...

Besides, we sometimes wants to "peek" the incoming records, to do some testing, without affecting the offsets, like the "peek" method provided in many data structures (ex: java Queue). So, in this "peek" method, we won't increase the position offset in the partition. That means, after peek, the next "poll"ed records will still include the records returned by `peek`. Under the `enable.auto.commit = true` (default setting) case, because the offsets are not incremented, so it won't affect the committed offsets. That means, after the consumer restarted or rebalanced, the next poll will always start from the offset before operating peek methods. (of course if user manually commit the offsets, the offsets will be incremented)

Imagine we have brokers up now, and producers are producing records. We're a team developing consumers to consume the data, and feed into another integration process. Before this KIP, we need to do a polling, to retrieve the data, and see if the integration works as expected. If luckily yes, then, we can seek the offset to the begining and start the new consumers to do the work, if no, we might need to poll more data, and do more troubleshooting cycle, but once the data are not producing fast enough, we might run into a situation that we need to seek the

So, we will have a `consumer#peek()` to allow consumers to:

...

This is a new added method in Consumer interface. There will be no impact to the existing users.

Rejected Alternatives

1. Could be easily realized on the user side by using manual offset commit + offset position rewind
→ 

That's true.
But I have the same thoughts as Sagar, which is that, it's for advanced users.
Another reason is for simplicity. If you've ever used the peek API from java collection (ex: Queue#peek), you should know what I'm talking about. When you have data in a queue, if you want to know what the first data is in the queue, you'd use peek(). You can also achieve it by remove() the 1st element from queue, and then added it back to the right position, but I believe that's not what you'd do.