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Compatibility, Deprecation, and Migration Plan

We can set the default to be -1. This is will be used a flag to signal using the same value as New brokers will start using the new default of 1 day to expire producer IDs. In typical usage, this should not cause any noticeable difference to users since out of sequence records should be very uncommon outside of the retry window. The value can also be configured to match transactional.id.timeout.ms. This will offer backwards compatibility and and continuity of behavior for the use. It will also cover the case where transaction.id.timeout.ms was also using its default.expiration.ms if it is necessary to keep the old behavior.

Rejected Alternatives

Have a default value of 7 days

This would be a safe option and match a configuration that did not customize transactional.id.expiration.ms. In the case of a custom configuration for transactional expiration, it would more easily reveal that the new configuration existed. However, since this is still not totally apparent, it is better to default to not changing current behavior.

Have a default value

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of -1 to use the transaction.id.expiration.ms configured value

Although this would be the most seamless transition for compatibility, it doesn't allow users to see benefits from the new configuration. The default of 1 day should not cause issues with typical clients, so it seems fair to set the default lower

The motivation for this change is to have a different (and shorter) period for expiration so that the broker is not easily overwhelmed with all the new idempotent producers. Since this configuration will silently drop idempotency guarantees, it seems safer to make the default more conservative.

Have the flag value as 0 instead of -1

0 would allow us to not have to worry about a valid configuration of 0 ms (immediately expired). However, 0 seems less intuitive as a flag and 0 ms is not all that different in practice to 1, which is a valid value for transactional.id.expiration.ms.