{
  create: <collection name>,
  clusteredIndex: { 
    	// <key pattern>
     	// (a) uses existing index key specification structure -
     	//  	all fields must be ascending (1), descending (-1) or "hashed"
  	// (b) multi-key cluster keys are prohibited
        key: <key pattern>, // required
        unique: <bool>, // required, must be ‘true’
        name: <string>, // optional, otherwise automatically generated
        v: <int>, // optional, must be ‘2’ if provided
  },	
   // expireAfterSeconds only available when <cluster key> is a single field.   expireAfterSeconds: <String or NumberLong> // optional
}