Question

I have 3 replica sets, each in its own shard. Each replica set has 2 Mongo servers (master and slave).

Which server(s) should I connect to when using the connection string for Mongo()?

Thanks!

Answer 1

If you have a sharded environment you should be running one or more mongos servers (typically one per application server) rather than connecting to the replica sets directly.

Excerpt from the MongoDB documentation:

The mongos process can be thought of as a routing and coordination process
that makes the various components of the cluster look like a single system.
When receiving client requests, the mongos process routes the request to the
appropriate server(s) and merges any results to be sent back to the client.

The connection to the mongos server(s) should be specified the same as you would connect to a mongod server, eg:

<?php

    $m = new Mongo("mongodb://mymongos1:27017");

?>