We have a set of contextual singletons that exist in a clear hierarchy. They all live in src/mongo/db.

Notable classes include:

• ServiceContext
• Client
• OperationContext

It's probably worth mentioning the OperationKiller and Interruptible interface with reference to the OperationContext.

All of these pieces live in a single directory but they are used across the codebase, thus this documentation could live in a number of places:

• docs/contexts.md would make this document immediately accessible.
• src/mongo/db/readme.md as a large section.
• src/mongo/db/contexts.md as a separate document.

This is intended to be abstract documentation, describe relationships and state transitions not code in common language.