kirito632 commented on PR #3504:
URL: https://github.com/apache/kvrocks/pull/3504#issuecomment-4585445595
Note on ACKED semantics:
The Redis documentation states that ACKED only deletes entries that were
"read and
acknowledged by all consumer groups", but it does not explicitly describe
how
`last_delivered_id` interacts with PEL state when determining whether an
entry can be
deleted.
I verified Redis 8.2 behavior with several scenarios:
* A consumer group created with `$` does not block deletion of entries
that predate its
delivery cursor, even if it never acknowledged those entries.
* A consumer group whose `last_delivered_id` is still behind the entry
does block deletion,
even when no PEL entry exists.
* A consumer group that has already delivered the entry but still has a
PEL entry for it
also blocks deletion.
* Once the PEL entry is acknowledged and removed, the same group no longer
blocks deletion,
even if its `last_delivered_id` is far beyond the entry.
These results suggest that ACKED is not determined solely by PEL
membership. Redis 8.2
appears to treat the following conditions as independently preventing
deletion:
* The group has not yet reached the entry (`entry_id > last_delivered_id`).
* The group still has a pending reference, meaning a PEL entry exists for
the entry.
The additional `last_delivered_id` check is intended to preserve this
Redis 8.2 behavior by
distinguishing two no-PEL cases: a group that has not reached the entry
yet must still
block deletion, while a group that was created past the entry, for example
using `$`,
should not block deletion.
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