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BSON - Binary JSON


The BSON ("Binary JSON") specification defines a binary format for storing JSON data in a more efficient and type safe way, including:

  • associative arrays (also known as name-value pairs)
  • integer indexed arrays
  • a suite of fundamental scalar types.

Data types and syntax

The following information is heavily based on .

The topmost element in a BSON structure must be of type BSON object and contains 1 or more elements, where an element consists of a field name, a type, and a value. Field names are strings. Types include:

  • Unicode string (using the UTF-8 encoding)
  • 32 bit integer
  • 64 bit integer
  • double (64-bit IEEE 754 floating point number)
  • decimal128 (128-bit IEEE 754-2008 floating point number; Binary Integer Decimal (BID) variant), suitable as a carrier for - decimal-place sensitive financial data and arbitrary precision numerics with 34 decimal digits of precision, a max value of approximately 106145
  • datetime w/o time zone (long integer number of milliseconds since the Unix epoch)
  • byte array (for arbitrary binary data)
  • boolean (true and false)
  • null
  • BSON object
  • BSON array
  • JavaScript code
  • MD5 binary data
  • Regular expression (Perl compatible regular expressions ("PCRE") version 8.41 with UTF-8 support.

EJSON (Extended JSON)

An important differentiator to JSON is that BSON contains types not present in JSON (e.g. datetime and byte array) and offers type-strict handling for several numeric types instead of a universal "number" type. For situations where these additional types need to be represented in a textual way, MongoDB's Extended JSON format can be used.


Compared to JSON, BSON is designed to be efficient both in storage space and scan-speed. Large elements in a BSON document are prefixed with a length field to facilitate scanning. In some cases, BSON will use more space than JSON due to the length prefixes and explicit array indices.


BSON originated in 2009 at MongoDB. Several scalar data types are of specific interest to MongoDB and the format is used both as a data storage and network transfer format for the MongoDB database, but it can be used independently outside of MongoDB.

Implementations are available in a variety of languages such as C, C++, C#, D, Delphi, Erlang, Go, Haskell, Java, JavaScript, Julia, Lua, OCaml, Perl, PHP, Python, Ruby, Rust, Scala, Smalltalk, and Swift.


A JSON "document" such as

{"hello": "world"}

will be stored as:

\x16\x00\x00\x00          // total document size
\x02 // 0x02 = type String
hello\x00 // field name
\x06\x00\x00\x00world\x00 // field value (size of value, value, null terminator)
\x00 // 0x00 = type EOO ('end of object')