Python SQLAlchemy - "MySQL server has gone away"
mysql, python, sqlalchemy
Solution
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)
Problem
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:puss123@10.0.51.5/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