Aleksey Shakhmatov
13d7b79f2c
The M0 implementation review found three of D1-D9 wrong or incomplete. The originals stay in place with pointers to a new amendments section, so a later session can see what changed rather than reading a rewritten history. A1: D4 as written is unsound. The doc and overflow slabs are append-only, so replay repairs them, but B+tree node pages are mutated in place -- after a crash the file holds an arbitrary mix of written-back and not-written-back pages, and once D6.3 truncates the log the data file is the only copy below the watermark. A half-persisted tree is unrecoverable. So the checkpoint needs shadow paging: no page below the last watermark's allocation mark is ever stored into, and the watermark write is the atomic switch. Knock-on: node ids cannot be page numbers, because Node.parent/next/prev are back-pointers by id and copy-on-write would cascade; an in-RAM id->page table per index keeps every persisted id at its current width and gives COW one pointer to fix. A2: follows from A1 -- a page free list is a prerequisite, not the defense-in-depth D6.2 assumed, because COW abandons every page it touches in every epoch. The churn gate stays, retargeted at document garbage. A3: section 5 step 4's trap was misidentified. store_record already copies keys, so "entries must own their key bytes" is work that does not need doing. The real problem is that a leaf record has nowhere to put a slab offset; the resolution is to make the payload that offset, in every index, and delete Entry.id rather than re-own it. A4: making _id_ a unique index keys uniqueness on the canonical encode_key rather than serialize_value, so int32 1 / int64 1 / double 1.0 collide as they do in MongoDB -- a compatibility improvement, with a documented one-way migration hazard for a database that already holds two such documents. Also records that Engine.seq is never restored on open (harmless today, silent data loss once a watermark exists), and two bugs the new spec harness found. AGENTS.md carries the same rules into the operating guide: ground rules grow from 7 to 9, and the old rule 6 is corrected with a note saying why.
199 lines
9.6 KiB
Markdown
199 lines
9.6 KiB
Markdown
# AGENTS.md — operating guide for AI agents working in this repo
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Read this first. It exists so any agent — in any session, without the
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original conversation's context — can work here safely and verifiably.
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## What this is
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MultiforaDB: a lightweight, embedded, MongoDB-compatible document database
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in Zig 0.16. It speaks the MongoDB wire protocol (OP_MSG, maxWireVersion 8)
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so real drivers (`mongosh`, Node, PyMongo) connect over TCP. Everything
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lives in a single-file LZ4-block log with an in-memory engine, B+tree
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indexes, and TTL/unique/sparse/compound index support.
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**Forward plan**: the project's direction — a full-fledged embedded,
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tens-of-GB, maximally MongoDB-compatible database — is decided and written
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in **[PLAN.md](PLAN.md)**. The current milestone is **M0 (mmap + WAL
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storage foundation)**. Before starting any work, read PLAN.md; its decision
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record (D1-D9) and ground rules are binding.
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## Read first, in order
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1. **PLAN.md** — the plan: decisions, milestones M0-M9, gates, work
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breakdown, deferred designs. The next session always starts here.
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2. **README.md** — current state, features, benchmark table, known gaps,
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working-with-large-collections notes.
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3. **ROADMAP.md** — design write-ups of the completed performance work
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(B+tree, ordered `_id` index, compressed log, byte storage, decomposed
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locks). Contains the project's ground rules and verification recipes.
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## Toolchain
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- **Zig 0.16.0** — pinned. Do not assume a newer Zig is compatible.
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- **Node + the official `mongodb` driver** (in `tests/e2e/node_modules`)
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for e2e. `npm install` in `tests/e2e` if node_modules is missing.
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## Build / test / run
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```sh
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zig build # builds zig-out/bin/multiforadb (ReleaseFast by default)
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zig build test # unit tests, ReleaseFast
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zig build test -Doptimize=ReleaseSafe # unit tests with safety checks on
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zig-out/bin/multiforadb --port 27017 --db data.log
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```
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**Critical traps:**
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- **`zig build test` does NOT refresh the server binary.** After any change
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under `src/`, rebuild with `zig build` before restarting the server, or
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e2e will run against stale code and report failures that the source no
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longer explains. (This has bitten repeatedly; it is documented in
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`tests/e2e/README.md`.)
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- **Debug builds are 10-200x slower** than ReleaseFast on every path. Never
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benchmark (or time out waiting for) a Debug server. `zig build` defaults
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to ReleaseFast on purpose.
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## E2E discipline (any change to src/)
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```sh
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zig build # rebuild the server first!
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zig build test # ReleaseFast
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zig build test -Doptimize=ReleaseSafe
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```
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Then, against a server on port 27020 (TTL suites need `--ttl-sweep-secs 1`):
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```sh
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node tests/e2e/e2e.js # CRUD + operators + aggregate + errors
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node tests/e2e/e2e2.js concurrent
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node tests/e2e/e2e2.js crash-a # write 50 docs, kill -9 the server
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node tests/e2e/e2e2.js crash-b # restart, verify all 50 survived
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node tests/e2e/e2e3.js # secondary indexes
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node tests/e2e/e2e4.js # TTL indexes (server must run --ttl-sweep-secs 1)
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node tests/e2e/e2e6.js # self-contained full lifecycle (spawns its own server, incl. kill -9)
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```
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Which suites to run for a given change:
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- anything touching the write path or log format → the crash pair
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(e2e2 crash-a/b) and e2e6
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- anything touching indexes → e2e3.js and e2e4.js
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- everything → all of the above
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`tests/e2e/README.md` has the full matrix, ports, and harness docs
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(`big.js` for multi-GB loads, `compare-run.sh`/`bench-run.sh` for the
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benchmark gate).
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## Spec tests (the compatibility gate — PLAN D2)
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```sh
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bash tests/spec/fetch.sh # pinned mongodb/specifications suites
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node tests/spec/run.js --scorecard # run + re-record tests/spec/scorecard.txt
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node tests/spec/run.js --file find.json --verbose
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```
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`tests/spec/scorecard.txt` is committed and re-recorded every milestone (PLAN
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D9), so progress is verifiable across sessions. It is a *measuring* tool: a red
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baseline is the expected state at M0, `run.js` exits 0 as long as it ran, and
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nothing unimplemented is ever counted as a pass. `tests/spec/README.md` has the
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scope, the pins, and what is not yet asserted.
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## Ground rules (binding — from ROADMAP.md and PLAN.md)
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1. **Measure A/B on one harness. Do not trust the model.** Several
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predictions made during the existing work were wrong in both directions.
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The cheap A/B is: flip one line, rebuild, run, flip it back.
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2. **Mutation-check any test guarding an invariant.** Break the thing the
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test is supposed to catch and confirm it goes red before trusting it.
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3. **The index invariant is absolute** (`src/index.zig` header): an index
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only *generates candidates*; the full filter is re-applied afterwards.
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Over-approximating is slow. Under-approximating is a wrong answer.
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4. **The database must always open.** Replay never refuses to start over
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recoverable damage (e.g., a unique index finding duplicates in existing
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data warns and keeps going).
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5. **The checkpoint never describes a state ahead of the durable log
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tail** (M0 invariant): watermark advancement is the last step of the
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checkpoint protocol, after the data-file fsync.
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6. **No page below the stable mark is ever stored into** (M0, PLAN
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Amendment A1). The whole crash story is downstream of this one rule: a
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mutation that would dirty a page belonging to the last durable
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checkpoint copies it to a fresh page first. Enforced structurally by a
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copy-on-write page accessor, and mechanically by `mprotect`-ing the
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stable prefix read-only in ReleaseSafe/test builds — a missed COW then
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segfaults in the suite instead of corrupting a database silently.
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7. **A checkpoint never renumbers slab offsets** (M0, Amendment A3). Index
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leaves hold physical document offsets; only a full rebuild may move
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documents, and it rebuilds every index in the same pass.
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8. **Write-then-extend discipline for mmap**: ftruncate before touching
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new pages; never fault past the end of the mapped file (SIGBUS).
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9. **`Engine.seq` is restored on open** as `max(watermark.seq, max replayed
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record.seq)`, and a log rewrite never emits a record whose seq is ≤ the
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watermark. Today `seq` restarts at 0 on every open — harmless without a
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watermark, silent data loss with one.
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Note for anyone reading an older revision: rule 6 previously read "index
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entries own their key bytes". That was a misdiagnosis — `store_record`
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already copies keys. See PLAN Amendment A3 for what the actual trap was.
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## Code style
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Binding reference: **`docs/TIGER_STYLE.md`** (TigerBeetle's TigerStyle,
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adopted verbatim). Read it before writing code. Its mechanical rules are
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enforced: `zig fmt` clean, 4-space indent, hard 100-column line limit, no
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`if` without braces unless it fits on one line, snake_case for functions/
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variables/files, units last in names (`latency_ms_max`), and always-on
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assertions (the repo's `assert.zig`, see below).
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Project-specific rules and deliberate deviations from TigerStyle:
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- Zig 0.16 idioms: `std.Io` threaded through everything, unmanaged
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containers, arena-threaded allocators where the code does.
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- House style: user-declared functions use `snake_case`; see existing
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`src/` for the pattern. Keep `src/lib.zig` as the library root and keep
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the server (`src/server.zig`) a thin front-end — the C API seam (PLAN
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D1) depends on that staying clean.
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- Public code invariants are asserted with the repo's own `assert.zig`
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(always active, including ReleaseFast) rather than `std.debug.assert`,
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so they do not silently vanish in optimized builds.
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- **Deviation (allocation):** TigerStyle's "statically allocated at
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startup, no allocation after init" is not yet this repo's architecture —
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the engine is arena-threaded. Do not introduce new *long-lived* dynamic
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allocations on hot paths; arenas scoped to a request are fine. Static
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allocation is a deferred design item (PLAN).
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- **Deviation (recursion):** TigerStyle bans recursion. `bson.zig`'s
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document parser is still recursive (nested docs/arrays); keep new code
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iterative and do not extend the recursion depth usage without a design
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note. Bounding parser depth is tracked work.
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- **Functions stay under 70 lines** (TigerStyle hard limit). A handful of
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pre-existing offenders remain (see the reflow/refactor backlog in
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ROADMAP notes); new code must not add to the count.
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## Source map
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## Source map
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```
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src/bson.zig BSON parse/serialize, ObjectId, canonical comparison/key encoding
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src/wire.zig OP_MSG/OP_QUERY framing, message + reply builders
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src/commands.zig command dispatch (hello, CRUD, aggregate, admin, indexes)
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src/server.zig TCP accept loop, per-connection handlers, TTL sweep monitor
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src/db.zig engine: db → collection → _id → document maps, slab storage
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src/storage.zig append-only log: blocks, LZ4, XxHash3, replay, compaction
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src/query.zig filter matcher, regex engine, sort, projection
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src/index.zig B+tree indexes: entries, search, query planner
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src/update.zig update operators with dot-path navigation
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src/main.zig CLI: --port, --bind, --db, --ttl-sweep-secs, --compact-threshold
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```
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## Working on a milestone
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1. Re-read PLAN.md (and this file). Confirm which milestone is current.
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2. Per-milestone gates are in PLAN.md; the M0 gate is 6 items ending with
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benchmark parity (phase8 baseline) and the spec-test runner red
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baseline.
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3. Commit scorecard and benchmark results with each milestone (PLAN D9) so
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progress stays verifiable across sessions.
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4. Deferred designs (cursors, aggregation, transactions, change streams,
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C API) are deliberately *not* specified yet — grill the design with the
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human before implementing (PLAN section 6).
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