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.
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AGENTS.md — operating guide for AI agents working in this repo
Read this first. It exists so any agent — in any session, without the original conversation's context — can work here safely and verifiably.
What this is
MultiforaDB: a lightweight, embedded, MongoDB-compatible document database
in Zig 0.16. It speaks the MongoDB wire protocol (OP_MSG, maxWireVersion 8)
so real drivers (mongosh, Node, PyMongo) connect over TCP. Everything
lives in a single-file LZ4-block log with an in-memory engine, B+tree
indexes, and TTL/unique/sparse/compound index support.
Forward plan: the project's direction — a full-fledged embedded, tens-of-GB, maximally MongoDB-compatible database — is decided and written in PLAN.md. The current milestone is M0 (mmap + WAL storage foundation). Before starting any work, read PLAN.md; its decision record (D1-D9) and ground rules are binding.
Read first, in order
- PLAN.md — the plan: decisions, milestones M0-M9, gates, work breakdown, deferred designs. The next session always starts here.
- README.md — current state, features, benchmark table, known gaps, working-with-large-collections notes.
- ROADMAP.md — design write-ups of the completed performance work
(B+tree, ordered
_idindex, compressed log, byte storage, decomposed locks). Contains the project's ground rules and verification recipes.
Toolchain
- Zig 0.16.0 — pinned. Do not assume a newer Zig is compatible.
- Node + the official
mongodbdriver (intests/e2e/node_modules) for e2e.npm installintests/e2eif node_modules is missing.
Build / test / run
zig build # builds zig-out/bin/multiforadb (ReleaseFast by default)
zig build test # unit tests, ReleaseFast
zig build test -Doptimize=ReleaseSafe # unit tests with safety checks on
zig-out/bin/multiforadb --port 27017 --db data.log
Critical traps:
zig build testdoes NOT refresh the server binary. After any change undersrc/, rebuild withzig buildbefore restarting the server, or e2e will run against stale code and report failures that the source no longer explains. (This has bitten repeatedly; it is documented intests/e2e/README.md.)- Debug builds are 10-200x slower than ReleaseFast on every path. Never
benchmark (or time out waiting for) a Debug server.
zig builddefaults to ReleaseFast on purpose.
E2E discipline (any change to src/)
zig build # rebuild the server first!
zig build test # ReleaseFast
zig build test -Doptimize=ReleaseSafe
Then, against a server on port 27020 (TTL suites need --ttl-sweep-secs 1):
node tests/e2e/e2e.js # CRUD + operators + aggregate + errors
node tests/e2e/e2e2.js concurrent
node tests/e2e/e2e2.js crash-a # write 50 docs, kill -9 the server
node tests/e2e/e2e2.js crash-b # restart, verify all 50 survived
node tests/e2e/e2e3.js # secondary indexes
node tests/e2e/e2e4.js # TTL indexes (server must run --ttl-sweep-secs 1)
node tests/e2e/e2e6.js # self-contained full lifecycle (spawns its own server, incl. kill -9)
Which suites to run for a given change:
- anything touching the write path or log format → the crash pair (e2e2 crash-a/b) and e2e6
- anything touching indexes → e2e3.js and e2e4.js
- everything → all of the above
tests/e2e/README.md has the full matrix, ports, and harness docs
(big.js for multi-GB loads, compare-run.sh/bench-run.sh for the
benchmark gate).
Spec tests (the compatibility gate — PLAN D2)
bash tests/spec/fetch.sh # pinned mongodb/specifications suites
node tests/spec/run.js --scorecard # run + re-record tests/spec/scorecard.txt
node tests/spec/run.js --file find.json --verbose
tests/spec/scorecard.txt is committed and re-recorded every milestone (PLAN
D9), so progress is verifiable across sessions. It is a measuring tool: a red
baseline is the expected state at M0, run.js exits 0 as long as it ran, and
nothing unimplemented is ever counted as a pass. tests/spec/README.md has the
scope, the pins, and what is not yet asserted.
Ground rules (binding — from ROADMAP.md and PLAN.md)
- Measure A/B on one harness. Do not trust the model. Several predictions made during the existing work were wrong in both directions. The cheap A/B is: flip one line, rebuild, run, flip it back.
- Mutation-check any test guarding an invariant. Break the thing the test is supposed to catch and confirm it goes red before trusting it.
- The index invariant is absolute (
src/index.zigheader): an index only generates candidates; the full filter is re-applied afterwards. Over-approximating is slow. Under-approximating is a wrong answer. - The database must always open. Replay never refuses to start over recoverable damage (e.g., a unique index finding duplicates in existing data warns and keeps going).
- The checkpoint never describes a state ahead of the durable log tail (M0 invariant): watermark advancement is the last step of the checkpoint protocol, after the data-file fsync.
- No page below the stable mark is ever stored into (M0, PLAN
Amendment A1). The whole crash story is downstream of this one rule: a
mutation that would dirty a page belonging to the last durable
checkpoint copies it to a fresh page first. Enforced structurally by a
copy-on-write page accessor, and mechanically by
mprotect-ing the stable prefix read-only in ReleaseSafe/test builds — a missed COW then segfaults in the suite instead of corrupting a database silently. - A checkpoint never renumbers slab offsets (M0, Amendment A3). Index leaves hold physical document offsets; only a full rebuild may move documents, and it rebuilds every index in the same pass.
- Write-then-extend discipline for mmap: ftruncate before touching new pages; never fault past the end of the mapped file (SIGBUS).
Engine.seqis restored on open asmax(watermark.seq, max replayed record.seq), and a log rewrite never emits a record whose seq is ≤ the watermark. Todayseqrestarts at 0 on every open — harmless without a watermark, silent data loss with one.
Note for anyone reading an older revision: rule 6 previously read "index
entries own their key bytes". That was a misdiagnosis — store_record
already copies keys. See PLAN Amendment A3 for what the actual trap was.
Code style
Binding reference: docs/TIGER_STYLE.md (TigerBeetle's TigerStyle,
adopted verbatim). Read it before writing code. Its mechanical rules are
enforced: zig fmt clean, 4-space indent, hard 100-column line limit, no
if without braces unless it fits on one line, snake_case for functions/
variables/files, units last in names (latency_ms_max), and always-on
assertions (the repo's assert.zig, see below).
Project-specific rules and deliberate deviations from TigerStyle:
- Zig 0.16 idioms:
std.Iothreaded through everything, unmanaged containers, arena-threaded allocators where the code does. - House style: user-declared functions use
snake_case; see existingsrc/for the pattern. Keepsrc/lib.zigas the library root and keep the server (src/server.zig) a thin front-end — the C API seam (PLAN D1) depends on that staying clean. - Public code invariants are asserted with the repo's own
assert.zig(always active, including ReleaseFast) rather thanstd.debug.assert, so they do not silently vanish in optimized builds. - Deviation (allocation): TigerStyle's "statically allocated at startup, no allocation after init" is not yet this repo's architecture — the engine is arena-threaded. Do not introduce new long-lived dynamic allocations on hot paths; arenas scoped to a request are fine. Static allocation is a deferred design item (PLAN).
- Deviation (recursion): TigerStyle bans recursion.
bson.zig's document parser is still recursive (nested docs/arrays); keep new code iterative and do not extend the recursion depth usage without a design note. Bounding parser depth is tracked work. - Functions stay under 70 lines (TigerStyle hard limit). A handful of pre-existing offenders remain (see the reflow/refactor backlog in ROADMAP notes); new code must not add to the count.
Source map
Source map
src/bson.zig BSON parse/serialize, ObjectId, canonical comparison/key encoding
src/wire.zig OP_MSG/OP_QUERY framing, message + reply builders
src/commands.zig command dispatch (hello, CRUD, aggregate, admin, indexes)
src/server.zig TCP accept loop, per-connection handlers, TTL sweep monitor
src/db.zig engine: db → collection → _id → document maps, slab storage
src/storage.zig append-only log: blocks, LZ4, XxHash3, replay, compaction
src/query.zig filter matcher, regex engine, sort, projection
src/index.zig B+tree indexes: entries, search, query planner
src/update.zig update operators with dot-path navigation
src/main.zig CLI: --port, --bind, --db, --ttl-sweep-secs, --compact-threshold
Working on a milestone
- Re-read PLAN.md (and this file). Confirm which milestone is current.
- Per-milestone gates are in PLAN.md; the M0 gate is 6 items ending with benchmark parity (phase8 baseline) and the spec-test runner red baseline.
- Commit scorecard and benchmark results with each milestone (PLAN D9) so progress stays verifiable across sessions.
- Deferred designs (cursors, aggregation, transactions, change streams, C API) are deliberately not specified yet — grill the design with the human before implementing (PLAN section 6).