numinnex commented on code in PR #3564:
URL: https://github.com/apache/iggy/pull/3564#discussion_r3480644735


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core/metadata/src/stm/stream.rs:
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Review Comment:
   Thanks — you're right that the comment overclaims, and I want to separate 
two things:
   
   This comment block and the `round_robin_counter` ordering it refers to come 
from #3527 (the consumer-group port), not this PR. The only snapshot change 
here is `purge_generation`, appended as the last `#[serde(default)]` field of 
`PartitionSnapshot` — exactly the safe trailing-append case the comment 
prescribes, so this PR doesn't introduce a new compatibility hazard.
   
   On the underlying concern itself: agreed — an older snapshot that still 
carried `round_robin_counter` mid-array would break `rmp_serde` (it would hit 
the integer where a `Vec` is now expected); `serde(default)` only rescues 
*trailing* gaps, not removed/reordered positions. But server-ng has no shipped 
or versioned snapshot format yet, so there are no such files to migrate. The 
#3527 comment is worth tightening to not claim general compatibility, but 
that's outside this PR's scope.



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