home
01 Jul 2013 - By Karl Seguin

Golang hot configuration reload

Like most, I've always appreciated a software package that lets me hot-reload the configuration without having to restart. Nginx immediately comes to mind, as does Postgresql (not for all settings, but most). This is something I've never done; so I was pretty happy when the opportunity presented itself.

I'm not sure how anyone else does this, but it's pretty basic stuff. When you get past the meaningless fact that you're reloading a configuration, it's just about providing concurrent read and write access to a shared object. There are a lot of ways to do that and a lot will depend on your language/framework. In Go, our approach was to use a read/write mutex.

First, our basic configuration structure, a global (to the package) variable and a similar lock:

import (
  "os"
  "log"
  "sync"
  "syscall"
  "os/signal"
  "io/ioutil"
  "encoding/json"
)

type Config struct {
  Mode string
  CacheSize int
}

var (
  config *Config
  configLock = new(sync.RWMutex)
)

Next we need a function that loads our config:

func loadConfig(fail bool){
  file, err := ioutil.ReadFile("config.json")
  if err != nil {
    log.Println("open config: ", err)
    if fail { os.Exit(1) }
  }

  temp := new(Config)
  if err = json.Unmarshal(file, temp); err != nil {
    log.Println("parse config: ", err)
    if fail { os.Exit(1) }
  }
  configLock.Lock()
  config = temp
  configLock.Unlock()
}

Whether we exit on failure depends on whether this is the first time loading the configuration or if it's a reload. We use a temp variable to hold our new reference so that we can minimize the duration of our write lock to the simplist possible code.

Next we need is a way to access the current configuration:

func GetConfig() *Config {
  configLock.RLock()
  defer configLock.RUnlock()
  return config
}

Finally we need to do an initial load as well as be able to cause a reload. To do this, we listen for a signal:

// go calls init on start
func init() {
  loadConfig(true)
  s := make(chan os.Signal, 1)
  signal.Notify(s, syscall.SIGUSR2)
  go func() {
    for {
      <-s
      loadConfig(false)
      log.Println("Reloaded")
    }
  }()
}

Like I said, not hard but at least a little neat.

post tags: devopsgolang
blog comments powered by Disqus