anchor.diff:  * ADDREPLICA
   * RESTORE
   * SPLITSHARD
  +* UTILIZENODE
  +* MOVEREPLICA
   
   In the future, the policy and preferences will be used by the Autoscaling framework to automatically change the cluster in response to events such as a node being added or lost.
  
positive.code: [source,json]
  {"replica": "#ALL", "freedisk": ">500"}
  
  Do not use more than `50%` of diskspace in any node
  [source,json]
  {"replica": "#ALL", "freedisk": ">50%"}
  
  
negative.code: 
  For the above rule to work, all nodes must the started with a system property called `"zone"`
  
  == example 3
  
  Distribute replicas equally in each zone
  
  [source,json]
  {"replica": "#EQUAL", "shard": "#EACH", "sysprop.zone": ["east", "west"]}
  
  or simply as follows
  
  [source,json]
  {"replica": "#EQUAL", "shard": "#EACH", "sysprop.zone": "#EACH"}
  
  
  ==== Place Replicas Based on Node Role
  Do not place any replica on a node which has the overseer role. Note that the role is added by the `addRole` collection API. It is *not* automatically the node which is currently the overseer.
  