Python SQLAlchemy - "MySQL server has gone away"

Question

Lets have a look at the next snippet -

@event.listens_for(Pool, "checkout")
def check_connection(dbapi_con, con_record, con_proxy):

cursor = dbapi_con.cursor()
try:
    cursor.execute("SELECT 1")  # could also be dbapi_con.ping(),
                                # not sure what is better
except exc.OperationalError, ex:
    if ex.args[0] in (2006,   # MySQL server has gone away
                      2013,   # Lost connection to MySQL server during query
                      2055):  # Lost connection to MySQL server at '%s', system error: %d
        # caught by pool, which will retry with a new connection
        raise exc.DisconnectionError()
    else:
        raise


engine = create_engine('mysql://user:[email protected]/dbname', pool_recycle = 3600,pool_size=10, listeners=[check_connection])

session_factory = sessionmaker(bind = engine, autoflush=True, autocommit=False)
db_session = session_factory()

...
some code that may take several hours to run
...

db_session.execute('SELECT * FROM ' + P_TABLE + " WHERE id = '%s'" % id)        

I thought that registering the checkout_connection function under the checkout event would solve it but it didnt now the question is how am i suppose to tell SQLAlchemy handle connection dropouts so every time i call execute() it will check if connection is available and if not it will initiate it once again?

----UPDATE----

The version of SQLAlchemy is 0.7.4

----UPDATE----

def checkout_listener(dbapi_con, con_record, con_proxy):
    try:
        try:
            dbapi_con.ping(False)
        except TypeError:
            dbapi_con.ping()
    except dbapi_con.OperationalError as exc:
        if exc.args[0] in (2006, 2013, 2014, 2045, 2055):
            raise DisconnectionError()
        else:
            raise


engine = create_engine(CONNECTION_URI, pool_recycle = 3600,pool_size=10)
event.listen(engine, 'checkout', checkout_listener)
session_factory = sessionmaker(bind = engine, autoflush=True, autocommit=False)
db_session = session_factory()

session_factory is sent to every newly created thread

class IncidentProcessor(threading.Thread):

    def __init__(self, queue, session_factory):
        if not isinstance(queue, Queue.Queue):
            raise TypeError, "first argument should be of %s" (type(Queue.Queue))
        self.queue = queue
        self.db_session = scoped_session(session_factory)
        threading.Thread.__init__(self)

    def run(self):

    self.db_session().execute('SELECT * FROM ...')

    ...
        some code that takes alot of time
    ...

    self.db_session().execute('SELECT * FROM ...')

now when execute runs after a big period of time i get the "MySQL server has gone away" error

Answer 1

There was a talk about this, and this doc describes the problem pretty nicely, so I used their recommended approach to handle such errors: http://discorporate.us/jek/talks/SQLAlchemy-EuroPython2010.pdf

It looks something like this:

from sqlalchemy import create_engine, event
from sqlalchemy.exc import DisconnectionError


def checkout_listener(dbapi_con, con_record, con_proxy):
    try:
        try:
            dbapi_con.ping(False)
        except TypeError:
            dbapi_con.ping()
    except dbapi_con.OperationalError as exc:
        if exc.args[0] in (2006, 2013, 2014, 2045, 2055):
            raise DisconnectionError()
        else:
            raise


db_engine = create_engine(DATABASE_CONNECTION_INFO,
                          pool_size=100,
                          pool_recycle=3600)
event.listen(db_engine, 'checkout', checkout_listener)