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Most of the time, the broker should only need to fetch the deltas, not the full state.  However, if the broker is too far behind the active controller, or if the broker has no cached metadata at all, the controller will send a full metadata image rather than a series of deltas.

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The broker will periodically ask for metadata updates from the active controller.  This request will double as a heartbeat, letting the controller know that the broker is alive.

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