Golang http.RoundTripper 笔记

msyxingfu

2019-06-28

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

接口定义

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

需要实现RoundTrip函数

type SomeClient struct {}

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

场景

原文: https://lanre.wtf/blog/2017/0...

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
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

Golang http.RoundTripper 笔记

限速(Rate limiting) 控制请求速率

实际的例子

实现http.RoundTripper 缓存 http response的逻辑。

一个http server的实现

import (
    "fmt"
    "net/http"
)

func main() {
    // server/main.go
    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 get here
        fmt.Println("The request actually got here")

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

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

http client中创建新的 http.Transport 实现 http.RoundTripper接口

主程序main实现
https://github.com/adelowo/ro...

func main() {
    cachedTransport := newTransport()

    // cachedTransport 是自定义实现http.RoundTripper接口的 Transport
    client := &http.Client{
        Transport: cachedTransport,
        Timeout:   time.Second * 5,
    }

    // 每5秒清除缓存
    cacheClearTicker := time.NewTicker(time.Second * 5)

    //每秒请求一次，可以看出response是从缓存获取还是从服务器请求
    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 {
        panic("Whoops")
    }

    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))
        }
    }
}

cacheTransport 中 RoundTrip 函数实现读取缓存中的reponse

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
}

运行结果

Golang http.RoundTripper 笔记