Status

Current state: AcceptedUnder Discussion

Discussion thread: https://www.mail-archive.com/dev@kafka.apache.org/msg81074.html

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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 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.  During this brief time period, we will be able to fetch records from the new leader, but not find the latest offset for the partition.

This behavior will apply only when answering requests from clients.  Therefore, there will be no impact on ReplicaFetcherThread, or other parts of the broker that make ListOffsetsRequests to other brokers.  We can distinguish broker requests from client requests by looking at the replicaId field of the RPC.  (Hence, other brokers can still use ListOffsetsRequests to figure out which replicas has the longest log for a partition.)

When unclean leader elections are enabled, data loss is possible.  So we cannot guarantee that offsets will always go forwards, even in theory, in this mode.  Therefore, when unclean leader elections are enabled on the broker, the KIP-207 behavior will not apply.

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, OffsetNotAvailableException, for a partition.  This new exception will be a subclass of RetriableException.

The KIP-207 behavior applies to all ListOffsetsRequests, whether they are for the latest offset, the earliest offset, or a time-based offset.  Since leader changes are rare, the performance impact should be very small.  Treating all ListOffsetsRequests the same simplifies the code.  This also avoids creating awkward situations where we can locate the offset for a certain time T, and then cannot locate the offset for that time following a leader change.

Compatibility

The new OffsetNotAvailableException mentioned earlier will be sent only to post-KIP-207 clients.  As mentioned earlier, pre-KIP-207 clients will receive LeaderNotAvailableException.  Therefore, older clients should be able to communicate with servers implementing KIP-207.  Similarly, because the client changes are limited to handling a single additional exception, post-KIP-207 clients can communicate with pre-KIP-207 brokers.

Rejected Alternatives

Rather than returning a retriable exception, the broker could simply put the ListOffsetsRequest into a purgatory structure until the offset was available.  This avoids the need for the client to poll the server.  We would create a new version of the ListOffsetsRequest RPC which adds a maximum timeout field.

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