Watch mongo update events using oplogs - mongodb

Question

I have to Listen to changes in particular field in mongodb documents and send data to client accordingly
I have tried looking for solutions, but only thing I could find is to query oplogs
(operation logs) of mongodb.
db.collection("oplog.rs", function(err, oplog) {})
Questions:
How are we actually going to make decisions based on oplogs (Capped Collections) and how frequently are we going to query oplogs to see the changes ?
Do we have any alternative solution to this problem may be using mongoose ?

Answer 1

MongoDB keeps track of all the database changes in a collection called "oplogs" (Operation Logs). Oplogs are used when you need to keep track of EACH and EVERY collection of the database. However if you need to keep track of just one collection, you may create tailable cursor on that particular collection using the below code. (Note: Only capped collections can have tailable cursor)
However if you want to use oplogs, In case of multiple MongoDB server instance, oplogs are enabled by default but if you are working on a single instance MongoDB server, you need to enable oplogs in mongoDB by this command in command prompt:
mongod --replSet rs0
Then to get the records from oplogs collection using Mongoose, you can try the following code.
var mongoose = require('mongoose');
var db = mongoose.connect('mongodb://localhost/local'); //local db has oplogs collection
mongoose.connection.on('open', function callback() {
var collection = mongoose.connection.db.collection('oplog.rs'); //or any capped collection
var stream = collection.find({}, {
tailable: true,
awaitdata: true,
numberOfRetries: Number.MAX_VALUE
}).stream();
stream.on('data', function(val) {
console.log('Doc: %j',val);
});
stream.on('error', function(val) {
console.log('Error: %j', val);
});
stream.on('end', function(){
console.log('End of stream');
});
});
Now I guess this may help. The above code is how you implement tailable cursor in Mongoose.