can I start simple, small, single mongodb instance, and switch to replicaset later easily, or shall I make it "replicable" from day one?

Question

I need to get things done, quick and fast and now.

Later on, down the road, say in a month or less, or more, I will want to have my database replicated in few more machines.

I wonder, if I can simply start working on my single instance, and start worrying about replication later on, or there are things I need to take into account now to make it easier to replicate later on.

Answer 1

You mostly can worry about it later on. If you switch to replica sets you will have to restart your mongod process and set up a replica set at that time though. If you want to avoid that you can also consider simply running a 3 member replication set on a single machine to save yourself any headaches later on.

This should give you a good overview of the pros/cons : http://www.mongodb.org/display/DOCS/Upgrading+to+Replica+Sets

Answer 2

To add to @Remon's answer, you can even convert your system from a single node (or a single replica set) to a cluster, without even restarting your mongod instances (you'll have to restart your app, though).