mongo-light

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.
• 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: one mutex serializes commands end-to-end (command-level atomicity); the Io worker pool only overlaps connection I/O with command execution — there is no read parallelism. 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)
  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
  update.zig   update operators with dot-path navigation
  main.zig     CLI: --port, --bind, --db

Not (yet) implemented

• Authentication (SCRAM) — run without credentials
• Real cursors (all results are returned in one batch, cursor id 0)
• Indexes (O(n) scans)
• Transactions, change streams, replicasets
• Compression (OP_COMPRESSED)

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.