title: How to get notified when a redis key expired?
body: <p>Why can't get notified when a key expired with the following code? </p>
  
  <p>I want to use redis, and when a key is expired then notify me. Then i can do something.</p>
  
  <pre><code>var Redis = require('ioredis')
  var sub = new Redis()
  var pub = new Redis()
  
  var subKey = '__keyevent@0__:del'
  
  sub.subscribe(subKey, function () {
    console.log('subscribe success !')
  })
  
  sub.on('message', function (channel, message) {
    console.log(channel, message, '======')
  })
  
  var testKey = 'test'
  setTimeout(function () {
    pub.multi()
        .set(testKey, 'test redis notify')
        .expire(testKey, 5)
        .exec(function (err) {
          if (err) {
            console.log(err, 'why err ?')
            return
          }
          console.log('.....')
        })
  }, 2000)
  </code></pre>