How to Fix Running out of DB Connections in FastAPI

running-out-of-db-connections-in-fastapi.md

Ran Out of Database Connections on Your SQLModel App? Here’s a Quick Fix!

Problem:

You might have run into a situation where your SQLModel app unexpectedly runs out of database connections. If that sounds familiar, you probably have something like this in your code:

# utils/db.py
def get_db():
    """A utility function to grab a database session."""
    engine = create_engine(settings.PG_CONNECTION_STRING)
    return Session(engine)

And you use it in a route like this:

# routes/some_route.py
engine = get_db()

@router.post("/search")
def query_stuff(
    query_params: StuffQueryParams = Body(...),
    with Session(engine) as session:
        # do my query stuff here
        ...
):

While this works, it inadvertently creates a new session for every single call. After around 95 calls (or whatever your database's connection limit is), Postgres will start refusing new connections. Oops!

Solution:

The good news is that there's a simple fix! You can adjust your code to yield a session instead of creating a new one each time:

# utils/db.py
from sqlmodel import SQLModel, Session, create_engine
from shared.config import settings

engine = create_engine(settings.PG_CONNECTION_STRING)

def get_db():
    """Yield a session instead of returning."""
    with Session(engine) as session:
        yield session

And in your route, modify it to reuse the session:

# routes/some_route.py
@router.post("/search")
def query_stuff(
    query_params: QueryParams = Body(...),
    session: Session = Depends(get_db),  # <-- Reuse the session by adding the call to get_db here.
):
    # do my query stuff here
    ...

By making these changes, you'll prevent your app from running out of database connections and ensure it can handle more requests without a hitch. Happy coding!