How to listen for changes to a MongoDB collection?

Question

I'm creating a sort of background job queue system with MongoDB as the data store. How can I "listen" for inserts to a MongoDB collection before spawning workers to process the job?

Do I need to poll every few seconds to see if there are any changes from last time, or is there a way my script can wait for inserts to occur?

This is a PHP project that I am working on, but feel free to answer in Ruby or language agnostic.

Answer 1

MongoDB has what is called capped collections and tailable cursors that allows MongoDB to push data to the listeners.

A capped collection is essentially a collection that is a fixed size and only allows insertions. Here's what it would look like to create one:

db.createCollection("messages", { capped: true, size: 100000000 })

MongoDB Tailable cursors (original post by Jonathan H. Wage)

Ruby

coll = db.collection('my_collection')
cursor = Mongo::Cursor.new(coll, :tailable => true)
loop do
  if doc = cursor.next_document
    puts doc
  else
    sleep 1
  end
end

PHP

$mongo = new Mongo();
$db = $mongo->selectDB('my_db')
$coll = $db->selectCollection('my_collection');
$cursor = $coll->find()->tailable(true);
while (true) {
    if ($cursor->hasNext()) {
        $doc = $cursor->getNext();
        print_r($doc);
    } else {
        sleep(1);
    }
}

Python (by Robert Stewart)

from pymongo import Connection
import time

db = Connection().my_db
coll = db.my_collection
cursor = coll.find(tailable=True)
while cursor.alive:
    try:
        doc = cursor.next()
        print doc
    except StopIteration:
        time.sleep(1)

Perl (by Max)

use 5.010;

use strict;
use warnings;
use MongoDB;

my $db = MongoDB::Connection->new;
my $coll = $db->my_db->my_collection;
my $cursor = $coll->find->tailable(1);
for (;;)
{
    if (defined(my $doc = $cursor->next))
    {
        say $doc;
    }
    else
    {
        sleep 1;
    }
}

Additional Resources:

Ruby/Node.js Tutorial which walks you through creating an application that listens to inserts in a MongoDB capped collection.

An article talking about tailable cursors in more detail.

PHP, Ruby, Python, and Perl examples of using tailable cursors.