Flask SQLAlchemy: application crashed on postgres server reboot - postgresql

Question

I have a standard setup of flask_sqlalchemy and an AWS RDS with postgresql.
I initialize the DB adapter the standard way:
from flask_sqlalchemy import SQLAlchemy
from flask import Flask
app = Flask(__name__)
db = SQLAlchemy()
db.init_app(app)
class Example(db.Model):
id = db.Column(db.Integer, primary_key=True, autoincrement=True)
username = db.Column(db.String(256))
If I reboot the RDS server and a SELECT starts during reboot I get:
psycopg2.OperationalError: terminating connection due to administrator
command SSL connection has been closed unexpectedly
The above exception was the direct cause of the following exception:
sqlalchemy.exc.OperationalError: (psycopg2.OperationalError)
terminating connection due to administrator command SSL connection has
been closed unexpectedly
That I understand. The DB is unavailable.
BUT: After that the application cannot recover from the situation because:
sqlalchemy.exc.InvalidRequestError: Can't reconnect until invalid
transaction is rolled back
The above exception was the direct cause of the following exception: sqlalchemy.exc.StatementError:
(sqlalchemy.exc.InvalidRequestError) Can't reconnect until invalid
transaction is rolled back
I understand the application throws exceptions during reboot.
But what puzzles me that it never recovers.
There is no write transaction to the DB! I don't understand what rollback the exception is describing.
Edit:
This is how all commits are implemented:
try:
db.session.commit()
except Exception as e:
db.session.rollback()
But there is no commit triggering the exception. Only SELECTS.

Answer 1

Do this and it should fix it for you session.rollback().
If you want to find this error, wrap it into a try/except-block and perform a session.rollback().
EDIT:
So I test some of the stuff and I believe the reason for the error lies within the try/except block. Ideally the doc says this is how you should be implementing inserts:
Create the Python object
Add it to the session
Commit the session
e.g.
>>> db.session.add(User(name="Flask", email="example#example.com"))
>>> db.session.commit()
Likewise for deletes:
>>> db.session.delete(me)
>>> db.session.commit()
For querying you do not need to commit exclusively, the query attribute in Flask-SQLAlchemy does it for you on the Model class.
so for selecting you just do:
users = User.query.all()
Which in your case is:
something = Example.query.all()
One more thing, can you please clarify or give more info on why you are implementing commits like this:
try:
db.session.commit()
except Exception as e:
db.session.rollback()
PS: I might have misread your last edit, my bad!