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A specific scenario where this really matters the current behavior causes date "loss" is when a MirrorMaker is subscribed to a regex pattern. If auto-topic creation is enabled on the cluster, and you start producing to a non-existent topic that matches the regex, then there will be a period of time where the producer is producing before the new topic's partitions have been picked up by the MirrorMaker. Those messages will never be consumed by the MirrorMaker because it will start from latest, ignoring those just-produced messages.

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This will be a silent breaking change since it flips the behavior around.. 

Existing mirrormakers will be unaffected for any topics they are currently consuming since they already have a saved offset.

Mirrormakers that start consuming topics for which they don't have a saved offset Anyone who starts a mirrormaker on an existing topic will start replicating the partitions from the beginning, rather than from the partition's current highwater mark. If this behavior is unexpectedly applied to the mirrormaker starts consuming a very large partition/topic, it will replicate far more data than expected.Existing mirrormakers will be unaffected since they already have a saved offsetpreviously. This has relatively low probability since most topics that get replicated are newly-created, in which case starting from the earliest simply prevents skipping the first few seconds/minutes of data written to the topic.

Since MirrorMaker 2.0 already behaves this way, this change will make future migrations from MM1 to MM2 easier for folks since the behavior will stop changing between them.

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