semaphore.js

Install: npm install semaphore

Limit simultaneous access to a resource.

// Create
var sem = require('semaphore')(capacity);

// Take
sem.take(fn[, n=1])
sem.take(n, fn)

// Leave
sem.leave([n])

// Available
sem.available([n])

// Limit concurrent db access
var sem = require('semaphore')(1);
var server = require('http').createServer(req, res) {
    sem.take(function() {
        expensive_database_operation(function(err, res) {
            sem.leave();

            if (err) return res.end("Error");

            return res.end(res);
        });
    });
});

// 2 clients at a time
var sem = require('semaphore')(2);
var server = require('http').createServer(req, res) {
    res.write("Then good day, madam!");

    sem.take(function() {
        res.end("We hope to see you soon for tea.");
        sem.leave();
    });
});

// Rate limit
var sem = require('semaphore')(10);
var server = require('http').createServer(req, res) {
    sem.take(function() {
        res.end(".");
        
        setTimeout(sem.leave, 500)
    });
});

License

MIT