[MongoDb][Node.js] A simple demo of read preference settings when reading documents from a replica set

read_preference.js

/**
 * A sample demo to demonstrate Read Preference settings when reading data from a replica set. 
 * Instructions:
 * 	1. Run your replica set servers.
 * 	2. Run this app (note: adjust the replica set's hosts & ports as you see them fit in your environments.)
 * 	3. Shutdown your replica set's primary node.
 * 	4. Watch the outputs of this running app. Confirm that the findOne were defered ( no errors being throws) when the primary node is downed and 
 * 		then come up again after a new primary node is elected in the replica set.
 */

var MongoClient = require('mongodb').MongoClient;
var ReadPreference = require('mongodb').ReadPreference;
var assert = require('assert');

// We'll set the read preference so that by default, this program would read data directly from secondary node in the replica set
var connection_string = "mongodb://KURONO:27018, KURONO:27019, KURONO:27020/demo_replica_set?readPreference=secondary";

MongoClient.connect(connection_string, function(err, db){
	if (err) throw err;

	// Insert a test document
	var test_doc = {'value': Math.random()};	
	var test_docs = db.collection("test_docs");

	test_docs.insert(test_doc, function(err, insertedDoc){
		if (err) throw err;		
		assert.ok(insertedDoc.length > 0);
		console.log("[INFO] - Successfully inserted "+JSON.stringify(insertedDoc)+" document.");

		function findOneDocument(){
			// Override the  default setting so that the next read would goes to Primary node in the replica set.
			var read_preference = {'readPreference': ReadPreference.PRIMARY };
			test_docs.findOne({_id: insertedDoc[0]._id},read_preference, function(err, foundDoc){
				if (err) throw err;
				//assert.ok(foundDoc.length > 0);
				console.log("[INFO] - Retrieved document: "+ JSON.stringify(foundDoc) + " where the read preference is "+JSON.stringify(read_preference));
			});

			console.log("[INFO] - Dispatched findOne operation...");

			// Delayed for 1 sec then dispatch another insert.
			setTimeout(findOneDocument, 1000);
		}

		findOneDocument();
	});
});

@ejazmafat999 - I'm hoping you figured it out by now, but you need to initialize your replicaSet.

mongo > rs.initialize();

Then, make it ok for the current replica to slave:

mongo > rs.slaveOk()