Systems

This section covers the cross-cutting building blocks that aren't user-visible features but show up everywhere in the codebase. They're shared between mongod, mongos, and jstestshell.

Pages

• BSON — the binary document format used on disk and on the wire.
• Transport — the network layer (ASIO, gRPC, named pipes).
• Executors — task executors, thread pools, batons, and futures.
• IDL — the YAML-driven interface definition compiler.
• logv2 — the structured logging system.
• Fail points — runtime hooks tests use to inject delays and errors.

Other shared infrastructure not covered in dedicated pages but worth knowing:

Subsystem	Where	One-line summary
Base error handling	src/mongo/base/	Status, StatusWith<T>, ErrorCodes registry, Status::OK().
Decorations	src/mongo/util/decorable.h	Typed slots attached to Service/Client/OperationContext/Collection.
Server parameters	docs/server_parameters.md	IDL-declared runtime knobs.
OpenTelemetry export	src/mongo/otel/	Tracing and metrics export.
Concurrency primitives	src/mongo/util/concurrency/	Mutexes, condition variables, latches with diagnostics, RWMutex.
Time and clocks	src/mongo/util/clock_source.h	Replaceable clock sources for tests.
FTDC (diagnostic data)	src/mongo/db/ftdc/	Periodic compressed snapshots of serverStatus/replSetGetStatus.
Watchdog	src/mongo/watchdog/	Detects stuck filesystem heartbeats.
Scripting (SpiderMonkey)	src/mongo/scripting/	The JS engine used by jstestshell and server-side $function.

The Patterns and conventions page covers how these pieces are typically used in everyday code.