目Api_Gateway中都添加Ocelot组件包。
Install-Package OcelotOcelot中最关键的就是配置路由信息，新建一个ocelot.json配置文件，将我们的两个API接口匹配规则放进去。
{ "Routes": [ //ApiA { "DownstreamPathTemplate": "/api/WeatherForecast", "DownstreamScheme": "http", "DownstreamHostAndPorts": [ { "Host": "localhost", "Port": 5050 } ], "UpstreamPathTemplate": "/ApiA/WeatherForecast", "UpstreamHttpMethod": [ "Get" ] }, { "DownstreamPathTemplate": "/api/ApiA", "DownstreamScheme": "http", "DownstreamHostAndPorts": [ { "Host": "localhost", "Port": 5050 } ], "UpstreamPathTemplate": "/ApiA", "UpstreamHttpMethod": [ "Get", "POST" ] }, { "DownstreamPathTemplate": "/api/ApiA/{id}", "DownstreamScheme": "http", "DownstreamHostAndPorts": [ { "Host": "localhost", "Port": 5050 } ], "UpstreamPathTemplate": "/ApiA/{id}", "UpstreamHttpMethod": [ "Get", "Put", "Delete" ] }, //ApiB { "DownstreamPathTemplate": "/api/WeatherForecast", "DownstreamScheme": "http", "DownstreamHostAndPorts": [ { "Host": "localhost", "Port": 5051 } ], "UpstreamPathTemplate": "/ApiB/WeatherForecast", "UpstreamHttpMethod": [ "Get" ] }, { "DownstreamPathTemplate": "/api/ApiB", "DownstreamScheme": "http", "DownstreamHostAndPorts": [ { "Host": "localhost", "Port": 5051 } ], "UpstreamPathTemplate": "/ApiB", "UpstreamHttpMethod": [ "Get", "POST" ] }, { "DownstreamPathTemplate": "/api/ApiB/{id}", "DownstreamScheme": "http", "DownstreamHostAndPorts": [ { "Host": "localhost", "Port": 5051 } ], "UpstreamPathTemplate": "/ApiB/{id}", "UpstreamHttpMethod": [ "Get", "Put", "Delete" ] } ], "GlobalConfiguration": { "BaseUrl": "https://localhost:44335" }}关于配置文件中的各项具体含义，可以参考官方文档中的介绍。主要就是将DownstreamPathTemplate模板内容转换为UpstreamPathTemplate模板内容进行接口的访问，同时可以指定HTTP请求的方式等等。GlobalConfiguration中的BaseUrl为我们暴漏出去的网关地址。
设置好ocelot.json后，需要在代码中使用它，在Program.cs中添加配置文件。
using Microsoft.AspNetCore.Hosting;using Microsoft.Extensions.Configuration;using Microsoft.Extensions.Hosting;namespace Api_Gateway{ public class Program { public static void Main(string[] args) { CreateHostBuilder(args).Build().Run(); } public static IHostBuilder CreateHostBuilder(string[] args) => Host.CreateDefaultBuilder(args) .ConfigureAppConfiguration((context, config) => { config.AddJsonFile("ocelot.json", optional: false, reloadOnChange: true); }) .ConfigureWebHostDefaults(webBuilder => { webBuilder.UseStartup<Startup>(); }); }}在Startup.cs中使用Ocelot。
using Microsoft.AspNetCore.Builder;using Microsoft.AspNetCore.Hosting;using Microsoft.AspNetCore.Http;using Microsoft.Extensions.DependencyInjection;using Microsoft.Extensions.Hosting;using Ocelot.DependencyInjection;using Ocelot.Middleware;namespace Api_Gateway{ public class Startup { public void ConfigureServices(IServiceCollection services) { services.AddOcelot(); } public void Configure(IApplicationBuilder app, IWebHostEnvironment env) { if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); } app.UseRouting(); app.UseEndpoints(endpoints => { endpoints.MapGet("/", async context => { await context.Response.WriteAsync("Hello World!"); }); }); app.UseOcelot().Wait(); } }}完成以上操作后，我们试着去调用接口看看能否正确获取预期数据。
curl -X GET "https://localhost:44335/ApiA"curl -X GET "https://localhost:44335/ApiB"curl -X POST "https://localhost:44335/ApiA" -H "Content-Type: multipart/form-data" -F "value=ApiA"curl -X POST "https://localhost:44335/ApiB" -H "Content-Type: multipart/form-data" -F "value=ApiB"curl -X GET "https://localhost:44335/ApiA/12345"curl -X GET "https://localhost:44335/ApiB/12345"curl -X PUT "https://localhost:44335/ApiA/12345" -H "Content-Type: multipart/form-data" -F "value=ApiA"curl -X PUT "https://localhost:44335/ApiB/12345" -H "Content-Type: multipart/form-data" -F "value=ApiB"curl -X DELETE "https://localhost:44335/ApiA/12345"curl -X DELETE "https://localhost:44335/ApiB/12345"curl -X GET "https://localhost:44335/ApiA/WeatherForecast"curl -X GET "https://localhost:44335/ApiB/WeatherForecast"可以看到，两个项目中的接口全部可以通过网关项目暴露的地址进行中转，是不是很方便？
本篇只是简单的应用，对于Ocelot的功能远不止于此，它非常强大，还可以实现请求聚合、服务发现、认证、鉴权、限流熔断、并内置了负载均衡器，而且这些功能都是只需要简单的配置即可完成。就不一一描述了，如有实际开发需求和问题，可以查看官方文档和示例。
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