John Lancaster
141 lines
3.8 KiB
Markdown
141 lines
3.8 KiB
Markdown
# Async SQLAlchemy Session Management
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Source:
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- https://docs.sqlalchemy.org/en/21/orm/extensions/asyncio.html
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- https://docs.sqlalchemy.org/en/21/orm/session_basics.html
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- https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/
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Status: adopted
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Decision level: mandatory
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Applies to: api-runtime, workers, tests
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Last reviewed: 2026-06-17
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---
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## Purpose
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Define one canonical session model for FastAPI + SQLAlchemy asyncio:
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- configure one shared session factory,
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- create one AsyncSession per request or per unit-of-work,
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- never share one AsyncSession across concurrent tasks.
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---
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## Scope and Non-Goals
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- In scope: session factory creation, FastAPI dependency wiring, request/task scoping, transaction demarcation.
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- Out of scope: ORM model design, query optimization strategy, schema migration tooling.
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---
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## Rules
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- Create `async_sessionmaker` once from app-owned AsyncEngine.
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- Use a fresh AsyncSession for each request or explicit unit-of-work.
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- Do not share AsyncSession across `asyncio.gather()` or parallel tasks.
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- Prefer direct dependency injection over global scoped-session patterns in new code.
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- Use explicit transaction boundaries (`async with session.begin():`) for writes.
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---
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## Canonical FastAPI Dependency Pattern
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```python
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from collections.abc import AsyncIterator
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from fastapi import Depends, Request
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from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
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def get_session_factory(request: Request) -> async_sessionmaker[AsyncSession]:
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return request.app.state.session_factory
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async def get_db_session(
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session_factory: async_sessionmaker[AsyncSession] = Depends(get_session_factory),
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) -> AsyncIterator[AsyncSession]:
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async with session_factory() as session:
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yield session
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```
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Route usage:
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```python
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from fastapi import APIRouter, Depends
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from sqlalchemy.ext.asyncio import AsyncSession
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router = APIRouter()
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@router.post("/items")
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async def create_item(session: AsyncSession = Depends(get_db_session)) -> dict:
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async with session.begin():
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# write operations here
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...
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return {"status": "ok"}
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```
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---
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## Configuration Guidance
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Typical session factory setup:
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```python
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from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
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session_factory = async_sessionmaker(
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engine,
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class_=AsyncSession,
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expire_on_commit=False,
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)
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```
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Notes:
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- `expire_on_commit=False` is commonly preferred in asyncio applications to reduce accidental post-commit reload behavior.
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- `AsyncSession.refresh()` is preferred over broad expiration patterns when state refresh is needed.
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---
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## Concurrency Rules
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- One session per concurrent task.
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- If work fans out into parallel tasks, each task receives its own AsyncSession.
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- Pass sessions explicitly to service functions; avoid mutable global session state.
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---
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## Anti-Patterns
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- A singleton/global AsyncSession reused across requests.
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- Sharing one AsyncSession across parallel tasks.
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- Hidden session creation in lower repository helpers with no caller control.
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- Mixing commit/rollback ownership across layers without a declared boundary.
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---
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## Operational Checks
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- Exactly one `async_sessionmaker` is registered in app lifecycle.
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- Request handlers receive sessions from one canonical dependency.
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- No code path creates AsyncSession in module import side effects.
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- Background jobs and API handlers each create task-local sessions.
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---
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## Testing Checks
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- Dependency override exists for test session factory.
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- Rollback behavior is verified for failed write units.
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- Parallel-task tests verify no shared AsyncSession instances.
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- Lifespan tests confirm session factory is initialized and teardown-safe.
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---
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## Migration Notes
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- If current code uses global/shared sessions, fix scope first before refactoring query style.
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- If legacy sync patterns are present, keep session boundary rules stable while migrating incrementally.
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