Go如何实现HTTP请求限流示例

算法描述：

用户配置的平均发送速率为r，则每隔1/r秒一个令牌被加入到桶中（每秒会有r个令牌放入桶中），桶中最多可以存放b个令牌。如果令牌到达时令牌桶已经满了，那么这个令牌会被丢弃；

实现用户粒度的限流

虽然在某些情况下使用单个全局速率限制器非常有用，但另一种常见情况是基于IP地址或API密钥等标识符为每个用户实施速率限制器。我们将使用IP地址作为标识符。简单实现代码如下：

package main
import (
  "net/http"
  "sync"
  "time"

  "golang.org/x/time/rate"
)

// Create a custom visitor struct which holds the rate limiter for each
// visitor and the last time that the visitor was seen.
type visitor struct {
  limiter *rate.Limiter
  lastSeen time.Time
}

// Change the the map to hold values of the type visitor.
var visitors = make(map[string]*visitor)
var mtx sync.Mutex
// Run a background goroutine to remove old entries from the visitors map.
func init() {
  go cleanupVisitors()
}

func addVisitor(ip string) *rate.Limiter {
  limiter := rate.NewLimiter(2, 5)
  mtx.Lock()
  // Include the current time when creating a new visitor.
  visitors[ip] = &visitor{limiter, time.Now()}
  mtx.Unlock()
  return limiter
}

func getVisitor(ip string) *rate.Limiter {
  mtx.Lock()
  v, exists := visitors[ip]
  if !exists {
    mtx.Unlock()
    return addVisitor(ip)
  }
  // Update the last seen time for the visitor.
  v.lastSeen = time.Now()
  mtx.Unlock()
  return v.limiter
}

// Every minute check the map for visitors that haven't been seen for
// more than 3 minutes and delete the entries.
func cleanupVisitors() {
  for {
    time.Sleep(time.Minute)
    mtx.Lock()
    for ip, v := range visitors {
      if time.Now().Sub(v.lastSeen) > 3*time.Minute {
        delete(visitors, ip)
      }
    }
    mtx.Unlock()
  }
}

func limit(next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    limiter := getVisitor(r.RemoteAddr)
    if limiter.Allow() == false {
      http.Error(w, http.StatusText(429), http.StatusTooManyRequests)
      return
    }
    next.ServeHTTP(w, r)
  })
}