title: MongoDB replication factors
body: <p>I'm new to Mongo, have been using Cassandra for a while.  I didn't find any clear answers from Mongo's docs.  My questions below are all closely related.</p>
  
  <p>1) How to specify the replication factors in Mongo?</p>
  
  <p>In C* you can define replication factors (how many replicas) for the entire database and/or for each table.  Mongo's newer replica set concept is similar to C*'s idea.  But, I don't see how I can define the replication factors for each table/collection in Mongo.  Where to define that in Mongo?</p>
  
  <p>2) Replica sets and replication factors?</p>
  
  <p>It seems you can define multiple replica sets in a Mongo cluster for your databases.  A replica set can have up to 7 voting members.  Does a 5-member replica set means the data is replicated 5 times?  Or replica sets are only for voting the primary?</p>
  
  <p>3) Replica sets for what collections?</p>
  
  <p>The replica set configuration doc didn't mention anything about databases or collections.  Is there a way to specify what collections a replica set is intended for?</p>
  
  <p>4) Replica sets definition?</p>
  
  <p>If I wanted to create a 15-node Mongo cluster and keeping 3 copies of each record, how to partition the nodes into multiple replica sets?</p>
  
  <p>Thanks.</p>