Mongoose - Keep local database in sync with remote database - node.js

Question

I have access to two separate databases that I'd like to keep in sync, a new one and an existing one, which will be in separate physical locations. The new one is going to be used to service an external API, so to cut down on request time, I think it makes sense to only query the local database for API requests.
My initial approach was to use mongoose.createConnection and limit the local collection to minor metadata and directly access the remote collection, but that's what I'm now looking to avoid.
Another approach might be to use mongoose.createConnection to periodically query the remote db and update the local one, but it could be costly if I want to do make frequent updates.
There are ways to cut down the cost - for example, there is a lastUpdated property in the relevant collection on the existing database, which could be used to limit the remote query to recently updated records such as:
RemoteCollection.find({
lastUpdated: {$gte: Date.now() - lookbackPeriod}
})
But I'm wondering if there's any native functionality of mongoose/mongoDB that can be used more efficiently make the updates. I also thought about mongodump and mongorestore to keep a full local copy of the records I needed, but that also seems costly.
Any help is appreciated.

Answer 1

After a bit of reading and thanks to Jake's comment, it looks like it's working. I need to do some more setup, but the code below should work and is based on this section from the docs:
https://mongoosejs.com/docs/models.html#change-streams
The first step would be start mongod with the --replSet flag:
mongod --replSet "rs0" --bind_ip localhost,<hostname(s)|ip address(es)>
Then close and restart mongo and run rs.initiate() on the database. You can then check the status of the replica set with rs.status(). If that command works and returns a result, the replica set functionality should be there.
Then within Node, you can do something like this:
// The docs reference creating a new model but you can just import an existing one
const RemotePerson = require('./models/RemotePerson');
const LocalPerson = require('./models/LocalPerson');
RemotePerson.watch().on('change', data => {
if (data.operationType === "insert") {
LocalPerson.create(data.fullDocument);
} else if (data.operationType === "update") {
LocalPerson.findByIdAndUpdate(data.documentKey, {
$set: data.updateDescription.updatedFields
});
}
});