Appendix B. MongoDB Internals

It is not necessary to understand MongoDB’s internals to use it effectively, but they may be of interest to developers who wish to work on tools, contribute, or simply understand what’s happening under the hood. This appendix covers some of the basics. The MongoDB source code is available at


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 representation of documents that is shared by all drivers, tools, and processes in the MongoDB ecosystem. That representation is called Binary JSON, or BSON (no one knows where the J went).

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:


BSON is designed to represent data efficiently, without using much extra space. In the worst case BSON is slightly less efficient ...

Get MongoDB: The Definitive Guide, 3rd Edition now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.