Tuesday, February 19, 2013

Automatically Connect to the Replica Set Primary with Mongo Shell

So you got your fancy MongoDB Replica Set running, did you, Mr. Fancypants? Before too long you might run into an issue: how do you connect to the primary node when the primary can change?

Luckily our applications don’t have this problem because their drivers are smart and automatically connect to the primary. The Mongo shell doesn’t do that, though.

Here’s a library to help with that. Throw this into a file called replicaSetConnector.js:

var ReplicaSetConnector = (function() { 
    var RSC = function(options) {
        // private method for handling the dirty work of connecting 
        // and authenticating to mongo
        var connectAndAuth = function(host) {
                print("Connecting to " 
                    + host + "/" + options.database 
                    + " as " + options.username + ":" + options.password);
            var connection = new Mongo(host);
            var database = connection.getDB(options.database);
            database.auth(options.username, options.password);    
            return database;    
        this.connect = function() {
            // db needs to be a global variable for subsequent shell commands to work :)
            // connect to the given host, which could be any node of the replica set
            db = connectAndAuth(options.initialHost);
            // load some basic replica set information, which will tell us 
            // if we're on the primary, and if not where it is
            var rsInfo = db.isMaster();

            // if we're already on the primary, we're done. Otherwise, change our
            // connection to the primary
                if(options.debug) print("You're not on master...");
                db = connectAndAuth(rsInfo.primary);
                if(options.debug) print("...or are you ;)");
    return RSC;

That’s a library we’ll reuse bunches of times. Now make another file for your environment, e.g. prod.js with this in it:

(new ReplicaSetConnector({ 
    initialHost: 'one-of-your-replica-set-nodes:27017', 
    database: "database-name", 
    username: "user", 
    password: "secret...",
    debug: true})).connect();

And we’re finally read to connect with a mongo shell like so:

%> mongo --shell --nodb replicaSetConnector.js prod.js
MongoDB shell version: 2.2.2
type "help" for help
loading file: replicaSetConnector.js
loading file: prod.js
Connecting to one-of-your-replica-set-nodes:27017/database-name as user:secret...
You're not on master...
Connecting to another-one-of-your-replica-set-nodes:27017/database-name as user:secret
...or are you ;)

Yes, now you are on master and the db object is set for you to begin executing commands. If you want to be extra terse on the command line, you can alias that command to something shorter, add the library to your mongorc file, or make Windows shortcut.