code: def _get_server_type(doc):
      
      if not doc.get('ok'):
          return SERVER_TYPE.Unknown
  
      if doc.get('isreplicaset'):
          return SERVER_TYPE.RSGhost
      elif doc.get('setName'):
          if doc.get('hidden'):
              return SERVER_TYPE.RSOther
          elif doc.get('ismaster'):
              return SERVER_TYPE.RSPrimary
          elif doc.get('secondary'):
              return SERVER_TYPE.RSSecondary
          elif doc.get('arbiterOnly'):
              return SERVER_TYPE.RSArbiter
          else:
              return SERVER_TYPE.RSOther
      elif doc.get('msg') == 'isdbgrid':
          return SERVER_TYPE.Mongos
      else:
          return SERVER_TYPE.Standalone
docs: Determine the server type from an ismaster response.