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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def execute(self):
 """Deploys and executes the physical dataflow."""

        self._collect_garbage()  # Make sure everything is clean
        # TODO (john): Check if dataflow has any 'logical inconsistencies'
        # For example, if there is a forward partitioning strategy but
        # the number of downstream instances is larger than the number of
        # upstream instances, some of the downstream instances will not be
        # used at all

        # Each operator instance is implemented as a Ray actor
        # Actors are deployed in topological order, as we traverse the
        # logical dataflow from sources to sinks. At each step, data
        # producers wait for acknowledge from consumers before starting
        # generating data.
        upstream_channels = {}
        for node in nx.topological_sort(self.logical_topo):
            operator = self.operators[node]
            # Generate downstream data channels
            downstream_channels = self._generate_channels(operator)
            # Instantiate Ray actors
            handles = self.__generate_actors(operator, upstream_channels,
                                             downstream_channels)
            if handles:
                self.actor_handles.extend(handles)
            upstream_channels.update(downstream_channels)
        logger.debug("Running...")
        return self.actor_handles