how to fix "OperationalError: (psycopg2.OperationalError) server closed the connection unexpectedly"

Same logic for sqlalchemy.orm, ( on which flask_sqlalchemy is based btw )

engine = sqlalchemy.create_engine(connection_string, pool_pre_ping=True)

More protection strategies can be setup such as it is described in the doc: https://docs.sqlalchemy.org/en/13/core/pooling.html#disconnect-handling-pessimistic

For example, here is my engine instantiation:

engine = sqlalchemy.create_engine(connection_string,                                      pool_size=10,                                      max_overflow=2,                                      pool_recycle=300,                                      pool_pre_ping=True,                                      pool_use_lifo=True)sqlalchemy.orm.sessionmaker(bind=engine, query_cls=RetryingQuery)

For RetryingQuery code, cf: Retry failed sqlalchemy queries

Building on the Solution in the answer and the info from @MaxBlax360's answer. I think the proper way to set these config values in Flask-SQLAlchemy is by setting app.config['SQLALCHEMY_ENGINE_OPTIONS']:

from flask import Flaskfrom flask_sqlalchemy import SQLAlchemyapp = Flask(__name__)# pool_pre_ping should help handle DB connection dropsapp.config['SQLALCHEMY_ENGINE_OPTIONS'] = {"pool_pre_ping": True}  app.config['SQLALCHEMY_DATABASE_URI'] = \    f'postgresql+psycopg2://{POSTGRES_USER}:{dbpass}@{POSTGRES_HOST}:{POSTGRES_PORT}/{POSTGRES_DBNAME}'db = SQLAlchemy(app)

See also Flask-SQLAlchemy docs on Configuration Keys

I'm posting my own answer to this, since none of the above addressed my particular setup (Postgres 12.2, SQLAlchemy 1.3).

To stop the OperationalErrors, I had to pass in some additional connect_args to create_engine:

create_engine(        connection_string,        pool_pre_ping=True,        connect_args={            "keepalives": 1,            "keepalives_idle": 30,            "keepalives_interval": 10,            "keepalives_count": 5,        }    )