Feature: Add option to set the starting offset for new consumer

[MitchellJeppson]

I have a use case where I would like to start up a kafka consumer that only processes new messages. Is there a config value that could be implemented to support this? I found begin_offset in brod. I'd be happy to take on the MR if I could get a little direction. Thanks!

[josevalim]

A PR to add support for such is indeed welcome.

[yordis]

Made a PR, not sure if it is the right spot

[danmarcab]

We have a similar use case, where we would like to always start from the :earliest offset. We've been testing the added begin_offset option and it doesn't seem to work. That option seems to be only used in :brod_consumers and BroadwayKafka is not using those (the produced is implementing the brod_group_member behaviour.

What do people think about adding a top-level config option:

@typep begin_offset :: :assigned | :reset | non_neg_integer()

Where:

• :assigned is the default and just uses the begin_offset from the partition assignment received from Kakfa (current behaviour)
• :reset forces reset the begin_offset using the configured offset_reset_policy after receiving assignments
• A particular offset forces the begin_offset to be that one

Now the last one is a bit problematic, since offsets are partition specific, so one integer might not cut it, might be better to use a timestamp and resolve offsets using brod... it gets complex

It seems that the main use cases are either to begin from the start or from the end so we might not need that last option. To simplify things, we could just keep :reset and :assigned

offset_reset_policy	begin_offset	outcome
:earliest	:reset	always read from start
:latest	:reset	always read from end
:earliest	:assigned	read from committed offset, or from start if not defined or expired (current behaviour)
:latest	:assigned	read from committed offset, or from end if not defined or expired (current behaviour)

Happy to open a new issue and open a PR for this if it makes sense

[josevalim]

@yordis are you using your patch? Is it working as expected?

It seems that the main use cases are either to begin from the start or from the end so we might not need that last option. To simplify things, we could just keep :reset and :assigned

This sounds good to me. A PR is welcome :)

[yordis]

That was long ago, but I remember we tested back then. I'm sorry I can't help you any more.

[josevalim]

Np, thanks! following the code path, it seems the option indeed is not used, so we can revert it. :)