writeonce/docs/plan/oop-vm/04-db-binding.md

DB binding — row format, id discipline, WAL layout, query subset

Normative companion to the engine plan (2026-08-01-db-engine-binding.md), the way 00-wob-format.md is normative for the image. Grows with the plan's tasks; this revision covers Task 1 (row storage). Memory-safety doctrine lives in the 9b design's section 6 (the copy bulkhead) — this doc is the format.

Two memory worlds, one crossing rule

Rows store no VM pointer, ever. Values cross from VM heap to row storage by copy on insert, and back by copy on read (wo_row_read allocates fresh VM values from the shard's runtime). The engine's own allocations are plain malloc — never the VM arena, so table growth cannot eat the program's heap cap, and a heap-exhausted program can still read its data.

Row format

row      := header slots
header   := id u64 | class_id u32 | flags u32          (16 bytes)
slots    := field_cnt × u64, declaration order          (the VM object shape)

One 8-byte slot per field, kind-driven — the same kind bytes the .wob class table carries, walked the same way the VM walks them:

kind	slot holds	engine-owned shape
SCALAR	the 8 bytes themselves	— (WO_NIL_SCALAR spells a ?scalar nil)
TEXT	pointer, 0 = nil	db_text { len u32; bytes[] }
OWNED	pointer, 0 = nil	db_rec { class_id u32; slots[] } — flattened by value, recursively through these same rules
MULTI	pointer, 0 = nil	db_multi { elem_kind u8; len u32; items[] }, elements encoded element-wise
MAP	pointer, 0 = nil	db_map { key_kind, val_kind u8; len u32; kv pairs }
GCREF	never stored	compile error upstream (the GC bulkhead); the engine refuses it defensively as an encode error

ref T is a SCALAR at this layer — the target row's id. The engine learns what it references only when the FK checks land (9b plan, Task 3).

Storage

Per shard, per class, created lazily on first insert:

• Slabs of 256 rows (DB_SLAB_ROWS), malloc'd, never moved or freed while the table lives — a row's address is stable for its lifetime, which is the property 9b's loop-scoped row views stand on.
• An occupancy bitmap (one bit per slot, slab-major) and a LIFO free-slot list: removal recycles the slot; a recycled slot is always used before a new slab grows. Ids are never reused; slots are.
• The primary index: an open-addressing hash, id → slot, splitmix64 finalizer, power-of-two capacity, 0.7 load, tombstoned deletes (ids are never 0 and never reused, so the all-ones sentinel cannot collide).

Id discipline

Per table, per shard: shard S of N allocates S+1, S+1+N, S+1+2N, … — the c-runtime plan's shipped interleave. Creation is coordination-free; a row's owner shard is (id-1) % N. Milestone 1 runs at N=1 and everything degenerates to 1, 2, 3, …. Id 0 does not exist (it is the hash's "empty" and the ?ref's nil).

Choke points

wo_row_insert and wo_row_remove are the only functions that mutate a table. Task 4's secondary indexes hook exactly these two sites (marked INDEX HOOK in database/src/table.c); the WAL (Task 2) stages its record beside the same calls. Anything else touching a slab is a defect by definition — the doctrine the Rust engine learned and this engine enforces.

WAL (Task 2) — database/src/wal.{c,h}

Record framing, replay-whole-or-not-at-all:

record  := len u32 | crc u32 | payload | mark u32
len      = payload bytes (never 0: a zero length is the preallocated tail)
crc      = CRC32 (poly 0xEDB88320) of the payload
mark     = 0x574F4C31 "WOL1", the last bytes of the record — a record
           without its mark is torn by definition
payload := kind u8 | class_id u32 | row_id u64 | body
kind     = 1 insert (body = fields), 2 remove (no body), 3 update (Task 5)

Body fields walk the class table's kinds: SCALAR 8 bytes; TEXT u32 len + bytes (0xFFFFFFFF = nil); OWNED presence u8 then class id + fields recursively; MULTI presence + elem kind + len + elements; MAP presence + both kinds + len + pairs. Little-endian, same platform note as the loader.

Commit order (doctrine, verbatim from the shipped phase-D pattern): RAM apply → stage record → wo_wal_commit (one pwrite of the batch + one fdatasync) → only then acknowledge. Group commit = everything staged since the last commit rides one sync.

Replay decodes payloads straight into engine-owned values — no VM heap involved, boot cannot depend on a VM existing — and rows re-enter through the choke-point row API, so Task 4's indexes rebuild for free. A torn tail (short record, bad CRC, missing mark, zero length) ends the intact prefix: everything from the tear on is dropped whole, and wo_wal_open positions its write offset AT the tear so the next commit overwrites it. A record that CRC-passes but does not decode is corruption, not a tear — replay fails loudly. A missing file is a fresh boot, not an error. After replay each table's next_id sits past every replayed id this shard owns.

Oracle: wo_wal_check(path) walks a file with no engine and reports the intact record count and prefix end — the crash battery's verifier (runtime/test/test_wal.c: five rounds of insert/commit/ack-over-pipe with SIGKILL mid-stream; every acked row present and exact after replay).

Still to come in this document

• Task 3: the insert statement's builtin ids (appended to 00-wob-format.md's builtin table) and execution contract.
• Task 4: secondary-index format, @unique trap code.
• Task 5: the select subset, update record semantics, and its builtins.