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:dev
docker run -d -p 5051:80 --name api_b api_b:dev
Api_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 Ocelot
Ocelot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。
