Code Architecture

FastStream Code Architecture

Public vs internal split

• faststream/_internal/ holds shared machinery: broker/ (abstract BrokerUsecase, registrator, router), endpoint/, di/ (fast-depends integration), context/, configs/, logger/, testing/, cli/, fastapi/, utils/.
• Broker packages (faststream/kafka/, rabbit/, nats/, redis/, confluent/, mqtt/) are thin public layers over _internal.
• Cross-broker public packages: faststream/middlewares/, params/, response/, specification/, message/, asgi/, opentelemetry/, prometheus/.

Rule: implement shared behavior in _internal/, expose it through broker packages. User-facing code (docs, examples, error messages) must never import from faststream._internal.

Broker package anatomy

Every broker package mirrors the same layout. Canonical reference: faststream/kafka/.

faststream/<broker>/
├── __init__.py        # public exports with explicit __all__
├── annotations.py     # broker-specific Annotated type aliases
├── broker/            # broker.py (BrokerUsecase subclass), router.py, registrator.py, logging.py
├── configs/           # @dataclass(kw_only=True) configs inheriting BrokerConfig
├── message.py         # StreamMessage subclass
├── parser.py          # message parser
├── publisher/         # publisher endpoint + producer.py
├── subscriber/        # subscriber endpoint (usecase.py; nats/redis split into usecases/)
├── response.py        # PublishCommand subclasses
├── security.py        # auth/security helpers
├── testing.py         # in-memory TestBroker
└── exceptions.py      # broker-specific exceptions

Brokers also carry optional integration subpackages where supported — kafka has fastapi/, helpers/, opentelemetry/, prometheus/, and schemas/ — follow kafka's structure when adding these to another broker.

Feature mirroring

All brokers expose the same surface: publish(), request(), ping(), start(), stop(), routers, publishers, message/response types. When adding a feature:

1. Find the closest analogue in another broker (kafka is usually the most complete) and follow its shape and naming.
2. Keep the public API identical across brokers unless the feature is inherently broker-specific.
3. Broker-specific features stay in the broker package — don't leak them into _internal/.

Typing

• mypy runs with strict = true (see [tool.mypy] in pyproject.toml): every function fully annotated, no implicit Optional, decorators typed. Checked paths: faststream/ and tests/mypy/.
• Generics are used for broker abstractions: BrokerUsecase[MsgType, ConnectionType, BrokerConfigType] (see faststream/_internal/broker/broker.py), BaseMiddleware[PublishCommandType, AnyMsg].
• Import Callable, Awaitable, Sequence, Mapping from collections.abc; newer typing features (Self, ParamSpec, TypedDict, ...) from typing_extensions.
• Connection kwargs use TypedDict (e.g. KafkaInitKwargs in faststream/kafka/broker/broker.py).
• Pydantic v1/v2 and Python-version differences go through faststream/_internal/_compat.py — never inline version checks elsewhere.

Configs

Config classes are @dataclass(kw_only=True) inheriting BrokerConfig (base in faststream/_internal/configs/). Example: faststream/kafka/configs/broker.py.

Public API

• Every __init__.py declares __all__ explicitly.
• Optional dependencies are guarded with try/except raising an ImportError that tells the user which extra to install — see faststream/kafka/__init__.py.

Style

• ruff uses select = ["ALL"] with curated ignores in ruff.toml — don't assume a rule is disabled; run just linter to check.
• Line length 90, double quotes, Google-style docstrings.
• just mypy must pass before a PR.

Comments

A comment carries the why — the constraint, the surprise, the reason this line is not the obvious one. What the code does is already on the screen.

• Two lines, maximum. A caveat needing more than that is usually a docstring, an ADR, or a sign the code should be clearer.
• Directly above the line it explains, so the reader meets the explanation at the moment the code surprises them. A caveat about one assertion belongs over that assertion, not in the docstring header several screens up.
• Name the issue when the code exists because of a reported bug: # ... which no broker can express (see issue #3056). The next reader gets the whole investigation for free.

Related skills

• testing-patterns — every source change needs tests following the base-testcase model.
• dev-workflow — full command reference (lint, mypy, docker brokers).
• documentation-writing — user-facing features need docs with tested snippets.