This legacy documentation does not necessarily reflect the current practices in MongoDB driver implementation, but may be useful for historical purposes. See the MongoDB Meta Driver for the current documentation of MongoDB driver implementation.


BSON is a binary-encoded serialization of JSON-like documents. BSON is designed to be lightweight, traversable, and efficient. BSON, like JSON, supports the embedding of objects and arrays within other objects and arrays. See bsonspec.org for the spec and more information in general.

BSON and MongoDB

MongoDB uses BSON as the data storage and network transfer format for “documents”.

BSON at first seems BLOB-like, but there exists an important difference: the MongoDB database understands BSON internals. This means that MongoDB can “reach inside” BSON objects, even nested ones, using dot notation. Among other things, this allows MongoDB to build indexes and match objects against query expressions on both top-level and nested BSON keys.

Language-Specific Examples

We often map from a language’s “dictionary” type, which may be its native objects, to BSON. The mapping is particularly natural in dynamically typed languages:

JavaScript: {"foo" : "bar"}
Perl: {"foo" => "bar"}
PHP: array("foo" => "bar")
Python: {"foo" : "bar"}
Ruby: {"foo" => "bar"}
Java: DBObject obj = new BasicDBObject("foo", "bar");


bson b;
bson_buffer buf;
bson_buffer_init( &buf )
bson_append_string( &buf, "name", "Joe" );
bson_append_int( &buf, "age", 33 );
bson_from_buffer( &b, &buf );
bson_print( &b );

See http://github.com/mongodb/mongo-c-driver/blob/master/src/bson.h for more information.


BSONObj p = BSON( "name" << "Joe" << "age" << 33 );
cout << p.toString() << endl;
cout << p["age"].number() << endl;

See the BSON section of the C++ Tutorial for more information.


BasicDBObject doc = new BasicDBObject();
doc.put("name", "MongoDB");
doc.put("type", "database");
doc.put("count", 1);
BasicDBObject info = new BasicDBObject();
info.put("x", 203);
info.put("y", 102);
doc.put("info", info);


The PHP driver includes bson_encode and bson_decode functions. bson_encode takes any PHP type and serializes it, returning a string of bytes:

$bson = bson_encode(null);
$bson = bson_encode(true);
$bson = bson_encode(4);
$bson = bson_encode("hello, world");
$bson = bson_encode(array("foo" => "bar"));
$bson = bson_encode(new MongoDate());

Mongo-specific objects (MongoId, MongoDate, MongoRegex, MongoCode) will be encoded in their respective BSON formats. For other objects, it will create a BSON representation with the key/value pairs you would get by running for ($object as $key => $value).

bson_decode takes a string representing a BSON object and parses it into an associative array.


>>> from bson import BSON
>>> bson_string = BSON.encode({"hello": "world"})
>>> bson_string
>>> bson_string.decode()
{u'hello': u'world'}

PyMongo also supports “ordered dictionaries” through the bson.son module. The BSON class can handle SON instances using the same methods you would use for regular dictionaries. Python 2.7’s collections.OrderedDict is also supported.


There are now two gems that handle BSON-encoding: bson and bson_ext. These gems can be used to work with BSON independently of the MongoDB Ruby driver.

>> require 'rubygems'
=> true
>> require 'bson'
=> true
>> doc = {:hello => "world"}
>> bson = BSON.serialize(doc).to_s
=> "\026\000\000\000\002hello\000\006\000\000\000world\000\000"
>> BSON.deserialize(bson.unpack("C*"))
=> {"hello" => "world"}

The BSON class also supports ordered hashes. Simply construct your documents using the OrderedHash class, also found in the MongoDB Ruby Driver.

MongoDB Document Types

MongoDB uses BSON documents for three things:

  1. Data storage (user documents). These are the regular JSON-like objects that the database stores for us. These BSON documents are sent to the database via the INSERT operation. User documents have limitations on the “element name” space due to the usage of special characters in the JSON-like query language.
    • A user document element name cannot begin with $.
    • A user document element name cannot have a . in the name.
    • The element name _id is reserved for use as a primary key id, but you can store anything that is unique in that field. The database expects that drivers will prevent users from creating documents that violate these constraints.
  2. Query “Selector” Documents: Query documents (or selectors) are BSON documents that are used in QUERY, DELETE and UPDATE operations. They are used by these operations to match against documents. Selector objects have no limitations on the “element name” space, as they must be able to supply special “marker” elements, like $where and the special “command” operations.
  3. “Modifier” Documents : Documents that contain ‘modifier actions’ that modify user documents in the case of an update.