# mongo-lite

A lightweight, embedded MongoDB-compatible document database written in
Zig 0.16. Like SQLite, it stores everything in a single file; unlike SQLite,
it speaks the MongoDB wire protocol, so real clients — `mongosh`, the Node.js
driver, PyMongo — connect over TCP and just work.

## Quick start

```sh
zig build                # build the server
zig build test           # run the unit test suite

zig-out/bin/mongo-lite --port 27017 --db data.log --compact-threshold 256m

# in another terminal:
mongosh --port 27017
> db.users.insertOne({name: "alice", age: 30})
> db.users.find({age: {$gt: 25}}).toArray()
> db.users.updateOne({name: "alice"}, {$set: {vip: true}})
> db.users.deleteOne({name: "bob"})
> db.sessions.createIndex({expireAt: 1}, {expireAfterSeconds: 3600})
```

## Features

- **Wire protocol**: OP_MSG (2013) plus legacy OP_QUERY/OP_REPLY (2004/2001)
  for the driver handshake; hello/isMaster with `maxWireVersion: 8`, so
  modern drivers (Node, Python, mongosh) connect without workarounds.
- **BSON**: full parse/serialize round-trip for all common types
  (including binary, regex, timestamps, ObjectId), canonical MongoDB
  comparison order for sorting and range queries.
- **CRUD**: `insert`, `find` (filter, sort, skip/limit, projection),
  `update` (multi/upsert), `delete`, `findAndModify`, `count`,
  `aggregate` (`$match`, `$sort`, `$skip`, `$limit`, `$project`, `$count`,
  `$group` with `$sum`), plus `create`/`drop`/`listCollections`/
  `listDatabases`/`dropDatabase`.
- **Query operators**: `$eq` `$ne` `$gt` `$gte` `$lt` `$lte` `$in` `$nin`
  `$exists` `$regex` (hand-rolled engine: anchors, `.`, `* + ?`, character
  classes, groups, alternation, `i`/`s` options) `$not` `$and` `$or` `$nor`
  `$size` `$all` `$elemMatch`, with dot paths and array multikey semantics.
- **Secondary indexes**: `createIndex`/`listIndexes`/`dropIndex` via the
  three driver commands, single-field and compound, with `unique`,
  `sparse` and `expireAfterSeconds` (TTL) options, persisted in the log
  and rebuilt on open (compaction re-emits them). A background sweeper
  expires TTL-indexed documents through the ordinary logged write path.
  The query planner turns equality / `$in` / range
  predicates into index lookups across `find`, `count`, `update`,
  `delete`, `findAndModify`, and a leading `$match` in `aggregate`; every
  candidate is re-checked against the full filter, so an index that
  over-approximates is merely slow, never wrong.
- **Update operators**: `$set` `$unset` `$inc` `$push` (`$each`) `$pull`
  `$rename`, with dot-path creation (including array indices).
- **Storage**: append-only record log (CRC32-checked, `fsync` per write,
  torn-tail tolerant) with in-memory indexes rebuilt on open and automatic
  compaction (rewrite + atomic rename when the log grows past
  `--compact-threshold`, default 16 MB). Killed mid-write (`kill -9`), the
  database recovers all committed writes; the log and compaction both work
  with relative or absolute `--db` paths. Records up to the announced 16 MB
  `maxBsonObjectSize` replay correctly.
- **Concurrency**: a writer-preferring read/write lock splits command
  execution — reads (`find`, `count`, `aggregate`, `list*`) run concurrently
  across connections, writes (CRUD, DDL) are exclusive and totally ordered,
  and handshake/no-op commands run lock-free. The log append + `fsync` still
  happen under the write lock, so the crash guarantees are unchanged. Fine
  for light workloads.

## Layout

```
src/
  bson.zig     BSON parse/serialize, ObjectId, canonical comparison order
  wire.zig     OP_MSG/OP_QUERY framing, message + reply builders
  commands.zig command dispatch (hello, CRUD, aggregate, admin, indexes)
  server.zig   TCP accept loop, per-connection handlers, TTL sweep monitor
  db.zig       in-memory engine: db → collection → _id → document maps
  storage.zig  append-only log: records, replay, CRC validation
  query.zig    filter matcher, regex engine, sort, projection
  index.zig    secondary indexes: entries, search, query planner
  update.zig   update operators with dot-path navigation
  main.zig     CLI: --port, --bind, --db, --ttl-sweep-secs, --compact-threshold
```

## Indexes

`collection.createIndex({field: 1})` works against every driver; the index
is persisted in the log, survives restarts and compaction, and is used by
the query planner to narrow scans.

- **Key patterns**: single-field and compound (up to 32 fields), each key
  `1` or `-1`. Descending order is metadata (entries are always stored
  value-ascending); the default index name is MongoDB's `a_1_b_-1`.
  `createIndex({_id: 1})` is an idempotent no-op — the docs map is the
  `_id_` index — and `dropIndex("_id_")` errors.
- **Options**: `unique` (a conflicting write fails with E11000 naming the
  index; per-document entries are deduped first, so `{a: [1,1]}` is legal)
  and `sparse` (documents missing an indexed field are skipped).
- **TTL**: `createIndex({expireAt: 1}, {expireAfterSeconds: 60})` deletes a
  document once its indexed date is that many seconds old. A background
  sweeper runs every `--ttl-sweep-secs` seconds (default 60, `0` disables
  it) and deletes through the ordinary write path, so each expiry is logged
  and fsynced and holds across a restart. As in MongoDB the option is
  single-field only (a compound key is `CannotCreateIndex`, code 67),
  `expireAfterSeconds` must be a whole number in `[0, 2147483647]` (`0`
  means "expire at the stored instant"), a non-date value at the path never
  expires, an array of dates expires on its earliest member, and expiry is
  coarse: a document stays visible until the next sweep. Re-creating an
  index with a different expiry is `IndexOptionsConflict` (85) and an
  expiry on `{_id: 1}` is `InvalidIndexSpecificationOption` (197), both as
  MongoDB has them.
- **Multikey**: an array at an indexed path is indexed as a whole *and*
  element-wise, mirroring the query matcher exactly, so both
  `{tags: "a"}` and `{tags: ["a","b"]}` hit the index. A compound index
  over two array paths rejects the document with MongoDB's "cannot index
  parallel arrays".
- **Planner**: picks the index covering the longest leading run of
  equality/`$in` predicates (cartesian product capped at 100 lookups),
  optionally with a range on the next key. Ranges with both bounds fall
  back to a scan on multikey indexes (a doc with `{a: [1,2]}` can satisfy
  `{a: {$gt: 5, $lt: 25}}` across two entries), and sparse indexes are
  never used for `null`-valued predicates. The `_id_` fast path resolves
  `{_id: ...}` through the docs map unless the value's compare class is
  serialization-ambiguous (int32 1, int64 1, double 1.0 compare equal but
  hash differently — those fall back to a scan, as do string/symbol/code).

v1 limits: no hashed/text/geo/partial indexes, and entry *insert* is O(n)
(a sorted array memmoves the tail) — fine for a light database, with a
B-tree as the follow-up. Removal is no longer a scan: entry generation is
a pure function of the document, so the entries to drop are regenerated
and found by binary search. A TTL sweep
walks every entry of every TTL index and holds the write lock for the
whole pass, so the interval is the tuning knob: expiry is never more
precise than `--ttl-sweep-secs`, and a very large TTL index wants a
longer one.

## Not (yet) implemented

- Authentication (SCRAM) — run without credentials
- Real cursors (all results are returned in one batch, cursor id 0)
- Transactions, change streams, replicasets
- Compression (OP_COMPRESSED)
- `collMod`, so an index's `expireAfterSeconds` cannot be changed in
  place — drop the index and re-create it with the new expiry
- `dropCollection`/`dropDatabase` write no log record, so a dropped
  collection (and its index definitions) resurrect on restart

## Working with large collections

Everything lives in RAM (db → collection → _id → document maps) and every
write command is logged with `fsync` before it is acknowledged (one sync per
command via group commit — a 500-doc `insertMany` syncs once, not 500
times), so multi-GB collections work, with cost/behavior notes measured by
the `tests/e2e/big.js` harness (12-core/32 GB Mac):

- **Build in ReleaseFast** — `zig build` defaults to it. A Debug server is
  10-200x slower on every path (the matcher alone was 70 µs/doc in Debug
  vs 0.4 µs in ReleaseFast), which dwarfed every other difference in the
  MongoDB comparison below.
- **Compaction no longer needs tuning for bulk loads.** It triggers on the
  share of the log that is garbage rather than on bytes appended, so a pure
  insert workload — which has no garbage — is never rewritten, and a
  rewrite-heavy one is reclaimed once about a fifth of the log is dead,
  keeping the file near 1.25x the live data. `--compact-threshold` is now
  only a floor below which small logs are left alone. (It used to fire
  every 16 MB regardless, rewriting the whole log each time: quadratic
  total traffic, and the reason bulk loads needed a raised threshold.)
- **`findOne({_id})` is O(1) only for ObjectId ids.** Integer, int64 and
  double ids compare equal but hash differently, so the docs-map fast path
  is skipped and every `_id` lookup becomes a full scan. Use the driver's
  default ObjectIds (or a secondary index) on big collections. The
  order-preserving key encoding already removes the ambiguity that forces
  this; lifting the restriction waits on an ordered `_id` index.
- **Secondary-index entry insert is O(n)** (sorted array — see v1 limits
  above), so inserting into a collection that already has an index is
  quadratic. Building an index over existing data is not: entries are
  appended unsorted and ordered once. Still cheapest to create indexes
  after the load.

## Performance vs MongoDB

`tests/e2e/compare-run.sh` runs the same driver workload (1 GB, 65,536 ×
16 KB docs, every write durable — mongo-lite fsyncs per command, mongod
runs with `j: true`) against each server and prints a side-by-side table.
With the ReleaseFast default build (MongoDB 8.3.7 on the same Mac):

| benchmark | mongo-lite | mongodb | winner |
|---|---|---|---|
| insertOne (sequential) | 0.19 ms | 4.1 ms | **mongo-lite ×22** |
| bulk insert (insertMany) | 810 MB/s | 714 MB/s | **mongo-lite ×1.1** |
| createIndex({k: 1}) | 51 ms | 82 ms | **mongo-lite** |
| countDocuments({}) | 1.5 ms | 13.8 ms | **mongo-lite ×9** |
| findOne({_id}) | 0.57 ms | 0.67 ms | mongo-lite |
| findOne indexed | 0.58 ms | 1.5 ms | **mongo-lite ×2.6** |
| range-scan count | 22 ms | 13 ms | mongodb ×1.7 |
| sort + limit(20), on `_id` | 2.4 ms | 2.2 ms | mongodb ×1.1 |
| sort + limit(20), indexed field | 1.0 ms | — | — |
| aggregate $group | 11.5 ms | 15.5 ms | **mongo-lite** |
| updateOne({_id}) | 0.17 ms | 0.19 ms | mongo-lite |
| updateMany (65 docs) | 1.6 ms | 6.4 ms | **mongo-lite ×4** |
| deleteOne + insert | 0.62 ms | 4.8 ms | **mongo-lite ×8** |
| server RSS | 2.0 GB | 1.5 GB | mongodb (×0.7) |
| kill -9 → reopen | 0.8 s | 1.3 s | **mongo-lite** |
| db on disk | 1.0 GB | 96 MB | mongodb (compressed) |

The remaining losses are structural rather than incidental. Disk size is
the big one: payloads are stored raw, so the log is 11x MongoDB's
compressed files. RSS trails because every document carries its own arena.
The range-scan gap is not the matcher — it is walking 65,536 documents
that each live in a separate allocation, one pointer chase apiece.

Reproduce the table with `bash tests/e2e/compare-run.sh 1g 16k`; the
pre-tree baseline is recorded in `tests/e2e/results/phase1.txt`, and the
runs with the B+tree and ordered `_id` index (roadmap items 1 and 2) in
`tests/e2e/results/phase2.txt` and `tests/e2e/results/phase3.txt`.

### What is left (highest impact first)

Each is written up with its design decisions, ordering constraints and
traps in [ROADMAP.md](ROADMAP.md).

1. **Compress the log** — the largest remaining gap (×11). Payloads are
   stored raw. A block-framed format with an LZ4 block codec would shrink
   highly compressible workloads massively; note Zig 0.16 ships zstd
   decompression only, and deflate would cap writes below the current
   insert rate.
2. **Stop giving every document its own arena** — the source of both the
   RSS gap and the range-scan gap. Storing canonical BSON bytes in a
   per-collection slab and matching against them (parsing only the fields a
   filter names) makes scans contiguous instead of a pointer chase.
3. **Decompose the global lock** — one reader/writer lock covers the whole
   engine and is held across fsync, compaction and reply construction.
   Per-collection locks plus cross-connection group commit are the path to
   using more than one core on writes.

Done so far, with the measurement that drove each:

- **Record integrity hash CRC32 → XxHash3.** `std.hash.Crc32` is
  table-driven and byte-at-a-time: 408 MB/s against XxHash3's 31 GB/s, or
  38 µs versus 0.5 µs on a 16 KB document — about two thirds of the entire
  bulk-insert cost. Insert 260 → 700 MB/s.
- **Compaction triggers on garbage, not on bytes written**, and syncs once
  per rewrite instead of once per document. Bulk load at the default
  threshold 41.6 → 703 MB/s.
- **Index builds append then sort once** instead of inserting into a sorted
  array. `createIndex` over 65,536 documents 649 → 44 ms.
- **Index entries hold encoded byte keys**, so comparing them is a memcmp
  rather than a walk over values in unrelated arenas.
- **A B+tree over the encoded keys** (roadmap item 1): fixed 4 KiB slotted
  pages in a flat u32-addressed node array, an overflow slab for long
  records, no rebalancing on delete, and bulk bottom-up packing. Entry
  insertion and removal are a descent plus a leaf-local edit instead of a
  tail memmove, so writes into an already-built index stopped being
  quadratic. `updateMany` 17.3 → 1.6 ms (2.8x slower than MongoDB → 4x
  faster); `createIndex` 62 → 51 ms.
- **An ordered `_id` index** (roadmap item 2): every collection carries an
  implicit `_id_` index (kept out of the secondary list, so the listing,
  drop and log-format surfaces are unchanged; rebuilt after replay like
  the secondaries). Its encoded keys are canonical, so the old
  serialization-guarded docs-map fast path is gone and integer/string
  `_id` point lookups, `$in` and ranges hit the tree instead of a full
  scan. `sort({_id: ...})` is now an index-ordered scan with an early stop:
  `sort+limit(20)` 6.2 → 2.4 ms (parity with MongoDB).
- **Entry removal is a binary search**, not a scan of the whole index.
  `updateMany` 15.4 → 5.5 ms.
- **Top-k sort selection** and an allocation-free decorate pass, plus
  **index-supplied ordering** when an index already holds candidates in the
  requested order. `sort+limit(20)` 40 → 4.3 ms, or 1.0 ms on an indexed
  field.
- **`limit` reaches the scan**, which used to materialize the whole
  collection before slicing, and `countDocuments` is answered by counting
  rather than by materializing and discarding every match.
- **Matching collects candidates on the stack**, resolves operators to an
  enum once per filter field rather than by string per document, and reuses
  one reply arena per connection.

Several real bugs surfaced while benchmarking:

- `plan_id` returned a pointer to a stack temporary (`&.{e}`) that dangled
  after the frame returned — Debug tolerated it, ReleaseFast read garbage,
  silently breaking every `findOne({_id: <ObjectId>})`. It now heap-copies
  the lookup value and frees it.
- Multi-doc writes fsynced once per document; they now group-commit (one
  fsync per command, same crash guarantees — verified by the kill -9
  crash suites).
- Compaction fsynced once per live document, because the log it wrote into
  never had deferred syncing enabled — 65,536 fsyncs to rewrite a 1 GB
  collection.
- `remove` never checked the compaction threshold, so a delete-heavy
  workload grew the log without bound.


## Code style

Zig 0.16 idioms (`std.Io` threaded through everything, unmanaged
containers); user-declared functions use `snake_case` per this repo's house
style.