Aleksey Shakhmatov
f2844e7894
Every reply came back in a single batch with `cursor.id = 0`, `getMore` was a
stub answering an empty `nextBatch` on the literal namespace `test.$cmd`, and
nothing read `batchSize`. That caps the useful collection size at what fits in
one 48 MiB message, which is the opposite of the tens-of-GB target and the
reason M0 made whole-index scans stream: the streaming candidate generator
existed with no consumer that could suspend.
## What a cursor is allowed to remember
A cursor holds no lock between requests, so everything it saves has to survive
arbitrary concurrent mutation. Nothing here is a pointer, and the two things
that look like stable addresses are not: `reset_tree` re-creates node ids 0 and
1 as different nodes, and `rebuild_collection` moves every document. Three
sources, chosen by query shape, each with a different memory contract:
- **stream** -- an index-ordered walk resumed from a `(key, off)` anchor plus a
`(leaf, slot)` hint. O(key) state, so this is what lets a cursor walk a
collection larger than memory. Survives a rebuild, because a repack changes
no key.
- **offsets** -- the matched slab offsets a narrowed plan already materialized,
8 bytes each. Killed by a rebuild with `QueryPlanKilled`, because those
offsets now name unrelated bytes.
- **buffered** -- canonical BSON copies, for a sort no index provides and for
aggregate/listing output. Depends on nothing, which is what lets a listing
hold a cursor over a `$cmd.*` namespace no collection backs.
`Collection.layout_epoch` and `Index.epoch` are the invalidation tokens, both
checked as error returns rather than assertions since a client reaches them by
keeping a cursor open across maintenance.
## Resume
`resume_forward`/`resume_reverse` are O(1) while the hint holds and fall back to
an exact-order band walk bounded by `resume_walk_max`. Without the hint, `seek`
lands at the *start* of an equal-key band, so `sort({status: 1})` over three
distinct values across 10M documents would cost ~5e10 comparisons to drain.
Two hazards found by draining a collection while writing to it, neither
predictable from reading the code:
- A deleted anchor must resume at its *band position*, or the rest of an
equal-key band is silently dropped -- most of the collection on a
low-cardinality index. Hence `band_index`.
- On a **unique** index a same-key entry can only be the anchor rewritten, so
resuming at it returned updated documents twice. Observed as duplicate `_id`s
while updating underneath a drain.
## Protocol
Measured against mongod 8.3.7 rather than recalled, which corrected three
assumptions: a bare `getMore` does *not* inherit the find's `batchSize` (4998 of
5000 documents come back), a namespace mismatch is `Unauthorized` (13) not
`CursorNotFound`, and `CursorInUse` is 143 not 12051.
`internalQueryFindCommandBatchSize` is 101, `cursorTimeoutMillis` 600000,
`clientCursorMonitorFrequencySecs` 4.
The rule everything follows is **never look ahead**: a batch that met its target
leaves the cursor open even when the source is in fact exhausted, so four
documents at `batchSize: 2` take three commands. `limit` acts as an EOF source,
which is what makes `batchSize == limit` close in one round trip. `skip` is
consumed once. `batchSize: 0` returns an empty batch with a live cursor.
Cursor ids are `(nonce << 20) | slot`, always positive. The nonce is not
decoration: without it a recycled slot serves one client another's documents.
Cursors are not connection-pinned, since the driver spec allows a `getMore` on
any connection to the same server; they end at exhaustion, `killCursors`, or the
idle sweep (a second monitor fiber, separate from the TTL one because the
cadences differ by an order of magnitude and a TTL failure must not stop
reclamation). The registry is fixed-capacity and evicts the least recently used
cursor, whose client sees the same 43 an idle timeout gives.
Fixed alongside, because cursors are what expose them:
- `listCollections` reported `"<db>."` with an *empty* collection part, which
makes the driver throw client-side -- so it would have broken the moment its
cursor stopped being id 0. Now `<db>.$cmd.listCollections`, as mongod uses.
- `count` ignored `skip` and `limit` entirely.
- `wire.end_message` now bounds a reply by the 48 MiB we advertise rather than
by `maxInt(u32)`; a reply past what we told the client to expect is not a large
reply, it is a desynchronized connection.
- Two `codeName` strings were wrong: 72 is `InvalidOptions` (MongoDB has no
`InvalidArgument`), and 40324 reports as `Location40324`.
## Verification
Unit 160/160 in ReleaseFast and ReleaseSafe; `tests/e2e/e2e7.js` adds 86 cursor
checks across five phases (batching/lifecycle/errors, streaming across churn,
aggregate+listings+count, expiry+capacity, restart) and is self-contained
because cursor behaviour is only observable with non-default flags. No
regressions: e2e 49, e2e3 16, e2e4 17, e2e2 2, e2e6 72. Spec 168 pass / 124
fail, +5 against the previous scorecard.
Mutation-checked, per the repo's second ground rule: `hint_slot + 1`, the
`band_index` off-by-one, both epoch bumps, the id nonce, the at-least-one-
document rule, and `stream_shape` returning null each turn the intended test
red. One claim was withdrawn rather than kept -- swapping `std.mem.order` for
`cmp_prefix` in the band walk changes nothing observable, so the comment now
says so instead of asserting a check that does not hold.
206 lines
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206 lines
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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)**. M0 (mmap + WAL storage foundation) has landed;
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the current milestone is **M1 (cursors + wire polish)**, whose cursor work is
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done — see `src/cursor.zig` and `tests/e2e/e2e7.js`. Before starting any
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work, read PLAN.md; its decision record (D1-D9) and ground rules are
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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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node tests/e2e/e2e7.js # self-contained cursors (spawns its own servers; needs no server running)
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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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- anything touching cursors, batching or the reply size → e2e7.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, scan resume
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src/cursor.zig server-side cursor state: registry, batch policy, expiry
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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 (aggregation, transactions, change streams, C API) are
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deliberately *not* specified yet — grill the design with the human before
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implementing (PLAN section 6). Cursors are no longer among them: the
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design was settled and implemented in M1, and `src/cursor.zig`'s module
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comment is where it is written down.
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