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

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

zig-out/bin/mongo-light --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"})

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 and sparse options, persisted in the log and rebuilt on open (compaction re-emits them). 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
  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

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).
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/TTL/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.

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)
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.