What is AsyncSession in SQLAlchemy and what are the rules for working with it in the context of a FastAPI microservice?

AsyncSession in SQLAlchemy is an asynchronous version of the session for working with the database, allowing input-output operations (such as database queries) to be performed without blocking the main thread. In the context of a FastAPI microservice, this is especially important for improving performance and scalability.

Main rules for working with AsyncSession in FastAPI:

• Creating a session: Usually, AsyncSession is created via async_sessionmaker, which is configured for an asynchronous engine (for example, using asyncpg for PostgreSQL).

• Lifecycle of the session: In FastAPI, it is recommended to use a dependency with yield to create the session during request processing and close it properly afterward.

• Asynchronous operations: All database operations (queries, commits) should be performed using await.

• Transactions: You can use the context manager async with session.begin(): for automatic transaction management.

Example of a dependency for AsyncSession in FastAPI:

from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
from fastapi import Depends

DATABASE_URL = "postgresql+asyncpg://user:password@localhost/dbname"
engine = create_async_engine(DATABASE_URL, echo=True)
async_session = async_sessionmaker(engine, expire_on_commit=False)

async def get_async_session() -> AsyncSession:
    async with async_session() as session:
        yield session

In FastAPI handlers, you can accept a session as follows:

from fastapi import APIRouter, Depends

router = APIRouter()

@router.get("/items/")
async def read_items(session: AsyncSession = Depends(get_async_session)):
    result = await session.execute("SELECT * FROM items")
    items = result.scalars().all()
    return items

Thus, AsyncSession provides non-blocking database operations, which is important for asynchronous web applications on FastAPI.