Python and Django OperationalError (2006, 'MyS

问题:

Original: I have recently started getting MySQL OperationalErrors from some of my old code and cannot seem to trace back the problem. Since it was working before, I thought it may have been a software update that broke something. I am using python 2.7 with django runfcgi with nginx. Here is my original code:

views.py

DBNAME = "test"
DBIP = "localhost"
DBUSER = "django"
DBPASS = "password"
db = MySQLdb.connect(DBIP,DBUSER,DBPASS,DBNAME)
cursor = db.cursor()

def list(request):
    statement = "SELECT item from table where selected = 1"
    cursor.execute(statement)
    results = cursor.fetchall()

I have tried the following, but it still does not work:

views.py

class DB:
    conn = None
    DBNAME = "test"
    DBIP = "localhost"
    DBUSER = "django"
    DBPASS = "password"
def connect(self):
    self.conn = MySQLdb.connect(DBIP,DBUSER,DBPASS,DBNAME)
def cursor(self):
    try:
        return self.conn.cursor()
    except (AttributeError, MySQLdb.OperationalError):
        self.connect()
        return self.conn.cursor()

db = DB()
cursor = db.cursor()

def list(request):
    cursor = db.cursor()
    statement = "SELECT item from table where selected = 1"
    cursor.execute(statement)
    results = cursor.fetchall()

Currently, my only workaround is to do MySQLdb.connect() in each function that uses mysql. Also I noticed that when using django's manage.py runserver, I would not have this problem while nginx would throw these errors. I doubt that I am timing out with the connection because list() is being called within seconds of starting the server up. Were there any updates to the software I am using that would cause this to break/is there any fix for this?

Edit: I realized that I recently wrote a piece of middle-ware to daemonize a function and this was the cause of the problem. However, I cannot figure out why. Here is the code for the middle-ware

def process_request_handler(sender, **kwargs):
    t = threading.Thread(target=dispatch.execute,
        args=[kwargs['nodes'],kwargs['callback']],
        kwargs={})
    t.setDaemon(True)
    t.start()
    return
process_request.connect(process_request_handler)

回答1:

As per the MySQL documentation, your error message is raised when the client can't send a question to the server, most likely because the server itself has closed the connection. In the most common case the server will close an idle connection after a (default) of 8 hours. This is configurable on the server side.

The MySQL documentation gives a number of other possible causes which might be worth looking into to see if they fit your situation.

An alternative to calling connect() in every function (which might end up needlessly creating new connections) would be to investigate using the ping() method on the connection object; this tests the connection with the option of attempting an automatic reconnect. I struggled to find some decent documentation for the ping() method online, but the answer to this question might help.

Note, automatically reconnecting can be dangerous when handling transactions as it appears the reconnect causes an implicit rollback (and appears to be the main reason why autoreconnect is not a feature of the MySQLdb implementation).

回答2:

Sometimes if you see "OperationalError: (2006, 'MySQL server has gone away')", it is because you are issuing a query that is too large. This can happen, for instance, if you're storing your sessions in MySQL, and you're trying to put something really big in the session. To fix the problem, you need to increase the value of the max_allowed_packet setting in MySQL.

The default value is 1048576.

So see the current value for the default, run the following SQL:

select @@max_allowed_packet;

To temporarily set a new value, run the following SQL:

set global max_allowed_packet=10485760;

To fix the problem more permanently, create a /etc/my.cnf file with at least the following:

[mysqld]
max_allowed_packet = 16M

After editing /etc/my.cnf, you'll need to restart MySQL or restart your machine if you don't know how.

回答3:

Check if you are allowed to create mysql connection object in one thread and then use it in another.

If it's forbidden, use threading.Local for per-thread connections:

class Db(threading.local):
    """ thread-local db object """
    con = None

    def __init__(self, ...options...):
        super(Db, self).__init__()
        self.con = MySQLdb.connect(...options...)

db1 = Db(...)


def test():
    """safe to run from any thread"""
    cursor = db.con.cursor()
    cursor.execute(...)

回答4:

I've been struggling with this issue too. I don't like the idea of increasing timeout on mysqlserver. Autoreconnect with CONNECTION_MAX_AGE doesn't work either as it was mentioned. Unfortunately I ended up with wrapping every method that queries the database like this

def do_db( callback, *arg, **args):
    try:
        return callback(*arg, **args)
    except (OperationalError, InterfaceError) as e:  # Connection has gone away, fiter it with message or error code if you could catch another errors
        connection.close()
        return callback(*arg, **args)

do_db(User.objects.get, id=123)  # instead of User.objects.get(id=123)

As you can see I rather prefer catching the exception than pinging the database every time before querying it. Because catching an exception is a rare case. I would expect django to reconnect automatically but they seemed to refused that issue.

回答5:

How old is this code? Django has had databases defined in settings since at least .96. Only other thing I can think of is multi-db support, which changed things a bit, but even that was 1.1 or 1.2.

Even if you need a special DB for certain views, I think you'd probably be better off defining it in settings.

回答6:

SQLAlchemy now has a great write-up on how you can use pinging to be pessimistic about your connection's freshness:

http://docs.sqlalchemy.org/en/latest/core/pooling.html#disconnect-handling-pessimistic

From there,

from sqlalchemy import exc
from sqlalchemy import event
from sqlalchemy.pool import Pool

@event.listens_for(Pool, "checkout")
def ping_connection(dbapi_connection, connection_record, connection_proxy):
    cursor = dbapi_connection.cursor()
    try:
        cursor.execute("SELECT 1")
    except:
        # optional - dispose the whole pool
        # instead of invalidating one at a time
        # connection_proxy._pool.dispose()

        # raise DisconnectionError - pool will try
        # connecting again up to three times before raising.
        raise exc.DisconnectionError()
    cursor.close()

And a test to make sure the above works:

from sqlalchemy import create_engine
e = create_engine("mysql://scott:tiger@localhost/test", echo_pool=True)
c1 = e.connect()
c2 = e.connect()
c3 = e.connect()
c1.close()
c2.close()
c3.close()

# pool size is now three.

print "Restart the server"
raw_input()

for i in xrange(10):
    c = e.connect()
    print c.execute("select 1").fetchall()
    c.close()

回答7:

I had this problem and did not have the option to change my configuration. I finally figured out that the problem was occurring 49500 records in to my 50000-record loop, because that was the about the time I was trying again (after having tried a long time ago) to hit my second database.

So I changed my code so that every few thousand records, I touched the second database again (with a count() of a very small table), and that fixed it. No doubt "ping" or some other means of touching the database would work, as well.

回答8:

In my opinion, the must common issue regarding such warning, is the fact that your application has reached the wait_timeout value of MySQL.

I had the same problem with a PythonFLASK app which I developed and I solved very easy.

P.S: I was using MySQL 5.7.14

$ grep timeout /etc/mysql/mysql.conf.d/mysqld.cnf 
# https://support.rackspace.com/how-to/how-to-change-the-mysql-timeout-on-a-server/
# wait = timeout for application session (tdm)
# inteactive = timeout for keyboard session (terminal)
# 7 days = 604800s / 4 hours = 14400s 
wait_timeout = 604800
interactive_timeout = 14400

One important observation: if you search for the variables via MySQL batch mode, the values will appear as it is. But If you perform "SHOW VARIABLES LIKE 'wait%';" or "SHOW VARIABLES LIKE 'interactive%';", the value configured for 'interactive_timeout', will appear to both variables, and I don't know why, but the fact is, that the values configured for each variable at '/etc/mysql/mysql.conf.d/mysqld.cnf', will be respected by MySQL process.

Regards!

回答9:

This error is mysterious because MySQL doesn't report why it disconnects, it just goes away.

It seems there are many causes of this kind of disconnection. One I just found is, if the query string too large, the server will disconnect. This probably relates to the max_allowed_packets setting.

回答10:

Firstly, You should make sure the MySQL session & global enviroments wait_timeout and interactive_timeout values. And secondly Your client should try to reconnect to the server below those enviroments values.

回答11:

This might be due to DB connections getting copied in your child threads from the main thread. I faced the same error when using python's multiprocessing library to spawn different processes. The connection objects are copied between processes during forking and it leads to MySQL OperationalErrors when making DB calls in the child thread.

Here's a good reference to solve this: Django multiprocessing and database connections