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Brokers that don't have a broker ID lease are said to be "fenced."  When a broker is fenced, it cannot process any client requests.  It will respond to all requests with a retryable error, which will lead the clients to re-fetch their metadata.  (Once clients have the most up-to-date metadata, they will not attempt to contact fenced brokers, of course.)  This prevents brokers which are not receiving metadata updates or that are not receiving and processing them fast enough from causing issues to clients.

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