code: def start_health_check(self, node_address):
          
          if self._stop_event.ready():
              return
  
          with self._health_lock:
              if self._address_mgr.is_address_known(node_address):
                  return  # already healthchecked
  
              node_address_hex = to_normalized_address(node_address)
              self.log.debug('Healthcheck', peer_address=node_address_hex)
  
              candidates = [
                  self._get_user(user)
                  for user in self._client.search_user_directory(node_address_hex)
              ]
              user_ids = {
                  user.user_id
                  for user in candidates
                  if validate_userid_signature(user) == node_address
              }
              self.whitelist(node_address)
              self._address_mgr.add_userids_for_address(node_address, user_ids)
  
              # Ensure network state is updated in case we already know about the user presences
              # representing the target node
              self._address_mgr.refresh_address_presence(node_address)
docs: Start healthcheck (status monitoring) for a peer
  
          It also whitelists the address to answer invites and listen for messages