# 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

# 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 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
```

## 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 index-accelerated sort, no hashed/text/geo/partial indexes,
and entry insert/removal is O(n) (a sorted array) — fine for a light
database, with a B-tree or id→entry map as the follow-up. 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; and
  compaction never resets `log_bytes`, so every write after the first
  compaction re-triggers the threshold check

## 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.