def _unregister_engine(self, uid):
        """Existing engine with ident `uid` became unavailable."""
        if len(self.targets) == 1:
            # this was our only engine
            pass

        # handle any potentially finished tasks:
        self.engine_stream.flush()

        # don't pop destinations, because they might be used later
        # map(self.destinations.pop, self.completed.pop(uid))
        # map(self.destinations.pop, self.failed.pop(uid))

        # prevent this engine from receiving work
        idx = self.targets.index(uid)
        self.targets.pop(idx)
        self.loads.pop(idx)

        # wait 5 seconds before cleaning up pending jobs, since the results might
        # still be incoming
        if self.pending[uid]:
            dc = ioloop.DelayedCallback(lambda : self.handle_stranded_tasks(uid), 5000, self.loop)
            dc.start()
        else:
            self.completed.pop(uid)
            self.failed.pop(uid)
Existing engine with ident `uid` became unavailable.