CODEWITHSUNDEEP
pythonflasksqlalchemyceleryflask-sqlalchemy
Balint
Balint

Reputation: 83

Flask + Celery + SQLAlchemy: database connection timeout

I have a Flask application to start long-running Celery tasks (~10-120 min/task, sometimes with slow queries). I use Flask-SQLAlchemy for ORM and connection management. My app looks like this:

app = Flask(__name__)
db = SQLAlchemy(app)
celery = make_celery(app)

@app.route('/start_job')
def start_job():
    task = job.delay()
    return 'Async job started', 202

@celery.task(bind=True)
def job(self):
    db.session.query(... something ...)
    ... do something for hours ...
    db.session.add(... something ...)
    db.session.commit()
    return

Unfortunately the MySQL server I have to use likes to close connections after a few minutes inactivity and the celery tasks can't handle the situation, so after a lot of waiting I get (2006, 'MySQL server has gone away') errors. AFAIK the connection pooling should take care of the closed connections. I read the docs, but it only writes about the SQLALCHEMY_POOL_TIMEOUT and SQLALCHEMY_POOL_RECYCLE parameters, so based on some random internet article I tried to change recycle to 3 minutes, but that didn't help.

How the connection (session ?) handling works with this configuration? What should I do to avoid such errors?

Upvotes: 0

Views: 2773

Answers (1)

Balint
Balint

Reputation: 83

I am not entirely sure about the goodness of the solution below, but it seems to solve the problem.

The session initialize a connection before the first query (or insert) statement and starts a transaction. Then it waits for a rollback or commit, but because of inactivity the MySQL server closes the connection after a few minutes. The solution is to close the session if you do not need it for a long time, and SQLAlchemy will open a new one for the next transaction.

@celery.task(bind=True)
def job(self):
    db.session.query(... something ...)
    db.session.close()
    ... do something for hours ...
    db.session.add(... something ...)
    db.session.commit()
    return

Upvotes: 2

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