Cache DNS responses and refresh them using a single goroutine on a 1 minute timer. Avoids having a spike of threads from cgo launched under load.

dnscache.go

package dnscache

import (
  "net"
  "sync"
  "time"
  "math/rand"
)

var (
  lock = new(sync.RWMutex)
  resolved = make(map[string]string, 32)
)

func init() {
  go refresh()
}

func Lookup(address string) (string, error) {
  lock.RLock()
  ip, exists := resolved[address]
  lock.RUnlock()
  if exists { return ip, nil }
  return lookup(address)
}

func refresh() {
  for {
    time.Sleep(time.Minute * 1)
    var addresses []string
    lock.RLock()
    for key, _ := range resolved {
      addresses = append(addresses, key)
    }
    lock.RUnlock()

    for _, address := range addresses {
      lookup(address)
      time.Sleep(time.Second * 5)
    }
  }
}

func lookup(address string) (string, error) {
  ips, err := net.LookupIP(address)
  if err != nil { return "", err }

  ip := ips[rand.Int31n(int32(len(ips)))].String()
  lock.Lock()
  resolved[address] = ip
  lock.Unlock()
  return ip, nil
}