Golang实现简单的API网关

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在最近的一个项目中，采用了微服务架构-go-kit进行后端的开发。在微服务架构风格中，一个大应用被拆分成为了多个小的服务系统提供出来，这些小的系统他们可以自成体系，也就是说这些小系统可以拥有自己的数据库，框架甚至语言等，因此我们需要设计一个API 网关(API Gataway)，其实网上已经有较多现成的实现框架，但是本项目的需求是比较简单的，因此将使用Golang自行实现。

实现

API网关是一个服务器，是系统的唯一入口。从面向对象设计的角度看，它与外观模式类似。API网关封装了系统内部架构，为每个客户端提供一个定制的API。它可能还具有其它职责，如身份验证、监控、负载均衡、缓存、请求分片与管理、静态响应处理。

用于实现API网关的技术有很多，大致分为这么几类：

  • 通用反向代理：Nginx、Haproxy、……
  • 网络编程框架：Netty、Servlet、……
  • API网关框架：Spring Cloud Gateway、Zuul、Zuul2、……

API网关最基本的功能就是反向代理。其实现方式有很多，本文将基于标准库net/http/httputil包中的ReverseProxy类型来实现实现一个简单的反向代理。反向代理的实现主要涉及到func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy和type ReverseProxy。

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func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy

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// NewSingleHostReverseProxy returns a new ReverseProxy that routes
// URLs to the scheme, host, and base path provided in target. If the
// target's path is "/base" and the incoming request was for "/dir",
// the target request will be for /base/dir.
// NewSingleHostReverseProxy does not rewrite the Host header.
// To rewrite Host headers, use ReverseProxy directly with a custom
// Director policy.
func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
	targetQuery := target.RawQuery
	director := func(req *http.Request) {
		req.URL.Scheme = target.Scheme
		req.URL.Host = target.Host
		req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
		if targetQuery == "" || req.URL.RawQuery == "" {
			req.URL.RawQuery = targetQuery + req.URL.RawQuery
		} else {
			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
		}
		if _, ok := req.Header["User-Agent"]; !ok {
			// explicitly disable User-Agent so it's not set to default value
			req.Header.Set("User-Agent", "")
		}
	}
	return &ReverseProxy{Director: director}
}

NewSingleHostReverseProxy返回一个新的ReverseProxy，将URLs请求路由到targe的指定的scheme, host, base path。

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// ReverseProxy is an HTTP Handler that takes an incoming request and
// sends it to another server, proxying the response back to the
// client.
type ReverseProxy struct {
	// Director must be a function which modifies
	// the request into a new request to be sent
	// using Transport. Its response is then copied
	// back to the original client unmodified.
	// Director must not access the provided Request
	// after returning.
	Director func(*http.Request)

	Transport http.RoundTripper

	FlushInterval time.Duration

	ErrorLog *log.Logger

	BufferPool BufferPool

	// ModifyResponse is an optional function that modifies the
	// Response from the backend. It is called if the backend
	// returns a response at all, with any HTTP status code.
	// If the backend is unreachable, the optional ErrorHandler is
	// called without any call to ModifyResponse.
	//
	// If ModifyResponse returns an error, ErrorHandler is called
	// with its error value. If ErrorHandler is nil, its default
	// implementation is used.
	ModifyResponse func(*http.Response) error

	ErrorHandler func(http.ResponseWriter, *http.Request, error)
}

ReverseProxy类型有两个重要的属性，分别是Director和ModifyResponse，这两个属性都是函数类型，在接收到客户端请求时，ServeHTTP函数首先调用Director函数对接受到的请求体进行修改，例如修改请求的目标地址、请求头等；然后使用修改后的请求体发起新的请求，接收到响应后，调用ModifyResponse函数对响应进行修改，最后将修改后的响应体拷贝并响应给客户端，这样就实现了反向代理的整个流程。

在NewSingleHostReverseProxy中源码已经对传入的URLs进行解析并且完成了Director的修改，我们只需要调用NewSingleHostReverseProxy函数并且传入目标服务器的URL即可，一个简单的反向代理就完成了啦。

代码

实例代码只涉及微服务中 user 与 auth模块，可以根据实际需求自行修改部分

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package main

import (
	"fmt"
	"log"
	"net/http"
	"net/http/httputil"
	"net/url"
	"strings"
)

type handle struct {
	host string
	port string
}

type Service struct {
	auth *handle
	user *handle
}

func (this *Service) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	var remote *url.URL
	if strings.Contains(r.RequestURI, "api/auth") {
		remote, _ = url.Parse("http://" + this.auth.host + ":" + this.auth.port)
	} else if strings.Contains(r.RequestURI, "api/user") {
		remote, _ = url.Parse("http://" + this.user.host + ":" + this.user.port)
	} else {
		fmt.Fprintf(w, "404 Not Found")
		return
	}
	proxy := httputil.NewSingleHostReverseProxy(remote)
	proxy.ServeHTTP(w, r)
}

func startServer() {
	// 注册被代理的服务器 (host， port)
	service := &Service{
		auth: &handle{host: "127.0.0.1", port: "8081"},
		user: &handle{host: "127.0.0.1", port: "8082"},
	}
	err := http.ListenAndServe(":8888", service)
	if err != nil {
		log.Fatalln("ListenAndServe: ", err)
	}
}

func main() {
	startServer()
}

参考链接