Brief description of the ask is mentioned in KIP-500's BrokerMetadataManagement
Instead of the controller pushing out updates to the other brokers, those brokers will fetch updates from the active controller via the new MetadataFetch API.
A MetadataFetch is similar to a fetch request. Just like with a fetch request, the broker will track the offset of the last updates it fetched, and only request newer updates from the active controller.
The broker will persist the metadata it fetched to disk. This will allow the broker to start up very quickly, even if there are hundreds of thousands or even millions of partitions. (Note that since this persistence is an optimization, we can leave it out of the first version, if it makes development easier.)
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.