Backend: investigate a simpler solution for SQLAlchemy session management in FastAPI

[benoit74]

In every API method, we need an SQLAlchemy session to interact with the database.

FastAPI does not like the @dbsession decorator we usually use because it makes it consider that we should pass args and kwargs query parameter.

Currently the backend is hence using dependency injection to inject the SQLAlchemy session. The drawback is that there is no more autocommit, we have to commit manually inside the API methods. And should we forget to commit, the transaction is rolled-back silently because the injected dependency is cleaned-up after the HTTP Response is sent AND it is cleaned-up with a GeneratorExit exception, which cause a transaction rollback. More details are provided in FastAPI documentation e.g. here.

The other solution mentionned in FastAPI documentation would be to use an ASGI Middleware. It works, but is not easier / cleaner than the dependency injection approach:

• the SQLAchemy session has to be injected inside the request scope, which makes the code a bit weird
• we cannot benefit from exception handler inside the middleware, looks like the response has already been sent or at least too much prepared once the exception handler is triggered. For instance we cannot have an exception handler managing all IntegrityError exceptions, which is kind of an issue

We have to investigate what could be a cleaner solution, maybe my ASGI / decorator experience is too limited or I missed an obvious solution. The solution would benefit other projects like openzim/nautilus-web (and next ones, or Zimfarm once we would have migrated it to FastAPI).

[benoit74]

Moving this to v2 since we have very few endpoints + they are all readonly so we do not really mind.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.