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The period when the offset is unavailable should be brief.  During this period, the broker should simply return a retriable exception when it is asked for the offset of the partition.  For current versions of ListOffsetsRequest, this exception can be NotLeaderForPartitionException LeaderNotAvailableException.  For new versions of ListOffsetsRequest, we can return a new, more precise exception.  The main advantage of creating a new exception is that the client knows it can avoid re-fetching metadata.  A second advantage is that the more precise error message may help with debugging on the client side.

Public Interfaces

There will be a new version of ListOffsetsResponse API.  This will be the same as the existing one, except that we can return a new exception, OffsetNotAvailableForPartitionException OffsetNotAvailableException, for a partition.  This new exception will be a subclass of RetriableException.

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