mongodb open connection issue

Question

I have the following log in my mongo console:

Tue Jul 23 17:20:01.301 [initandlisten] waiting for connections on port 27017
Tue Jul 23 17:20:01.401 [websvr] admin web console waiting for connections on port 28017
Tue Jul 23 17:20:01.569 [initandlisten] connection accepted from 127.0.0.1:58090 #1 (1 connection now open)
Tue Jul 23 17:20:01.570 [initandlisten] connection accepted from 127.0.0.1:58089 #2 (2 connections now open)
Tue Jul 23 17:20:21.799 [initandlisten] connection accepted from 127.0.0.1:58113 #3 (3 connections now open)
....
....
....

likewise the log goes on and now it is in 112. Each time when i start mongo server this happens. I only have a singleton connection in my code. What can be the issue here:

public static DB getConnection(String databaseName) throws AppConnectionException {

    if (null != db) {
        Logger.debug("Returning existing db connection...!");
        return db;
    }

    Logger.debug("Creating new db connection...!");
    final String connStr = PropertyRetreiver.getPropertyFromConfigurationFile("rawdata.url");

    try {

        final MongoClientURI uri = new MongoClientURI(connStr);
        final MongoClient client = new MongoClient(uri);
        db = client.getDB(databaseName);

    } catch (UnknownHostException e) {
        throw new AppConnectionException(
                "Unable to connect to the given host / port.");
    }

    return db;
}

Answer 1

MongoClient has internal connection pool. Maximum number of connections can be configured (default is 100). You can set it by using MongoClientOptions like this:

MongoClientOptions options = MongoClientOptions.builder()
                .connectionsPerHost(100)
                .autoConnectRetry(true)
                .build();

And then give these options to MongoClient (checked it in Mongo Java API v2.11.1). Connections in pool are maintained open (opening and closing connection is usually an expensive operation) so that they can be later reused.

I would also refactor your MongoDB client singleton using enum for example to avoid putting synchronized on this method.

Here is a sketch of what I mean:

public enum MongoDB {
    INSTANCE;

    private static final String MONGO_DB_HOST = "some.mongohost.com";
    private Mongo mongo;
    private DB someDB;

    MongoDB() {

        MongoClientOptions options = MongoClientOptions.builder()
                .connectionsPerHost(100)
                .autoConnectRetry(true)
                .readPreference(ReadPreference.secondaryPreferred())
                .build();

        try {
            mongo = new MongoClient(MONGO_DB_HOST, options);
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }

        someDB = mongo.getDB("someDB");
         //authenticate if needed
         //boolean auth = someDB.authenticate("username", "password".toCharArray());
         //if(!auth){
         //     System.out.println("Error Connecting To DB");
         //}        
    }

    public DB getSomeDB() {
        return someDB;
    }

    //call it on your shutdown hook for example 
    public void close(){
        mongo.close();
    }
}

Then, you can access your database via

MongoDB.INSTANCE.getSomeDB().getCollection("someCollection").count();