code: def mark_dead(self, connection):
          
          # mark as dead even when sniffing to avoid hitting this host during the sniff process
          self.connection_pool.mark_dead(connection)
          if self.sniff_on_connection_fail:
              self.sniff_hosts()
docs: Mark a connection as dead (failed) in the connection pool. If sniffing
          on failure is enabled this will initiate the sniffing process.
  
          :arg connection: instance of :class:`~elasticsearch.Connection` that failed