Databases Reference
In-Depth Information
APPENDIX C
MongoDB Internals
For the most part, users of MongoDB can treat it as a black box. When trying to un-
derstand performance characteristics or looking to get a deeper understanding of the
system, it helps to know a little bit about the internals of MongoDB, though.
BSON
Documents in MongoDB are an abstract concept—the concrete representation of a
document varies depending on the driver/language being used. Because documents are
used extensively for communication in MongoDB, there also needs to be a represen-
tation of documents that is shared by all drivers, tools, and processes in the MongoDB
ecosystem. That representation is called Binary JSON (BSON).
BSON is a lightweight binary format capable of representing any MongoDB document
as a string of bytes. The database understands BSON, and BSON is the format in which
documents are saved to disk.
When a driver is given a document to insert, use as a query, and so on, it will encode
that document to BSON before sending it to the server. Likewise, documents being
returned to the client from the server are sent as BSON strings. This BSON data is
decoded by the driver to its native document representation before being returned to
the client.
The BSON format has three primary goals:
Efficiency
BSON is designed to represent data efficiently, without using much extra space.
In the worst case BSON is slightly less efficient than JSON and in the best case
(e.g., when storing binary data or large numerics), it is much more efficient.
Traversability
In some cases, BSON does sacrifice space efficiency to make the format easier to
traverse. For example, string values are prefixed with a length rather than relying
