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Detect if event loop is busy.

The idea is not new, implementation probably too. I just saw this and found it useful. This projects does pretty the same but in pure javascript.

Things you can do

  • Log how busy is the loop
  • Stop accepting new tasks
  • Warn yourself
  • Scale up the app

Example (shameless copied from toobusy)

var busy = require('busy'),
    express = require('express');

var app = express();

var busyCheck = busy(function(amount) {
    console.log('Loop was busy for', amount, 'ms');

// middleware which blocks requests when we're too busy
app.use(function(req, res, next) {
    if (busyCheck.blocked) {
        res.send(503, "I'm busy right now, sorry.");
    } else {

app.get('/', function(req, res) {
    // processing the request requires some work!
    var i = 0;
    while (i < 1e5) i++;
    res.send("I counted to " + i);

var server = app.listen(3000);

process.on('SIGINT', function() {
    // calling .stop allows your process to exit normally

Api busy([options], [callback])

Optional options:

- `max` max time in ms alowed to be busy, default is 100ms
- `interval` how often to check the state in ms, default is 50ms

Optional callback is called every time the event loop was busy for longer amount of time than defined in max. Passed value is amount of ms the loop was blocked for.

var busyCheck = busy();

busyCheck.blocked; // Is true if by the last check, max was exceeded
busyCheck.blockedFor; // Number in ms the loop was blocked for, during the last check

busyCheck.stop(); // Stop doing checks