Mongoose Connection

open()	openSet()
close()	readyState

open()

Opens the connection to MongoDB.

Syntax:-
  open(connection_string, [database], [port], [options], [callback])

• connection_string <String> mongodb://uri or the host to which we are connecting
• [database] <String> database name //optional
• [port] <Number> database port //optional
• [options] <Object> options //optional
  • db, server, replset, user, pass, auth are the options
• [callback] <Function> //optional

Example:-

var mongoose=require('mongoose');
var con = mongoose.createConnection();
var cb = function(err){
  if(!err)
    console.log(err);
  else
  console.log("connection opened");
};
con.open("mongodb://localhost/tutorialtous",cb);

openSet()

Opens the connection to a replica set.

Syntax:-
   openSet(uris, [database], [options], [callback])

• uris <String> comma-separated mongodb:// URIs
• [database] <String> database name if not included in uris
• [options] <Object> passed to the internal driver
• [callback] <Function>

Note:-
Functionality of openSet() will be similar to open() except openSet() works with replicaSets

Check this Example

var mongoose=require('mongoose');
var con = mongoose.createConnection();
var cb = function(err){
  if(!err)
    console.log(err);
  else
  console.log("connection opened");
};
con.openSet("mongodb://localhost/tutorialtous,
mongodb://mongodb.tutorialtous.com:23396/tutorialtous",cb);

close()

Closes the connection, it doesn’t throw any error even the connection is not opened or already closed.

Syntax:-
    close([callbackfunction]);

Example:-

var mongoose=require('mongoose');
var con = mongoose.createConnection();
var cb = function(err){
  if(!err)
    console.log(err);
  else
  console.log("connection opened");
};
var ccb = function(err){
  if(!err)
    console.log(err);
  else
  console.log("connection closed");
};
con.open("mongodb://localhost/tutorialtous",cb);
con.close(ccb);
console.log("connection opened");
console.log(err);
console.log("connection closed");
console.log(err);

readyState

It is property of connection which holds state of the connection

When this concept (readyState) is useful:

Due to network interrupt at server/client side or server down time connection to the database may lose, in order to trace those situations we need to use the readyState concept.

Let check this example

var mongoose=require('mongoose');
var con = mongoose.createConnection();
var connecting = function(err){
console.log("Please Wait connecting \t"+con.readyState);
};
var disconnecting = function(err){
console.log("Please Wait Disconnecting \t"+con.readyState);
};
var connected = function(err){
console.log("Connection Connected\t"+con.readyState);
};
var disconnected= function(err){
console.log("Connection DisConnected \t"+con.readyState);
};
con.on("connecting",connecting);
con.on("disconnecting",disconnecting);
con.on("connected",connected);
con.on("disconnected",disconnected);
con.open("mongodb://localhost/tutorialtous");

If the mongo server is up state then we will get this output:-
Please Wait connecting 2
Connection Connected 1

Now disable net connection at client machine or stop the server if both client and server running in same machine. Then you will observe the following message

Connection DisConnected 0
While doing the practical application it is very helpful based on my work experience.

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