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。
func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy
// 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。
// 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
package mainimport (
"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.URLif 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()}
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