Reactive Mongo

This module allows you to subscribe to events, based on what's shakin' in your MongoDB.

NPM

npm install reactive-mongo --save

Usage

Create a connection to your database (local or remote) and begin reacting to changes in the oplog.

var Reactive = require('reactive-mongo');
var Db = new Reactive(); // assumes local, unless URI passed
 
Db.on('error', function(err) {
  console.log(err);
});
 
var reference = new Db('testdb.records');
 
reference.on('insert', function(doc) {
  console.log(doc);
});
 
reference.on('update', function(doc) {
  console.log(doc);
});
 
reference.on('delete', function(doc) {
  console.log(doc);
});
 
reference.on('op', function(op, doc) {
  console.log('operation: ' + op);
  console.log(doc);
});

On any reference, you can also retrieve the query and raw operation object.

reference.on('update', function(doc, query, op) {
  // attributes updated
  console.log(doc);
  // query used in find
  console.log(query);
  // raw oplog operation
  console.log(op);
});

How?

MongoDB uses an oplog (short for "operation log") to sync a master to its slaves. Most databases have some form of replication files.

Working on local

If you're on a local instance of MongoDB, you'll likely have to configure it to write to the oplog.

First run MongoDB with the following settings

$ sudo mongod --replSet rs0 --oplogSize 100

In the same terminal you'll probably see a repeating error.

2014-09-08T15:55:16.748-0700 [rsStart] replSet can't get local.system.replset config from self or any seed (EMPTYCONFIG)
2014-09-08T15:55:17.749-0700 [rsStart] replSet can't get local.system.replset config from self or any seed (EMPTYCONFIG)
2014-09-08T15:55:18.750-0700 [rsStart] replSet can't get local.system.replset config from self or any seed (EMPTYCONFIG)
2014-09-08T15:55:19.751-0700 [rsStart] replSet can't get local.system.replset config from self or any seed (EMPTYCONFIG)
2014-09-08T15:55:20.752-0700 [rsStart] replSet can't get local.system.replset config from self or any seed (EMPTYCONFIG)

This is fine. Just temporary.

Now, begin a Mongo client connection, and do the following.

$ mongo
> use local
switched to db local
> rs.initiate()
{
  "info2" : "no configuration explicitly specified -- making one",
  "me" : "Your-MacBook-Pro.local:27017",
  "info" : "Config now saved locally.  Should come online in about a minute.",
  "ok" : 1
}

Now, in the same client, if you run show collections you should see some new ones for the oplog.

Go back to the mongod terminal and restart it. The errors should go away, and you'll be ready to use Reactive Mongo.

Working on Compose (formerlly MongoHQ)

You must use a database that has elastic search turned on.

Use a database user who has oplogAccess enabled.

In the settings for the database, you'll see an example of how to connect to the database. It will look something like

mongodb://<user>:<password>@candidate.35.mongolayer.com:10491,candidate.34.mongolayer.com:10493/oplog_test

In this example oplog_test is the database name. Tweak that slightly. Instead of ending in /oplog_test change it to be /local?authSource=oplog_test.

mongodb://<user>:<password>@candidate.35.mongolayer.com:10491,candidate.34.mongolayer.com:10493/local?authSource=oplog_test

Use this path to connect to your database and tail the oplog.