MongoDb: sharded replica set: data distribution
Sometimes, the 3 config server are out of sync. What does it mean? How can I quickly get rid of these issues?


  Most convenient way would be to copy (cp or rsync) the configdb data directory content between hosts in cases of inconsistency


When should I choose MongoDb Shard with Replica Set vs MongoDb Replica set only?


  Replica ONLY: When you desire to make your mongo data highly available
  and add redundancy in your solution. The benefit can be to offload
  reporting requests to secondary nodes and/or make mongodb cluster
  sustain network partitions or disaster recovery
  
  Sharding ONLY: When you desire to scale your mongodb cluster
  horizontally to achieve load balancing and distribute traffic to
  multiple hosts you must choose sharding. For example, you can achieve
  partitioning benefits with sharding enabled on databases. 
  
  Sharding with replica set: When you desire to enable all benefits
  stated above i.e. scale you cluster horizontally to handle large
  volumes of data for load balancing and also achieve HA within each
  shard you must choose this approach