shoney.arickathil
566559cf70
- `resident: all | keys` replaces `resident: all | index`. Two reasons beyond taste: it kills the collision with the `index:` argument (`@table(index: [customer], resident: index)` read badly), and it puts both values on ONE axis — each now answers "what row data stays resident", where `all`/`index` mixed a quantity with a structure name - accurate as well as clearer: what stays resident is the id->offset map, the secondary indexes and the unique shadows — all key structures; row payloads are exactly what leaves. `resident: none` was rejected as overclaiming, since the indexes very much are resident - checked for collisions: neither `all` nor `keys` is a keyword or a builtin (`key_at`/`val_at` exist, bare `keys` does not) - the spec's wart note became a recorded decision; the rejected spelling is kept quoted so the rationale still reads - fixes a bug I introduced in the 2026-08-26 track move: all six moved iterations carried a banner reading "Part of [Story — the database beyond RAM]" whose link pointed at the LANGUAGE arc — correct target, lying text, the exact failure mode the link audit warned about. Banners now point at the databasev2 story, and the original "Part of" line says plainly which track the iteration was authored in before the move - linkcheck 0 broken / 0 anchors Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
105 lines
4.9 KiB
Markdown
105 lines
4.9 KiB
Markdown
---
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track: databasev2
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iteration: "3"
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was_language_iteration: "32"
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status: refine
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chain: 6
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---
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# databasev2 3 — WAL checkpoint: disk space reclamation and bounded replay
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> **Moved 2026-08-26** from the language track, where this was iteration 32.
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> Part of [Story — databasev2: the database beyond RAM](00-story.md). Content unchanged by
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> the move; its dependencies are restated in that track index.
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> Format: `product/story-iteration-template`. Part of
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> [Story — one language, one runtime, one database, one binary](../language-runtime-database/00-story.md)
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> — the track this iteration was authored in before the 2026-08-26 move.
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>
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> **Inserted 2026-08-21** (stage-3 guarantee refinement found the hole):
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> the WAL is append-only FOREVER — no checkpoint, no truncation exists
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> in the engine or anywhere on the roadmap. Disk grows without bound and
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> replay time grows with history, so restart cost rises with every write
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> the program ever made. RAM reclamation already exists (deleted rows
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> free their slot — [`04-db-binding.md`](../../plan/oop-vm/04-db-binding.md):
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> "Ids are never reused; slots are"); this iteration is the DISK half.
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> LAST in the concurrency chain:
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> **stage 3 → 22 → 31 → 24 → 23 → 32** — it wants 22's measured
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> replay/restart numbers to justify its policy and must compose with
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> 23's group-commit write path.
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## Goals
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- **Disk space is reclaimed.** A checkpoint writes the live store as a
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snapshot and truncates the WAL behind it; deleted rows and
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overwritten versions stop occupying disk forever.
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- **Replay is bounded.** Startup replays snapshot + WAL tail, not the
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program's whole write history — restart time becomes a function of
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store size, not store age.
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- **Every existing guarantee holds byte-for-byte.** Ack-after-durable,
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replay-whole-or-not-at-all, torn-tail drop, ids never reused — a
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checkpoint changes where bytes live, never what an ack means. A crash
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DURING checkpoint recovers from the previous snapshot + full tail:
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the old WAL is not truncated until the new snapshot is durable.
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## Acceptance Criteria (draft — the spec refines)
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- **Given** a store with N rows after many writes and deletes, **when**
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a checkpoint completes, **then** disk usage reflects the live rows
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(plus the WAL tail), and a restart replays snapshot + tail to the
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byte-identical store.
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- **Given** kill -9 at ANY instant during a checkpoint, **when** the
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process restarts, **then** recovery produces the same consistent
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store as if the checkpoint had never started — no acknowledged write
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lost, no partial snapshot ever read.
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- **Given** the iteration-22 restart benchmark re-run after checkpoint
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lands, **when** replay time is measured on an aged store, **then**
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the bounded-replay improvement is recorded as a before/after delta.
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- **Given** writes arriving while a checkpoint runs (the DB actor
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serializes statements; the checkpoint must not stall them beyond the
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stated budget), **when** the mixed load completes, **then** every ack
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held its durability contract and the tail contains exactly the
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post-snapshot writes.
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## Out Of Scope
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- MVCC / multi-version reads — the store is update-in-place RAM; "old
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versions" exist only as WAL history, which is exactly what truncation
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reclaims.
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- Incremental/streaming backup, point-in-time recovery — a snapshot is
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a recovery artifact here, not a backup product.
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- Cross-shard checkpoint coordination — the WAL is owner-shard-only
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(stage 3's rule); one shard, one checkpoint.
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- Compression, dedup, tiering — measure first (22), add only what a
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number justifies.
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## Info
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Forks the spec must settle:
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1. **Snapshot format** — a row-image dump of the live store (simple,
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O(live rows)) vs a rewritten-compacted WAL (reuses replay machinery,
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O(live rows) too but stays in one format). Leaning: row-image dump
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in the WAL's existing record grammar, so replay needs no second
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decoder.
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2. **Trigger policy** — size threshold (WAL bytes vs snapshot bytes
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ratio), boot-time compaction, explicit call, or some mix. Leaning:
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ratio threshold checked at commit, plus manual trigger for tests;
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decided against 22's numbers.
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3. **Write availability during checkpoint** — stop-the-world dump
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(simplest; the DB actor just runs one long "statement") vs
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fork-and-dump vs incremental copy. Leaning: measure the
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stop-the-world pause on the 1M-row store first (22); complexity only
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if the pause breaks a stated budget.
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4. **Composition with 23** — the snapshot's durability barrier rides
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the same per-shard ring (WRITE+FSYNC chain, then the truncate);
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ordering vs in-flight group commits must be stated normatively in
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[`04-db-binding.md`](../../plan/oop-vm/04-db-binding.md)'s WAL
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section.
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## Proposed Solution
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Brainstorm → spec → plan after 23 lands (the write path it composes
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with) using 22's aged-store replay numbers as the policy input; extend
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`04-db-binding.md`'s WAL section with the snapshot format the way the
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record grammar is documented today.
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