MongoDB Data Change
How to listen for changes to a MongoDB collection.
So you’re doing something real-time?
Introducing MongoDB Change Streams
New in MongoDB 3.6
https://docs.mongodb.com/manual/release-notes/3.6/
Released January 10, 2018
$ mongod --version
db version v3.6.2Change Streams are used in Database Mirroring
Database Mirroring: the replication of redundant copies of a database. The purpose is to ensure continuous data availability and minimize or avoid downtime.
Replication: a feature allowing multiple database servers to share the same data, thereby ensuring redundancy and facilitating load balancing. See Replication.
How it works:
- replication creates additional copies of the data and allows for automatic fail-over to another node. Replication can also help with horizontal scaling.
This architecture in Mongo is also called a Replication Set.
You can then also make a replica set part of a sharded cluster.
— That’s for another post.
Replication Sets
Replica Set: A cluster of MongoDB servers that implements master-slave replication and automated failover.
More: https://docs.mongodb.com/manual/replication/
- For this example there will only be a master.
To use changeStreams the Mongo Database must be a Replication Set.
By default your Database will be a “Standalone” database.
Standalone: An instance of
mongodthat is running as a single server and not as part of a replica set.
In order to use changeStreams you must convert your “Standalone” database into a “Replica Set” database.
How to Convert a Standalone to a Replica Set.
1. Shut down the Standalone mongod instance.
kill -2 <MONGOD_PROCESS_ID>2. Restart the instance as a Replica Set.
Use the --replSet option to specify the name of the new replica set.
The following command starts a standalone instance as a member of a new replica set named rs0.
mongod --replSet rs0All options:
mongod --port 27017 --dbpath /srv/mongodb/db0 --replSet rs0 --bind_ip localhost,<ip address of the mongod host>3. Connect to the mongodb with a mongo shell.
mongo4. Initiate the new replica set.
rs.initiate()About rs.initiate() command: https://docs.mongodb.com/manual/reference/method/rs.initiate/#rs.initiate
You should get the following, you can now exit mongo.
rs0:PRIMARY>Your database is now a member of a Replica Set!
You can check the status of the Replica Set with rs.status()
How To Use
The following example is a practical application for how you might use this.
The following snippet is taken from the LightBase project;
a full working server Framework using everything posted here and more, check it out here.
Example:
When the Mongo Collection User changes in any way, all users are resent over the Socket.io connection.
Using: Node.js, MongoDB, Mongoose.js, Express.js, Socket.io
How It Works
First is to put a watch on the Mongoose Model
const changeStream = User.watch();We can then use the event “Change” to get notified anytime the Collection changes.
changeStream.on(‘change’, function(change) {
console.log(‘User COLLECTION CHANGED’);
// CODE FROM BELOW GOES HERE
});Now every time the database changes we can query for all the data and resend it.
User.find({}, (err, data) => {
if (err) throw err; if (data) {
// RESEND ALL USERS
socket.emit(‘users’, data);
}
});
NOTE: you must start the mongodb with the --replSet rs0 everytime you start the database, otherwise it will default to a standalone.
