Mongodb – Config enable sharding collection in MongoDB cluster

Actually, it is a Very Bad Idea™ to do a load balancing this way with MongoDB. First of all, you can give multiple mongos instances to the driver. If one mongos goes down or becomes unavailable due to overload, the driver will be happy to access the next available mongos. Second, the config servers and mongod instances get unnecessary load for continuously updating the cluster "map". Third, they most likely will initiate and tear down connections continuously, too.

The solution would be to use CNAMEs at the DNS level instead of multiple A records for a host, but maintain unique hostnames on the OS level. That is, the hostname command should return a unique value on every host of the cluster, including the mongos instances.

A word on DNS based round robin load balancing: It's crap when used on it's own. You don't have any control over the load actually sent to each of the balanced servers. In case you have caching DNS servers between you and your biggest group of unique users, you actually might worsen the load for an individual server. You better use a "real" loadbalancer in front of your servers, something like HAProxy, varnish or nginx (depending on your application). In case you need / want to have more than one of those, you can use DNS based round robin load balancing on these, though from my experience, it is better to make one load balancer highly available to prevent a single point of failure.

Generally it is not recommended to use IP addresses to configure a cluster. You have not mentioned your environment, but it is common for IP addresses to change in many environments with a reboot, or perhaps you will need to move one or more of the nodes to a different host in the future.

Should that happen, your IP address will change but if you use hostnames (even those in the hosts file rather than DNS) it gives you a layer of abstraction and means you can have the hostname remain the same, not have to alter your database configuration for such a move.

This becomes particularly important for config servers in a sharded environment. An altered IP address is procedurally the same as an altered host name, and as you can see from this procedure, that means that moving a config server will require downtime for your cluster.

With all that said, for testing or for an environment where you can tear everything down and start again (or similar), these concerns do not really apply and you can use IP addresses in your configuration. For any system that will run long term, or any system that will run in production and require minimal interruption, then the use of hostnames over IP addresses is certainly recommended.