$source: {     
  connectionName: “myConnection”,
  db: “db1”,
  coll: “coll1”
  .. other existing fields ...
  config: {
     ...
     readPreference: "primary|primaryPreferred|secondary|secondaryPreferred|nearest"
     //readPreferenceTags is intended to be able to target one specific node in the        
     //cluster to read from. This allows users to set up a temporary analytics node       
     //to serve the queries without affecting normal application workloads       
     readPreferenceTags: { key1: value1,  ... }     
  }, 
}