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https://issues.apache.org/jira/browse/CASSANDRA-21452?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Jon Haddad updated CASSANDRA-21452:
-----------------------------------
Issue Type: Epic (was: Improvement)
> Complete cursor compaction coverage (complex columns, BTI, wide schemas),
> verified byte-for-byte against the iterator path
> --------------------------------------------------------------------------------------------------------------------------
>
> Key: CASSANDRA-21452
> URL: https://issues.apache.org/jira/browse/CASSANDRA-21452
> Project: Apache Cassandra
> Issue Type: Epic
> Components: Local/Compaction
> Reporter: Jon Haddad
> Assignee: Jon Haddad
> Priority: Normal
>
> h2. Background
> CASSANDRA-20918 introduced cursor-based compaction ({{CursorCompactor}},
> {{SSTableCursorReader}}, {{SSTableCursorWriter}}): a streaming merge that
> avoids the per-partition/row/cell object materialization of the iterator path
> ({{CompactionIterator}}). Reported results: 2–5x faster compaction, up to
> ~100x allocation reduction, with a fixed memory footprint per sstable.
> The motivating problem is CASSANDRA-20428: {{ByteArrayAccessor.read}} — the
> {{new byte[length]}} per value performed by cell and clustering
> deserialization — is a dominant allocation source wherever the iterator-based
> read machinery runs, and compaction pays it for every cell of every sstable
> it merges. The cursor path eliminates that allocation class entirely, but
> only for the schemas and configurations it supports; everything else silently
> falls back to the iterator path at {{AbstractCompactionPipeline.create}}.
> h2. What was missing
> As merged, {{CursorCompactor.isSupported()}} / {{unsupportedMetadata()}}
> reject most real-world schemas and configurations, so the allocation and
> throughput wins do not apply where they matter most:
> # *Multi-cell columns* — non-frozen collections and UDTs are unsupported. Any
> table with a {{map}}, {{set}}, {{list}}, or non-frozen UDT column falls back.
> # *BTI output format* — the cursor only writes the BIG format. Clusters on
> the trie format (the trunk default direction) never use the cursor path.
> # *Partial-range scanners* — token-subrange compactions fall back.
> # *Counters* — counter tables fall back.
> # *Garbage-skipping modes* — {{tombstoneOption != NONE}} falls back (default
> is NONE, so this is lower priority).
> Out of scope, by design: secondary indexes / SAI (index observers consume
> materialized rows and need their own design) and pre-current sstable versions
> (compaction rewrites to the current version, so the gap phases out on its
> own).
> Equally missing was *verification*: the existing cursor tests asserted each
> path's output against expected CQL results independently, but never compared
> the two paths against each other, and never at the file level. Two compaction
> implementations that must be interchangeable at scale need a stronger bar
> than "both look right in isolation" — divergence between cursor- and
> iterator-compacted replicas is silent and permanent.
> h2. What needs to be implemented
> Close the support gaps incrementally — complex columns, then BTI output, then
> partial-range scanners, then counters, with the garbage-skipper equivalent
> optional — with each increment landing only when it passes the verification
> bar below. Wide schemas (>64-column subset encodings), materialized-view
> tables (strict liveness), and multi-gigabyte partitions are part of the
> supported surface and must be covered, not special-cased away.
> Two standing constraints:
> * *Garbage-free is the point of the feature.* Every change must preserve the
> no-per-element-allocation property of the hot path, enforced by measurement
> rather than code review alone.
> * *Behavior must be identical, not merely equivalent.* Purge decisions,
> tie-breaks, stats, index structure — everything.
> h2. Required: byte-for-byte differential verification
> The two compaction implementations must be interchangeable, and the way to
> prove that is a test harness that compacts the same inputs through both
> production pipelines and compares the outputs *byte for byte* — every
> component of every output sstable, in both sstable formats. Any component
> permitted to diverge needs an explicit, justified exception; the expectation
> is that none are needed. (Experience so far supports the bar: byte
> divergences found this way have been bugs, not justified differences.)
> Coverage should span the supported surface — the tombstone, TTL, and
> timestamp edge cases where merge implementations classically disagree,
> randomized schemas and workloads, and large-scale shapes (wide schemas,
> many-sstable merges, multi-gigabyte partitions) — and should grow with each
> increment so a closed gap can never silently fall back or regress. The
> harness design beyond that is an implementation detail of the patch.
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