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AGENTS.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Stack

Python 3.14 async REST API. FastAPI + SQLAlchemy 2 (async) + PostgreSQL + Alembic. Served by Granian (uvloop). Dependencies managed by uv. Linted with ruff, type-checked with ty (not mypy). Observability via lite-bootstrap (OpenTelemetry, Sentry). DI via modern-di. Repositories via advanced-alchemy.

Common commands

The development workflow runs inside Docker via just (just --list / read the justfile for the full recipe list). The application service mounts the repo and depends on a db Postgres service. Bare just runs the default pipeline (install + lint + build + test).

Things the recipe names don't make obvious:

  • just test [args] recreates a clean DB first (alembic downgrade base → upgrade head) and tears the stack down afterward; [args] are passed through to pytest.
  • just test-migrations [args] runs the pytest-alembic suite in tests/migrations/ against an empty database: single head, upgrade, up/down consistency for every revision, and models matching the migrated schema (alembic check as a test). just test skips that directory (--ignore in addopts, omit in coverage) because it migrates the schema under the transaction-rollback fixture, so the recipe clears addopts.
  • just run applies migrations then starts the app on :8000.
  • just lint auto-fixes (eof-fixer + ruff format + ruff check --fix + ty check); CI (below) is the no-fix gate.

Running a single test (inside container or with DB available):

uv run pytest tests/test_decks.py::test_get_decks -x

CI (.github/workflows/main.yml) runs ruff format --check, ruff check --no-fix, ty check, then two Postgres jobs: alembic upgrade head followed by pytest, and the pytest-alembic suite. Match this locally before pushing.

Architecture

Request lifecycle

app/__main__.py boots Granian pointing at app.application:build_app (factory). build_app() in app/application.py:

  1. Creates a modern_di.Container with ioc.ALL_GROUPS from app/ioc.py.
  2. Builds a FastAPIBootstrapper from settings.api_bootstrapper_config, injecting the SQLAlchemy OpenTelemetry instrumentor.
  3. modern_di_fastapi.setup_di(app, container) wires DI scopes onto the FastAPI app.
  4. Includes one ROUTER per resource (app.api.decks.ROUTER, app.api.cards.ROUTER) under /api via include_routers. Add a new resource by creating app/api/<name>.py with its own APIRouter-based ROUTER and registering it there.
  5. Registers handlers from app/exceptions.py: DuplicateKeyError → 422, NotFoundError and ForeignKeyError → 404, and sqlalchemy.exc.TimeoutError and RepositoryError → database_unavailable_handler, which returns 503 for a pool timeout and re-raises anything else.

DI scopes (modern-di)

app/ioc.py splits providers into three groups, collected in ALL_GROUPS.

Database:

  • database_engine: singleton-ish primary AsyncEngine (create_primary_sa_engine, close_sa_engine finalizer).
  • database_read_engine: singleton-ish read AsyncEngine built from DB_REPLICA_DSN, or from DB_DSN when it is unset. It runs in AUTOCOMMIT (no BEGIN/ROLLBACK per request) and sets default_transaction_read_only=on, so Postgres rejects writes on it. Each statement sees its own snapshot, so a read that runs several queries can see a write that commits in between. Load related rows in the same query (fetch_with_cards uses joinedload). Without a replica it is a second pool on the primary, so a process can hold up to 2 × db_pool_size connections.
  • dynamic_engine: Scope.REQUEST; choose_sa_engine returns the read engine for GET/HEAD requests, the primary otherwise (including when there is no request). GET handlers cannot write, and a read right after a write may see replica lag.
  • session: Scope.REQUEST, bound to dynamic_engine, finalized by close_session.
  • transaction: Scope.REQUEST, a db_retry.Transaction over session. Leaving its block rolls back anything uncommitted and closes the session.
  • autocommit_transaction: the same Transaction, opened at AUTOCOMMIT (autocommit_isolation_level). CreateDeckUseCase and UpdateDeckUseCase use it because each sends exactly one statement, which Postgres runs atomically on its own. The asyncpg adapter then sends no BEGIN or COMMIT, so those writes cost one round trip instead of three. A use case with two or more statements, or one that relies on rolling back after a check, as UpdateCardsUseCase does, keeps transaction. The db_session fixture overrides autocommit_isolation_level to None, because switching the shared test connection to AUTOCOMMIT would roll back its outer transaction. tests/test_autocommit_writes.py runs both use cases against a real engine instead, through the committing_engine fixture. tests/test_card_locking.py uses the same fixture, because a lock wait needs separate connections; both delete the rows they commit.

Repositories (Scope.REQUEST): decks_repository and cards_repository, on Database.session with auto_commit=False. The use case commits through transaction.commit().

UseCases (Scope.REQUEST): one provider per use case in app/use_cases/. modern-di fills each dataclass field by its type, so a use case needs no kwargs.

Endpoints inject a use case with FromDI(UseCase) from modern_di_fastapi, call it, and convert the result to a schema. They never touch repositories or the session. Add new providers to the matching group rather than constructing services manually in routes.

Pool and server sizing

DB_POOL_SIZE (positive) and DB_MAX_OVERFLOW (non-negative) size the pool, and app/__main__.py sets Granian's backpressure to their sum, so each worker accepts only as many requests at once as it has connections. Requests beyond that wait in Granian's socket backlog instead of queueing on the pool. APP_WORKERS sets the Granian worker count, and each worker has its own pool. DB_POOL_TIMEOUT (default 30 s) bounds the wait for a connection, and a timeout returns 503 through exceptions.database_unavailable_handler. advanced-alchemy wraps the timeout in a RepositoryError, so the handler is also registered for RepositoryError and re-raises anything not caused by a pool timeout. DB_POOL_RECYCLE defaults to SQLAlchemy's -1 (never). Set it below the idle timeout of any proxy or load balancer in front of Postgres, so a connection it dropped is not handed out and retried after a 1-2 s backoff.

Use cases and lost connections

pool_pre_ping is off by default (DB_POOL_PRE_PING). A pooled connection that died while idle is handled by db_retry.postgres_retry instead, which saves a round trip on every checkout. That only works if the retried function owns its whole transaction, so every use case has this shape:

@dataclasses.dataclass(kw_only=True, frozen=True, slots=True)
class ListDecksUseCase:
    transaction: Transaction
    decks_repository: DecksRepository

    @postgres_retry
    async def __call__(self, *, after: int | None, limit: int) -> Sequence[models.Deck]:
        async with self.transaction:
            decks: typing.Final = await self.decks_repository.list_decks(after=after, limit=limit)
            await self.transaction.commit()
        return decks
  • Every query runs inside async with self.transaction:. A query before the block leaves the session in a failed transaction on a lost connection, and the retry then fails with PendingRollbackError.
  • Commit before leaving the block, reads included, and before any early return. Otherwise __aexit__ rolls back, which expires the loaded rows.
  • A retry runs the whole use case again. If the connection drops during COMMIT, the write may already have landed. A write that creates rows needs something that makes the second run harmless:
    • CreateDeckUseCase takes a client-generated idempotency_key (schemas.DeckCreate, unique on decks). DecksRepository.get_or_create sends one INSERT ... ON CONFLICT (idempotency_key) DO UPDATE SET idempotency_key = EXCLUDED.idempotency_key RETURNING *. On a conflict the no-op update leaves the row as it was and RETURNING hands it back (DO NOTHING would return no row), so a retry or a client resend gets the first deck. A concurrent request with the same key waits on the row lock and then gets the same deck. advanced-alchemy's own upsert and get_or_upsert do a SELECT followed by an INSERT, so they cost two round trips and lose that race.
    • CreateCardsUseCase relies on the (deck_id, front) unique constraint. A replay is rejected as a duplicate-key error rather than creating the cards twice.
    • Updates (UpdateDeckUseCase, UpdateCardsUseCase) write the same values again, so a replay is harmless.

app/use_cases/ is exempt from ruff's TC rules because modern-di reads the dataclass field annotations at runtime.

Database layer

  • app/models.py: BigIntAuditBase from advanced_alchemy (auto id, created_at, updated_at). The module aliases orm_registry.metadata onto orm.DeclarativeBase.metadata so Alembic autogenerate sees both. New models go here.
  • app/repositories.py: Subclass SQLAlchemyAsyncRepositoryService[Model] with a nested BaseRepository(SQLAlchemyAsyncRepository[Model]). Use cases call the service methods (get_many, get_one, create_many) for reads and plain inserts. Writes that advanced-alchemy would spread over several round trips are one statement on the session instead: its update reads the row twice and refreshes it after the flush, and upsert_many selects the rows before and after writing them. update_deck sends UPDATE ... RETURNING and get_or_create sends INSERT ... ON CONFLICT ... RETURNING. update_cards sends one WITH locked AS (SELECT id FROM cards WHERE id = ANY(:ids) AND deck_id = :deck_id ORDER BY id FOR UPDATE) UPDATE cards ... FROM unnest(:ids, :fronts, :backs, :hints) AS updates(...), locked WHERE cards.id = updates.id AND cards.id = locked.id AND cards.deck_id = :deck_id RETURNING *. It binds one array per column, so the SQL text stays the same for any number of cards: SQLAlchemy caches it and asyncpg prepares it once per connection, which a multi-row VALUES would not allow. Every updated row must match a locked row, and the CTE locks its rows in id order (lazily, one before each update), so concurrent PUTs on overlapping cards always lock them in the same order and cannot deadlock. Without it the lock order follows the plan, which can be the order the client sent; tests/test_card_locking.py forces that interleaving. card_deck_id_front_uc is DEFERRABLE INITIALLY IMMEDIATE, so uniqueness is checked at the end of the statement and one PUT can swap two cards' fronts. It raises NotFoundError (404) if fewer rows come back than ids were sent, which rolls the whole request back. PUT never inserts. An INSERT ... ON CONFLICT (id) upsert would move a card in from another deck and would insert client-chosen ids without advancing cards_id_seq, so a later POST would collide on the primary key. schemas.CardCreates and schemas.CardUpdates cap a request at MAX_CARDS_PER_REQUEST cards, and schemas.CardUpdates rejects duplicate ids, both with a 422 before any query runs. Ids in paths, after and card bodies are bounded to Postgres bigint (schemas.MAX_ID), so an oversized id is a 422 and never reaches asyncpg. A POST to a deck that does not exist fails on the foreign key, and ForeignKeyError maps to 404. A statement that can hit a unique constraint runs inside wrap_sqlalchemy_exception so that it still raises DuplicateKeyError.
  • app/resources/db.py: engine factories live here and are DI creators: modern-di resolves their parameter annotations at runtime, so pyproject.toml exempts app/resources/ from ruff's TC rules. create_session passes join_transaction_mode="create_savepoint". This is inert in production (the session binds to an engine) but enables the test rollback pattern below: when a test binds the session to a connection already in a transaction, the session owns its own savepoint so the outer transaction survives commits. Do not "fix" it.
  • migrations/env.py swaps the asyncpg driver for the sync postgresql driver and uses app.models.METADATA as target_metadata.

Settings

app/settings.py: pydantic_settings.BaseSettings. Env vars are unprefixed (DB_DSN, DB_REPLICA_DSN, SERVICE_DEBUG, SERVICE_ENVIRONMENT, LOG_LEVEL, APP_HOST, APP_PORT, OPENTELEMETRY_ENDPOINT, SENTRY_DSN, CORS_ALLOWED_ORIGINS, APP_WORKERS, DB_POOL_TIMEOUT, DB_POOL_RECYCLE, ...). api_bootstrapper_config produces a FastAPIConfig for lite-bootstrap. LOG_LEVEL is case-insensitive and sets both Granian's log level and the app's root logger.

Tests

tests/conftest.py provides the test isolation pattern. Read it before adding fixtures:

  • app fixture builds a fresh app via LifespanManager.
  • db_session opens a connection, begins a transaction, and overrides Database.dynamic_engine with the connection itself, so reads and writes share one rolled-back transaction. Each session built against that connection uses join_transaction_mode="create_savepoint", so transaction.commit() releases the session's own savepoint while the outer transaction is rolled back at teardown, so each test starts clean.
  • set_async_session_in_base_sqlalchemy_factory wires db_session into SQLAlchemyFactory.__async_session__ so polyfactory factories in tests/factories.py (DeckModelFactory, CardModelFactory) persist via the rolled-back session. Test modules that use these factories opt in with pytestmark = [pytest.mark.usefixtures("set_async_session_in_base_sqlalchemy_factory")].

pytest.ini_options sets asyncio_mode = "auto", so async tests do not need @pytest.mark.asyncio. Coverage runs by default with --cov-fail-under=100, so just test fails unless tests cover every line of new code.

Conventions

  • Ruff is configured with select = ["ALL"] and a curated ignore list in pyproject.toml. Don't sprinkle # noqa; prefer fixing or extending the project ignore list if a rule is genuinely wrong for the codebase.
  • Routes convert ORM objects to schemas explicitly, never via typing.cast. Single objects: schemas.X.model_validate(instance). Collections: schemas.Xs.from_models(objects) (the Collection[T] seam in app/schemas.py). Both rely on from_attributes=True.
  • List endpoints page by id (keyset). They take limit (1 to schemas.MAX_PAGE_LIMIT, default schemas.DEFAULT_PAGE_LIMIT) and after, the last id the client has seen, and return a Page[T] subclass (schemas.DecksPage, schemas.CardsPage) with items and next_after. The repository filters id > after and fetches limit + 1 rows ordered by id. Page.from_models(rows, limit=limit) drops the extra row and sets next_after to the last returned id when that row existed, or null on the last page. Don't page with OFFSET: Postgres reads and discards every skipped row.
  • Deck responses are deliberately two-shaped: list_decks/create_deck/update_deck return the light schemas.Deck (no cards), while get_deck returns schemas.DeckWithCards. The split mirrors loading (lists and writes use noload, with no cards query; detail uses joinedload via fetch_with_cards), so the type states exactly what each endpoint loads.
  • Line length is 120.

Agent skills

Issue tracker

GitHub issues on modern-python/fastapi-sqlalchemy-template, via gh. See docs/agents/issue-tracker.md.

Triage labels

The five canonical roles, each label string equal to its name. See docs/agents/triage-labels.md.

Domain docs

Single-context: CONTEXT.md and docs/adr/ at the repo root. See docs/agents/domain.md.