Flask-SQLAlchemy: Can't reconnect until invalid transaction is rolled back

问题

So I am using Amazon Web Services RDS to run a MySQL server and using Python's Flask framework to run the application server and Flask-SQLAlchemy to interface with the RDS.

My app config.py

SQLALCHEMY_DATABASE_URI = '<RDS Host>'
SQLALCHEMY_POOL_RECYCLE = 60

My __ init __.py

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy

application = Flask(__name__)
application.config.from_object('config')
db = SQLAlchemy(application)

I have my main application.py

from flask import Flask
from application import db
import flask.ext.restless
from application.models import Person

application = Flask(__name__)
application.debug=True
db.init_app(application)

@application.route('/')
def index():
    return "Hello, World!"

manager = flask.ext.restless.APIManager(application, flask_sqlalchemy_db=db)
manager.create_api(Person, methods=['GET','POST', 'DELETE'])

if __name__ == '__main__':
    application.run(host='0.0.0.0')

The models.py

class Person(db.Model):
    __bind_key__= 'people'
    id = db.Column(db.Integer, primary_key=True)
    firstName = db.Column(db.String(80))
    lastName = db.Column(db.String(80))
    email = db.Column(db.String(80))

    def __init__(self, firstName=None, lastName=None, email=None):
        self.firstName = firstName
        self.lastName = lastName
        self.email = email

I then have a script to populate the database for testing purposes after db creation and app start:

from application import db
from application.models import Person

person = Person('Bob', 'Jones', 'bob@website.net')
db.session.add(person)
db.session.commit()

Once I've reset the database with db.drop_all() and db.create_all() I start the application.py and then the script to populate the database.

The server will respond with correct JSON but if I come back and check it hours later, I get the error that I need to rollback or sometimes the 2006 error that the MySQL server has gone away.

People suggested that I change timeout settings on the MySQL server but that hasn't fixed anything. Here are my settings:

innodb_lock_wait_timeout = 3000
max_allowed_packet       = 65536
net_write_timeout        = 300
wait_timeout             = 300

Then when I look at the RDS monitor, it shows the MySQL server kept the connection open for quite a while until the timeout. Now correct me if I'm wrong but isn't the connection supposed to be closed after it's finished? It seems that the application server keeps making sure that the database connection exists and then when the MySQL server times out, Flask/Flask-SQLAlchemy throws an error and brings down the app server with it.

Any suggestions are appreciated, thanks!

回答1:

I think what did it was adding

db.init_app(application)

in application.py, haven't had the error since.

回答2:

Everytime checking rollback or not is troublesome..

I made insert, update functions which need commit.

@app.teardown_request
def session_clear(exception=None):
    Session.remove()
    if exception and Session.is_active:
        Session.rollback()

回答3:

Here you missing pool recycle as MySql closes session after some time so you need to add pool recycle so that connections in pool get reconnect after pool recycle time.

app.config['SQLALCHEMY_POOL_RECYCLE'] = 3600