shoney.arickathil
a873cf7331
- `randread N R` in the sample: fill N rows, read R across the WHOLE range - Weyl order `i*2654435761 mod n` — no RNG in the language, none needed; both legs read the SAME key order so residency is the only variable - `randread` driver leg: control (256 MiB, does not bind) vs over-cap (6 MiB + swap), sizes kept modest — quick resolves it in ~5s - gates the RATIO, not the absolutes: over-cap reads/sec belongs to the box's swap device, the factor between two runs belongs to the engine - reads must all resolve (hits == R) or the leg fails; a collapse measured over unresolved reads is noise - 133 checks, 0 failures; gate bites on a doctored collapse_x Measured — this closes the gap the swap leg left: - resident 1 851 166 reads/sec, p50 0us p99 1us - over-cap 6 771 reads/sec, p50 128us p99 487us - 273x throughput, ~480x p99, all 20 000 reads resolving in both - so the two access patterns sit ~270x apart under identical pressure: append-mostly insert ~1%, random read 273x - departure is a STEP not a curve (1us -> 487us, nothing between), which is why p99_departure_decile finds no knee — there is none - caveat recorded, NOT inherited: this is demand-paged anonymous memory through swap (4 KiB/fault, no readahead). `resident: keys` preads via the page cache — should be better, but databasev2 2 task 7 must measure its own read path. New criterion added there Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
190 lines
12 KiB
Markdown
190 lines
12 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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**Corrected 2026-08-27 by measurement.** This section used to open "an
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allocation failure is a catchable trap, not a crash", and that is true only of
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the VM arena. Table storage has no ceiling, and with `vm.overcommit_memory = 0`
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its `malloc` never fails — the process is **SIGKILLed** (rc=137, measured at
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360 000 rows under a 64 MiB cap). The checked path below is real, but it is the
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arena's, not the store's. See [iteration 1](01-ram-ceiling-measurement.md).
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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.**
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**Iteration [1](01-ram-ceiling-measurement.md) has now measured this
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(2026-08-27), and it strengthened the statement rather than softening it.** A row
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costs **96.5–100 B** Int-only and **320.6–324 B** text-heavy (3.3× apart, so no
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single per-row number can bound RAM). At the ceiling the engine has exactly two
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behaviours and **neither one tells anybody**: without swap the process is
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**SIGKILLed on signal 9** — table storage has no checked ceiling, and under
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`vm.overcommit_memory = 0` its `malloc` succeeds and the kernel kills on page
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touch — and with swap it **keeps returning 0 while serving from disk**, finishing
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900 000 rows in 148 s against 150 s uncapped. Durability is the one thing that
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does hold: acked writes came back as an intact prefix across an OOM kill.
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And when swap does absorb it, the price depends entirely on access pattern:
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inserting pays **~1%**, while reading randomly across the table pays **273×**
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(1 851 166 reads/s resident against 6 771 over-cap, p99 1 µs against 487 µs).
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That second number is the one this track must respect, because it is the access
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pattern [2](02-table-storage-modes.md)'s `resident: keys` creates by design.
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That is why "back-pressure at exhaustion" is not a design option. Exhaustion
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either kills without warning or never arrives — and the latency signal offers no
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early warning either, since departure is a **step** (1 µs to 487 µs, nothing in
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between) rather than a curve. Only a **declared threshold** can speak in time.
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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` moves the decision into the language, where the compiler can
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act on it. **Two keys, not one enum** — the developer is answering two
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independent questions, and an enum would need a name for every combination:
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- **`durable: true | false`** (default `true`). `false` skips the WAL append
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entirely: no record, no fsync, ack from RAM, table empty after restart. The
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compiler can then refuse a program that stores a durable `ref` into such a
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table, because that id would dangle across a restart (WO-E224).
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- **`resident: all | keys`** (default `all`). `keys` keeps the id map, the
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secondary indexes and the unique shadows resident and reads rows back from
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the log by offset. This is the key that raises the ceiling — and the
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arithmetic is why it works: 240M rows × 16 B of index ≈ 3.8 GB resident for a
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120 GB table.
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`durable: false` with `resident: keys` is refused: rows would be neither logged
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nor resident, so there would be nowhere to read them from.
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The grammar change was small, as predicted — `Ast.table_cfg` gained two fields
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and the parser's argument match two arms. The *semantics* were the work, which
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is why iteration 2 is 7 tasks rather than one.
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**Does this break principle 7?** It amends it, deliberately, and the amendment
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is applied: the **log** is authoritative and residency is a declared per-table
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policy. Durability is untouched and unconditional — ack after fsync, replay
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whole-or-nothing, torn tails dropped by CRC. What stays rejected is a *second*
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engine: a paged B-tree with its own buffer pool. Reading rows from the log we
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already write is not that.
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An earlier draft of this section proposed a three-valued `mode:` enum including
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`cold`. That name conflated durability with residency and could not be defined
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before its mechanism existed; the history is in
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[iteration 2](02-table-storage-modes.md).
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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) | 🔄 **measured 2026-08-27**: footprint per shape (3.3× apart), the two silent exits (SIGKILL vs swap-serving-from-disk at ~uncapped speed), and ack-after-fsync surviving an OOM kill. Also measured: the **273× random-read collapse** over an oversized table. Outstanding: a replay baseline | nothing; extends iteration 22's harness |
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| 2 | [per-table storage](02-table-storage-modes.md) | the grammar: `durable: true\|false` and `resident: all\|keys`, per table, replacing the global `WO_DATA` all-or-nothing. **In progress — the `durable` half is done** | 1 for the budget default |
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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) | ⚠ **largely superseded by 2** — `resident: keys` is the ceiling-raiser. Its premise (a user-space resident working set) was rejected in favour of the kernel page cache. Revisit only with a measurement showing the page cache insufficient | — |
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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 the budget default should follow from a
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measurement, not a guess. **3 and 4 matter to 2** for the same reason tiering
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onto a never-truncating log would have: `resident: keys` rebuilds its offset map
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by scanning the whole log at boot until 3's snapshot persists it.
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Amended 2026-08-27: the original rationale sequenced **6** as the ceiling-raiser
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after 3, 4 and 5. `resident: keys` took that role into iteration 2, so 6 is
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largely superseded and 5 is no longer a prerequisite for anything on the
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critical path.
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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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