shoney.arickathil
746dc2b42b
- docs/stories/databasev2/, numbered from 1. Six PENDING database iterations
moved from the language track and renumbered, keeping the old id in
`was_language_iteration:` so a search for "iteration 32" still finds it:
32 -> 3 WAL checkpoint, 23 -> 4 io_uring commit, 33 -> 7 single-file store,
27 -> 8 query grammar, 20 -> 9 cross-program, 21 -> 10 keypair auth.
Done work (9, 9b, 22) stays as v1 history; language 18 left whole
- the problem, read off the engine not guessed: rows are malloc'd slabs with
addresses stable forever, NO eviction/spill/paging anywhere in database/src,
the WAL never checkpoints so boot replays all history, and durability is one
process-global WO_DATA so no table can say it matters more than another.
An allocation failure IS a clean catchable WO_T_OOM — but swap thrash
arrives first and carries no error signal at all, which is the real hazard
- four new iterations:
1 measure the ceiling FIRST (curve not cliff; the three exits; kill -9 at
exhaustion) — every later default should follow from a number
2 `@table(mode: ram | durable | cold)` — the grammar ask. Small surface
(Ast.table_cfg gains a key, the parser already rejects unknown args), big
semantics: `durable` defaults so nothing changes silently, and the
compiler refuses a durable row holding a `ref` into a ram table
5 bounded tables + refuse/evict/back-pressure, shedding BEFORE the OS acts
6 cold tiering — mostly forks, incl. whether the language surfaces the
fault cost and whether @unique on cold is refused outright. A paged
B-tree stays rejected: if tiering needs one, reject tiering
- 39 links repointed, link TEXT renumbered to track-local ids; arc gains one
pointer row replacing the six moved; board + board-views cover three tracks
- linkcheck 0 broken / 0 anchors; no code blocks in any story
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
152 lines
9.1 KiB
Markdown
152 lines
9.1 KiB
Markdown
# Story — databasev2: the database beyond RAM
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The third track. `language-runtime-database/` built the engine;
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[`porch/`](../porch/00-story.md) is the framework on top; this track answers the
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question v1 deliberately deferred: **what happens when the data does not fit in
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memory.**
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Numbering restarts at 1, local to this track. Frontmatter carries
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`track: databasev2`, and iterations moved here keep their old id in
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`was_language_iteration:` so a search for "iteration 32" still finds the WAL
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checkpoint. Status stays where it belongs — the `status:` key, never a directory.
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## The problem, stated honestly
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Principle 7 says **RAM is authoritative; the WAL makes it durable.** That is a
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real design, not a shortcut: reads never touch disk, so latency is predictable,
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and durability is a sequential append rather than a storage engine bolted to the
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side. Iteration 22 measured what it buys — reads at 1.3M ops/s after the index
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probe landed, p50 1µs.
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The bill comes due at the ceiling. Read from the engine as it stands:
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- **Rows live in `malloc`'d slabs of 256, and their addresses are stable
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forever** (`database/src/table.c`, `DB_SLAB_ROWS`). Slabs are allocated as a
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table grows and freed only when the table is destroyed. The free-slot list
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recycles removed slots, so a delete-heavy table plateaus — but a growing table
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only grows.
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- **The ceiling is process RSS, not a configured number.** `WO_HEAP_MB` (default
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64 MiB) bounds the VM object arena; table storage is separate `malloc`, so
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nothing in the system declares a maximum dataset size. There is no knob that
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says "this database may use at most N".
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- **There is no eviction, no spill, no paging, no LRU.** Grep
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`database/src/` for any of them and nothing comes back. Every row ever
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inserted and not deleted is resident.
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- **The WAL is append-only with no checkpoint.** Boot replays every record ever
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written, so startup time is O(all writes in the file's history) and disk grows
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without bound. That is databasev2 [3](03-wal-checkpoint.md).
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- **Durability is process-global.** `WO_DATA` is one environment variable that
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turns on one `shard-0.wal` for the whole process (`runtime/src/main.c`). There
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is no way to say "this table matters, that one is scratch".
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### What actually breaks first
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Worth being precise, because the failure mode determines the fix — and the good
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news is that the engine's own behaviour is clean:
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**An allocation failure is a catchable trap, not a crash.** Every `malloc` in
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the row encoder is checked and jumps to an `oom` label; `DB_ERR_OOM` maps to
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`WO_T_OOM`, which a program can `try`/`catch`. So a writeonce program that runs
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out of memory *refuses the insert* rather than corrupting or dying. That is a
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much better starting position than most engines have.
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**But the trap is almost never what a real deployment hits first.** Long before
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`malloc` returns NULL, the box starts swapping, and a RAM-authoritative database
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on swap is the worst of both worlds: it has paid for in-memory data structures
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and is now serving them from disk with no read path designed for that. On a
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cgroup-limited host the OOM killer arrives instead, and an external `SIGKILL` is
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the one shutdown path that skips every guarantee the WAL was written to provide —
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though ack-after-fsync means acked writes still survive; iteration 22's `kill -9`
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battery proves that much.
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So the honest problem statement is not "malloc fails". It is: **there is no
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declared budget, no back-pressure as the budget is approached, and no way to
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distinguish data that must be resident from data that merely is.** Iteration
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[1](01-ram-ceiling-measurement.md) exists to replace this paragraph with
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numbers before anything is designed on top of it.
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## The lever: per-table storage modes
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The developer's ask, and the reason this track has a grammar iteration.
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Today every `@table` is identical: resident, and durable if and only if
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`WO_DATA` is set for the whole process. Real applications are not uniform —
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a session table, a rate-limit counter and a page cache want *resident and
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disposable*; an orders table wants *resident and durable*; an audit log wants
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*durable and rarely read*. One global switch cannot express that, so it forces
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either "everything is precious" or "nothing is".
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Extending `@table` with a storage mode moves the decision into the language,
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where the compiler can act on it:
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- **`ram`** — resident, never WAL-logged, gone on restart. The compiler knows
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no durability code is needed; the engine knows these rows are the first
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candidates to shed under pressure; and — the part that matters — a program
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that expects a `ram` table to survive a restart is now stating something the
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compiler can refuse.
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- **`durable`** — today's behaviour: resident and WAL-logged, ack after fsync.
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- **`cold`** — durable, and *not* required to be resident. This is the mode that
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actually raises the ceiling, and it is the one with real design work behind it
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(iteration [6](06-cold-tiering.md)).
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The grammar change is small and the surface is already the right shape:
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`Ast.table_cfg` is `{ table_name; indexes }`, the parser's argument match
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already rejects unknown keys with a catalogued diagnostic
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(`unknown @table argument ... (supported: name, index)`), and adding one more
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key follows the path `index` already cut. The *semantics* are the work, not the
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syntax — which is exactly why it gets its own iteration
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([2](02-table-storage-modes.md)) and why it comes after the measurement.
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This is also the honest answer to "does this break principle 7?" It does not.
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RAM stays authoritative **for the tables that say so**. `cold` is a declared
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exception a developer opts into per table, with the trade written at the
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declaration site rather than buried in an operations runbook.
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## The sequence
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| # | Iteration | Delivers | Needs |
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| --- | --- | --- | --- |
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| 1 | [RAM ceiling: measure the breaking point](01-ram-ceiling-measurement.md) | what actually happens from 50% RAM to OOM — swap onset, latency cliff, trap behaviour, `kill -9` survival | nothing; extends iteration 22's harness |
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| 2 | [`@table` storage modes](02-table-storage-modes.md) | the grammar: `mode: ram \| durable \| cold`, per table, replacing the global `WO_DATA` all-or-nothing | 1 for its defaults |
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| 3 | [WAL checkpoint](03-wal-checkpoint.md) *(was language 32)* | snapshot + truncate: disk reclaimed, replay bounded | 4 composes |
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| 4 | [io_uring group commit](04-io-uring-commit.md) *(was language 23)* | close the 66× durable/RAM write gap (4.5k vs 297k inserts/s) | the arc (landed) |
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| 5 | [Bounded tables and eviction](05-bounded-tables-eviction.md) | a capacity a `ram` table may not exceed, and what happens when it does | 2 |
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| 6 | [Cold tiering](06-cold-tiering.md) | rows that leave RAM and come back — the iteration that raises the ceiling | 2, 3, 5 |
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| 7 | [Single-file store](07-single-file-db.md) *(was language 33)* | `WO_DATA=<path>.db` — a file path IS the store | independent |
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| 8 | [Query grammar from corpora](08-query-grammar-corpus.md) *(was language 27)* | whole-query `count`, `exists` | independent |
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| 9 | [Cross-program tables](09-cross-program-tables.md) *(was language 20)* | attach to a running program's database over local IPC | independent |
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| 10 | [Keypair attach auth](10-keypair-attach-auth.md) *(was language 21)* | program identity as a keypair; mutual challenge–response | 9 |
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```
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1 ──▶ 2 ──▶ 5 ──▶ 6
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│ ▲
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3 ──▶ 4 ─────┘
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7, 8 independent
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9 ──▶ 10
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```
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Order rationale: **1 before 2** because a mode's default should follow from a
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measurement, not a guess. **3 and 4 before 6** because tiering onto a log that
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never truncates would make the disk problem worse, not better. **5 before 6**
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because eviction from a bounded resident table is the simpler half of the same
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mechanism, and getting the policy right there de-risks the hard half.
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## What this track does NOT own
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| Not databasev2's | Owner |
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| --- | --- |
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| `transaction { }` and `@table` feature flags | language [iteration 18](../language-runtime-database/18-memory-db-features.md) — approved spec, left whole on purpose |
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| the TTL cache middleware | also language 18 (and [porch 1](../porch/01-store-backed-middleware.md) points there) |
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| typed binding of rows into app classes | language [iteration 29 `@derive`](../language-runtime-database/29-compile-time-metaprogramming.md) |
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| `fs` mutation verbs, outbound sockets | language [iteration 38](../language-runtime-database/38-content-platform-capabilities.md) |
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| benchmark harness and CI | iteration 22 (landed) built the harness; per-change CI is language iteration 30 |
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| a paged B-tree storage engine | **nobody, deliberately.** Recorded as rejected in [`discarded.md`](../../plan/discarded.md): the disk story is the WAL. `cold` tiering is not a licence to build SQLite. |
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## Review protocol
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The language track's, unchanged: one iteration read and approved before the next
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starts; every iteration an unsplittable slice with phases, per-phase tasks,
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Given/When/Then criteria and an out-of-scope list. Every engine change is gated
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by `just employee`, `just db-actor` and `just db-bench` against
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`bench/baseline.json` — and any iteration that claims a performance change must
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move a number in that baseline, or it did not happen.
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