Storing the return value of node.js setTimeout in redis

Question

Storing the return value of node.js setTimeout in redis

I'm using setTimeout in Node.js and it seems to behave differently from client-side setTimeout in that it returns an object instead of a number. I want to store this in redis, but since redis only stores strings, I need to convert the object to a string. However, using JSON.stringify throws a circular reference error. How can I store this object in redis if I want to be able to fetch it from redis and call clearTimeout on it?

javascript
node.js
redis
settimeout

Answer 1

You cannot store the object in Redis. The setTimeout method returns a Handler (object reference).

One idea would be to create your own associative array in memory, and store the index in Redis. For example:

var nextTimerIndex = 0;
var timerMap = {};

var timer = setTimeout(function(timerIndex) {
    console.log('Ding!');

    // Free timer reference!
    delete timerMap[timerIndex];
}, 5 * 1000, nextTimerIndex);

// Store index in Redis...

// Then, store the timer object for later reference
timerMap[nextTimerIndex++] = timer;

// ...
// To clear the timeout
clearTimeout(timerMap[myTimerIndex]);

Answer 2

This code is used when the timeouts need not be persistent across server restarts

var timeouts = {};

app.get('/', function (req, res) {
  var index = timeouts.length;
  timeouts[index] = setTimeout(console.log, 1000000, req.user.name);

  redis.set('timeout:' + req.user.name, index, function (err, reply) {
    res.end();
  });
});

app.get('/clear', function (req, res) {
  redis.get('timeout:' + req.user.name, function (err, index) {
   clearTimeout(timeouts[index]);
   delete timeouts[index];
   redis.delete('timeout:' + req.user.name);
   res.end();
  });
});

If you need timeouts to be persistent across server restarts, then you might need to store _idleStart and _idleTimeout values for every timer in the redis, and load them up everytime you server restarts

app.get('/', function (req, res) {
  var timeout = setTimeout(console.log, 1000000, req.user.name);
  var time = timeout._idleStart.getTime() + timeout._idleTimeout;

  redis.set('timeout:' + req.user.name, time, function (err, reply) {
    res.end();
  });
});

app.get('/clear', function (req, res) {
  redis.delete('timeout:' + req.user.name);
  res.end();
});

// Load timeouts on server start
// *I know this is not the correct redis command*
// *It's not accurate, only approx*
redis.get('timeout:*', function (err, vals) {
  vals.forEach(function (val) {
    var time = val - new Date().getTime();
    setTimeout(console.log, time, username)
  });
});

Answer 3

I was attempting to do the same thing as the OP. My solution was to set the timeout with a conditional check on a new key inside the timeout in my disconnect handler:

redis.hset("userDisconnecting:" + userId, "disconnect", 1);

setTimeout(function() {
    redis.hget("userDisconnecting:" + userId, "disconnect",
     function(err, result) {
        if (result.toString() === "1") {
           //do stuff, like notify other clients of the disconnect.
        }
    });
}, 10000);

Then, when the client connects again, I set that key to 0, so the stuff that needs to fire on true disconnect doesn't happen:

redis.hset("userDisconnecting:" + userId, "disconnect", 0);

The timeouts themselves aren't persistent across server restarts, but you could solve that by kicking off a sweeper method on startup. Connected clients would come back "online" pretty quickly.