BSON

BSON (“Binary JSON”) is a binary-encoded extension of JSON, developed by 10gen / MongoDB for storing documents on disk and transmitting them on the wire. It is specified at bsonspec.org. Every BSON document can be converted to a JSON document; many JSON documents, however, cannot be represented as BSON without normalisation, because BSON has a smaller set of valid strings and numeric ranges.

What BSON adds over JSON

• Native date type ($date or ISODate).
• 64-bit integers (NumberLong) — JSON only has a single “number” type that typical parsers map to a double.
• 128-bit decimals (NumberDecimal) — for financial precision.
• Binary blobs (BinData) — for embedded bytes without base64.
• ObjectId — MongoDB’s 12-byte primary-key type.
• Regular expression type.
• Minimum / Maximum key sentinels — used internally by MongoDB for index ordering.

Trade-offs

BSON sacrifices text-based debuggability (you cannot cat it and read it) in exchange for compact storage, fast parsing (field lengths are stored inline, so a parser can skip over values without decoding them), and the extra types above.

Where you encounter it

Practically only inside MongoDB:

• The on-disk storage format for MongoDB collections.
• The wire format between MongoDB drivers and the server.
• The format of mongodump output (.bson files).

The MongoDB shell and mongoexport convert BSON to a JSON dialect (“Extended JSON”) when emitting text output. That dialect uses $date, $numberLong, and similar keys to represent types that plain JSON cannot.

JSON ↔ BSON conversion

• bson on npm (Node.js driver) — serialize(obj) / deserialize(buf).
• bson on PyPI (part of pymongo) — bson.encode(obj) / bson.decode(buf).
• In Go, go.mongodb.org/mongo-driver/bson.

Most applications never touch BSON directly. The driver handles the conversion and you work with idiomatic in-memory data structures.