Golang http.RoundTripper

参考
Golang http.RoundTripper 笔记

RoundTripper is an interface representing the ability to execute a single HTTP transaction, obtaining the Response for a given Request.

对于http客户端，可以使用不同的实现了 RoundTripper 接口的Transport实现来配置它的行为。RoundTripper 有点像 http.Client 的中间件

一、接口定义

type RoundTripper interface { 
       RoundTrip(*Request) (*Response, error)
}

实现比如

type SomeClient struct {}

func (s *SomeClient) RoundTrip(r *http.Request)(*http.Response, error) {
    //Something comes here...Maybe
    return nil,nil
}

二、应用场景

参考https://lanre.wtf/blog/2017/07/24/roundtripper-go/
1.缓存 responses，比如 app需要访问 Github api，获取 trending repos，这个数据变动不频繁，假设30分钟变动一次，你显然不希望每次都要点击api都要来请求Github api，解决这个问题的方法是实现这样的http.RoundTripper

• 有缓存时从缓存取出response数据
• 过期，数据通过重新请求api获取

2.根据需要设置http header, 一个容易想到的例子go-github一个Github的 api的go客户端。某些github api不需要认证，有些需要认证则需要提供自己的http client，比如 ghinstallation,下面是ghinstallation 的 RoundTrip 函数实现，设置 Authorization 头

image.png

3.限速(Rate limiting) 控制请求速率

三、实例https://github.com/adelowo/rounder

1.server.go

package main

import (
    "fmt"
    "net/http"
)

func main() {

    mux := http.NewServeMux()

    mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        // This is here so we can actually see that the responses 
        // that have been cached don't actually get here
        fmt.Println("The request actually got here")

        w.Write([]byte("You got here"))
    })

    http.ListenAndServe(":8000", mux)
}

2.client.go

package main

import (
    "bufio"
    "bytes"
    "errors"
    "fmt"
    "io/ioutil"
    "log"
    "net/http"
    "net/http/httputil"
    "os"
    "os/signal"
    "strings"
    "sync"
    "syscall"
    "time"
)

func main() {

    cachedTransport := newTransport()

    //Create a custom client so we can make use of our RoundTripper
    //If you make use of http.Get(), the default http client located at http.DefaultClient is used instead
    //Since we have special needs, we have to make use of our own http.RoundTripper implementation
    client := &http.Client{
        Transport: cachedTransport,
        Timeout:   time.Second * 5,
    }

    //Time to clear the cache store so we can make request to the original server
    cacheClearTicker := time.NewTicker(time.Second * 5)

    //Make a new request every second
    //This would help demonstrate if the response is actually coming from the real server or from the cache
    reqTicker := time.NewTicker(time.Second * 1)

    terminateChannel := make(chan os.Signal, 1)

    signal.Notify(terminateChannel, syscall.SIGTERM, syscall.SIGHUP)

    req, err := http.NewRequest(http.MethodGet, "http://localhost:8000", strings.NewReader(""))

    if err != nil {
        log.Fatalf("An error occurred ... %v", err)
    }

    for {
        select {
        case <-cacheClearTicker.C:
            // Clear the cache so we can hit the original server
            cachedTransport.Clear()

        case <-terminateChannel:
            cacheClearTicker.Stop()
            reqTicker.Stop()
            return

        case <-reqTicker.C:

            resp, err := client.Do(req)

            if err != nil {
                log.Printf("An error occurred.... %v", err)
                continue
            }

            buf, err := ioutil.ReadAll(resp.Body)

            if err != nil {
                log.Printf("An error occurred.... %v", err)
                continue
            }

            fmt.Printf("The body of the response is \"%s\" \n\n", string(buf))
        }
    }
}

func cacheKey(r *http.Request) string {
    return r.URL.String()
}

type cacheTransport struct {
    data              map[string]string
    mu                sync.RWMutex
    originalTransport http.RoundTripper
}

func (c *cacheTransport) Set(r *http.Request, value string) {
    c.mu.Lock()
    defer c.mu.Unlock()
    fmt.Println("cache key:",cacheKey(r))
    fmt.Println("cache value:",value)
    c.data[cacheKey(r)] = value
}

func (c *cacheTransport) Get(r *http.Request) (string, error) {
    c.mu.RLock()
    defer c.mu.RUnlock()

    if val, ok := c.data[cacheKey(r)]; ok {
        return val, nil
    }

    return "", errors.New("key not found in cache")
}

// There be dragons!!!
func (c *cacheTransport) RoundTrip(r *http.Request) (*http.Response, error) {

    // Check if we have the response cached..
    // If yes, we don't have to hit the server
    // We just return it as is from the cache store.
    if val, err := c.Get(r); err == nil {
        fmt.Println("Fetching the response from the cache")
        return cachedResponse([]byte(val), r)
    }

    // Ok, we don't have the response cached, the store was probably cleared.
    // Make the request to the server.
    resp, err := c.originalTransport.RoundTrip(r)

    if err != nil {
        return nil, err
    }

    // Get the body of the response so we can save it in the cache for the next request.
    buf, err := httputil.DumpResponse(resp, true)

    if err != nil {
        return nil, err
    }

    // Saving it to the cache store
    c.Set(r, string(buf))

    fmt.Println("Fetching the data from the real source")
    return resp, nil
}

func (c *cacheTransport) Clear() error {
    c.mu.Lock()
    defer c.mu.Unlock()

    c.data = make(map[string]string)
    return nil
}

func cachedResponse(b []byte, r *http.Request) (*http.Response, error) {
    buf := bytes.NewBuffer(b)
    return http.ReadResponse(bufio.NewReader(buf), r)
}

func newTransport() *cacheTransport {
    return &cacheTransport{
        data:              make(map[string]string),
        originalTransport: http.DefaultTransport,
    }
}

四、Go 标准库剖析 1（transport http 请求的承载者）