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MongoDB Specifications

This repository holds in progress and completed specification for features of MongoDB, Drivers, and associated products. Also contained is a rudimentary system for producing these documents.

Driver Mantras

When developing specifications -- and the drivers themselves -- we follow the following principles:

Strive to be idiomatic, but favor consistency

Drivers attempt to provide the easiest way to work with MongoDB in a given language ecosystem, while specifications attempt to provide a consistent behavior and experience across all languages. Drivers should strive to be as idiomatic as possible while meeting the specification and staying true to the original intent.

No Knobs

Too many choices stress out users. Whenever possible, we aim to minimize the number of configuration options exposed to users. In particular, if a typical user would have no idea how to choose a correct value, we pick a good default instead of adding a knob.

Topology agnostic

Users test and deploy against different topologies or might scale up from replica sets to sharded clusters. Applications should never need to use the driver differently based on topology type.

Where possible, depend on server to return errors

The features available to users depend on a server's version, topology, storage engine and configuration. So that drivers don't need to code and test all possible variations, and to maximize forward compatibility, always let users attempt operations and let the server error when it can't comply. Exceptions should be rare: for cases where the server might not error and correctness is at stake.

Minimize administrative helpers

Administrative helpers are methods for admin tasks, like user creation. These are rarely used and have maintenance costs as the server changes the administrative API. Don't create administrative helpers; let users rely on "RunCommand" for administrative commands.

Check wire version, not server version

When determining server capabilities within the driver, rely only on the maxWireVersion in the hello response, not on the X.Y.Z server version. An exception is testing server development releases, as the server bumps wire version early and then continues to add features until the GA.

When in doubt, use "MUST" not "SHOULD" in specs

Specs guide our work. While there are occasionally valid technical reasons for drivers to differ in their behavior, avoid encouraging it with a wishy-washy "SHOULD" instead of a more assertive "MUST".

Defy augury

While we have some idea of what the server will do in the future, don't design features with those expectations in mind. Design and implement based on what is expected in the next release.

Case Study: In designing OP_MSG, we held off on designing support for Document Sequences in Replies in drivers until the server would support it. We subsequently decided not to implement that feature in the server.

The best way to see what the server does is to test it

For any unusual case, relying on documentation or anecdote to anticipate the server's behavior in different versions/topologies/etc. is error-prone. The best way to check the server's behavior is to use a driver or the shell and test it directly.

Drivers follow semantic versioning

Drivers should follow X.Y.Z versioning, where breaking API changes require a bump to X. See for more.

Backward breaking behavior changes and semver

Backward breaking behavior changes can be more dangerous and disruptive than backward breaking API changes. When thinking about the implications of a behavior change, ask yourself what could happen if a user upgraded your library without carefully reading the changelog and/or adequately testing the change.

Writing Documents

Write documents using GitHub Flavored Markdown, following the MongoDB Documentation Style Guidelines.

Store all source documents in the source/ directory.


This repo uses pre-commit for managing linting. pre-commit performs various checks on the files and uses tools that help follow a consistent style within the repo.

To set up pre-commit locally, run:

brew install pre-commit
pre-commit install

To run pre-commit manually, run pre-commit run --all-files.

To run a manual hook like shellcheck manually, run:

pre-commit run --all-files --hook-stage manual shellcheck

Prose test numbering

When numbering prose tests, always use relative numbered bullets (1.). New tests must be appended at the end of the test list, since drivers may refer to existing tests by number.

Outdated tests must not be removed completely, but may be marked as such (e.g. by striking through or replacing the entire test with a note (e.g. Removed).

Building Documents

We build the docs in text mode in CI to make sure they build without errors. We don't actually support building html, since we rely on GitHub to render the documents. To build locally, run:

pip install sphinx
cd source 
sphinx-build -W -b text . docs_build index.rst

Converting to JSON

There are many YAML to JSON converters. There are even several converters called yaml2json in NPM. Alas, we are not using yaml2json anymore, but instead the js-yaml package. Use only that converter, so that JSON is formatted consistently.

Run npm install -g js-yaml, then run make in the source directory at the top level of this repository to convert all YAML test files to JSON.


All the specs in this repository are available under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.