Invalid transaction persisting across requests

Question

Summary

One of our threads in production hit an error and is now producing InvalidRequestError: This session is in \'prepared\' state; no further SQL can be

Answer 1

A surprising thing is that there's no exception handling around that self.session.commit. And a commit can fail, for example if the connection to the DB is lost. So the commit fails, session is not removed and next time that particular thread handles a request it still tries to use that now-invalid session.

Unfortunately, Flask-SQLAlchemy doesn't offer any clean possibility to have your own teardown function. One way would be to have the SQLALCHEMY_COMMIT_ON_TEARDOWN set to False and then writing your own teardown function.

It should look like this:

@app.teardown_appcontext
def shutdown_session(response_or_exc):
    try: 
        if response_or_exc is None:
            sqla.session.commit()
    finally:
        sqla.session.remove()
    return response_or_exc

Now, you will still have your failing commits, and you'll have to investigate that separately... But at least your thread should recover.