[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();
	});
});

i have implemented this demo, if i am doing read and write query simaltaniously the speed of query execution is depend on how another query executed. if replica set use different member for read then speed of query not increases.

for overcome from this issue, if i am use different replica set member for read and write, if i am read from secondary then it will give error "error: MongoError: not master and slaveOk=false", how can i overcome from this issue. please help me

@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()