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)
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.
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).
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
For me this was happening in debug mode.
So I tried Persistent connections in debug mode, checkout the link: Django - Documentation - Databases - Persistent connections.
In settings:
'default': {
'ENGINE': 'django.db.backends.mysql',
'NAME': 'dbname',
'USER': 'root',
'PASSWORD': 'root',
'HOST': 'localhost',
'PORT': '3306',
'CONN_MAX_AGE': None
},
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(...)
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.
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.
This error may occur when you try to use the connection after a time-consuming operation that doesn't go to the database. Since the connection is not used for some time, MySQL timeout is hit and the connection is silently dropped.
You can try calling close_old_connections() after the time-consuming non-DB operation so that a new connection is opened if the connection is unusable. Beware, do not use close_old_connections() if you have a transaction.
The most 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 Flask app.
Here's how I solved:
$ 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
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.
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.
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()
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.
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.
Related
I'm running a Django project with Peewee in Python 3.6 and trying to track down what's wrong with the connection pooling. I keep getting the following error on the development server (for some reason I never experience this issue on my local machine):
Lost connection to MySQL server during query
The repro steps are reliable and are:
Restart Apache on the instance.
Go to my Django page and press a button which triggers a DB operation.
Works fine.
Wait exactly 10 minutes (I've tested enough to get the exact number).
Press another button to trigger another DB operation.
Get the lost connection error above.
The code is structured such that I have all the DB operations inside an independent Python module which is imported into the Django module.
In the main class constructor I'm setting up the DB as such:
from playhouse.pool import PooledMySQLDatabase
def __init__(self, host, database, user, password, stale_timeout=300):
self.mysql_db = PooledMySQLDatabase(host=host, database=database, user=user, password=password, stale_timeout=stale_timeout)
db_proxy.initialize(self.mysql_db)
Every call which needs to make calls out to the DB are done like this:
def get_user_by_id(self, user_id):
db_proxy.connect(reuse_if_open=True)
user = (User.get(User.user_id == user_id))
db_proxy.close()
return {'id': user.user_id, 'first_name': user.first_name, 'last_name': user.last_name, 'email': user.email }
I looked at the wait_timeout value on the MySQL instance and its value is 3600 so that doesn't seem to be the issue (and I tried changing it anyway just to see).
Any ideas on what I could be doing wrong here?
Update:
I found that the /etc/my.cnf configuration file for MySQL has the wait-timeout value set to 600, which matches what I'm experiencing. I don't know why this value doesn't show when I runSHOW VARIABLES LIKE 'wait_timeout'; on the MySQL DB (that returns 3600) but it does seem likely the issue is coming from the wait timeout.
Given this I tried setting the stale timeout to 60, assuming that if it's less than the wait timeout it might fix the issue but it didn't make a difference.
You need to be sure you're recycling the connections properly -- that means that when a request begins you open a connection and when the response is delivered you close the connection. The pool is not recycling the conn most likely because you're never putting it back in the pool, so it looks like its still "in use". This can easily be done with middleware and is described here:
http://docs.peewee-orm.com/en/latest/peewee/database.html#django
I finally came up with a fix which works for my case, after trying numerous ideas. It's not ideal but it works. This post on Connection pooling pointed me in the right direction.
I created a Django middleware class and configured it to be the first in the list of Django middleware.
from peewee import OperationalError
from playhouse.pool import PooledMySQLDatabase
database = PooledMySQLDatabase(None)
class PeeweeConnectionMiddleware(object):
CONN_FAILURE_CODES = [ 2006, 2013, ]
def __init__(self, get_response):
self.get_response = get_response
def __call__(self, request):
if database.database: # Is DB initialized?
response = None
try:
database.connect(reuse_if_open=True)
with database.atomic() as transaction:
try:
response = self.get_response(request)
except:
transaction.rollback()
raise
except OperationalError as exception:
if exception.args[0] in self.CONN_FAILURE_CODES:
database.close_all()
database.connect()
response = None
with database.atomic() as transaction:
try:
response = self.get_response(request)
except:
transaction.rollback()
raise
else:
raise
finally:
if not database.is_closed():
database.close()
return response
else:
return self.get_response(request)
Error OperationalError: (OperationalError) (2006, 'MySQL server has gone away') i'm already received this error when i coded project on Flask, but i cant understand why i get this error.
I have code (yeah, if code small and executing fast, then no errors) like this \
db_engine = create_engine('mysql://root#127.0.0.1/mind?charset=utf8', pool_size=10, pool_recycle=7200)
Base.metadata.create_all(db_engine)
Session = sessionmaker(bind=db_engine, autoflush=True)
Session = scoped_session(Session)
session = Session()
# there many classes and functions
session.close()
And this code returns me error 'MySQL server has gone away', but return it after some time, when i use pauses in my script.
Mysql i use from openserver.ru (it's web server like such as wamp).
Thanks..
Looking at the mysql docs, we can see that there are a bunch of reasons why this error can occur. However, the two main reasons I've seen are:
1) The most common reason is that the connection has been dropped because it hasn't been used in more than 8 hours (default setting)
By default, the server closes the connection after eight hours if nothing has happened. You can change the time limit by setting the wait_timeout variable when you start mysqld
I'll just mention for completeness the two ways to deal with that, but they've already been mentioned in other answers:
A: I have a very long running job and so my connection is stale. To fix this, I refresh my connection:
create_engine(conn_str, pool_recycle=3600) # recycle every hour
B: I have a long running service and long periods of inactivity. To fix this I ping mysql before every call:
create_engine(conn_str, pool_pre_ping=True)
2) My packet size is too large, which should throw this error:
_mysql_exceptions.OperationalError: (1153, "Got a packet bigger than 'max_allowed_packet' bytes")
I've only seen this buried in the middle of the trace, though often you'll only see the generic _mysql_exceptions.OperationalError (2006, 'MySQL server has gone away'), so it's hard to catch, especially if logs are in multiple places.
The above doc say the max packet size is 64MB by default, but it's actually 16MB, which can be verified with SELECT ##max_allowed_packet
To fix this, decrease packet size for INSERT or UPDATE calls.
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()
from documentation you can use pool_recycle parameter:
from sqlalchemy import create_engine
e = create_engine("mysql://scott:tiger#localhost/test", pool_recycle=3600)
I just faced the same problem, which is solved with some effort. Wish my experience be helpful to others.
Fallowing some suggestions, I used connection pool and set pool_recycle less than wait_timeout, but it still doesn't work.
Then, I realized that global session maybe just use the same connection and connection pool didn't work. To avoid global session, for each request generate a new session which is removed by Session.remove() after processing.
Finally, all is well.
One more point to keep in mind is to manually push the flask application context with database initialization. This should resolve the issue.
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()
app = Flask(__name__)
with app.app_context():
db.init_app(app)
https://docs.sqlalchemy.org/en/latest/core/pooling.html#disconnect-handling-optimistic
def sql_read(cls, sql, connection):
"""sql for read action like select
"""
LOG.debug(sql)
try:
result = connection.engine.execute(sql)
header = result.keys()
for row in result:
yield dict(zip(header, row))
except OperationalError as e:
LOG.info("recreate pool duo to %s" % e)
connection.engine.pool.recreate()
result = connection.engine.execute(sql)
header = result.keys()
for row in result:
yield dict(zip(header, row))
except Exception as ee:
LOG.error(ee)
raise SqlExecuteError()
I have a seemingly straight-forward situation, but can't find a straight-forward solution.
I'm using sqlalchemy to query postgres. If a client timeout occurs, I'd like to stop/cancel the long running postgres queries from another thread. The thread has access to the Session or Connection object.
At this point I've tried:
session.bind.raw_connection().close()
and
session.connection().close()
and
session.close
and
session.transaction.close()
But no matter what I try, the postgres query still continues until it's end. I know this from watching pg in top. Shouldn't this be fairly easy to do? I'm I missing something? Is this impossible without getting the pid and sending a stop signal directly?
This seems to work well, so far:
def test_close_connection(self):
import threading
from psycopg2.extensions import QueryCanceledError
from sqlalchemy.exc import DBAPIError
session = Session()
conn = session.connection()
sql = self.get_raw_sql_for_long_query()
seconds = 5
t = threading.Timer(seconds, conn.connection.cancel)
t.start()
try:
conn.execute(sql)
except DBAPIError, e:
if type(e.orig) == QueryCanceledError:
print 'Long running query was cancelled.'
t.cancel()
source
For those MySQL folks that may have ended up here, a modified version of this answer that kills the query from a second connection can work. Essentially the following, assuming pymysql under the hood:
thread_id = conn1.connection.thread_id()
t = threading.Timer(seconds, lambda: conn2.execute("kill {}".format(thread_id)))
The original connection will raise pymysql.err.OperationalError. See this other answer for a neat way to create a long running query for testing.
Found on MYSQL that you can specify the query optimiser hints.
One such hint is MAX_EXECUTION_TIME to specify how long query should execute before termination.
You can add this in your app.py
#event.listens_for(engine, 'before_execute', retval=True)
def intercept(conn, clauseelement, multiparams, params):
from sqlalchemy.sql.selectable import Select
# check if it's select statement
if isinstance(clauseelement, Select):
# 'froms' represents list of tables that statement is querying
table = clauseelement.froms[0]
'''Update the timeout here in ms (1s = 1000ms)'''
timeout_ms = 4000
# adding filter in clause
clauseelement = clauseelement.prefix_with(f"/*+ MAX_EXECUTION_TIME({timeout_ms}) */", dialect="mysql")
return clauseelement, multiparams, params
SQLAlchemy query API not working correctly with hints and
MYSQL reference
I am working on a web service with Twisted that is responsible for calling up several packages I had previously used on the command line. The routines these packages handle were being prototyped on their own but now are ready to be integrated into our webservice.
In short, I have several different modules that all create a mysql connection property internally in their original command line forms. Take this for example:
class searcher:
def __init__(self,lat,lon,radius):
self.conn = getConnection()[1]
self.con=self.conn.cursor();
self.mgo = getConnection(True)
self.lat = lat
self.lon = lon
self.radius = radius
self.profsinrange()
self.cache = memcache.Client(["173.220.194.84:11211"])
The getConnection function is just a helper that returns a mongo or mysql cursor respectively. Again, this is all prototypical :)
The problem I am experiencing is when implemented as a consistently running server using Twisted's WSGI resource, the sql connection created in init times out, and subsequent requests don't seem to regenerate it. Example code for small server app:
from twisted.web import server
from twisted.web.wsgi import WSGIResource
from twisted.python.threadpool import ThreadPool
from twisted.internet import reactor
from twisted.application import service, strports
import cgi
import gnengine
import nn
wsgiThreadPool = ThreadPool()
wsgiThreadPool.start()
# ensuring that it will be stopped when the reactor shuts down
reactor.addSystemEventTrigger('after', 'shutdown', wsgiThreadPool.stop)
def application(environ, start_response):
start_response('200 OK', [('Content-type','text/plain')])
params = cgi.parse_qs(environ['QUERY_STRING'])
try:
lat = float(params['lat'][0])
lon = float(params['lon'][0])
radius = int(params['radius'][0])
query_terms = params['query']
s = gnengine.searcher(lat,lon,radius)
query_terms = ' '.join( query_terms )
json = s.query(query_terms)
return [json]
except Exception, e:
return [str(e),str(params)]
return ['error']
wsgiAppAsResource = WSGIResource(reactor, wsgiThreadPool, application)
# Hooks for twistd
application = service.Application('Twisted.web.wsgi Hello World Example')
server = strports.service('tcp:8080', server.Site(wsgiAppAsResource))
server.setServiceParent(application)
The first few requests work fine, but after mysqls wait_timeout expires, the dread error 2006 "Mysql has gone away" error surfaces. It had been my understanding that every request to the WSGI Twisted resource would run the application function, thereby regenerating the searcher object and re-leasing the connection. If this isn't the case, how can I make the requests processed as such? Is this kind of Twisted deployment not transactional in this sense? Thanks!
EDIT: Per request, here is the prototype helper function calling up the connection:
def getConnection(mong = False):
if mong == False:
connection = mysql.connect(host = db_host,
user = db_user,
passwd = db_pass,
db = db,
cursorclass=mysql.cursors.DictCursor)
cur = connection.cursor();
return (cur,connection)
else:
return pymongo.Connection('173.220.194.84',27017).gonation_test
i was developing a piece of software with twisted where i had to utilize a constant MySQL database connection. i did run into this problem and digging through the twisted documentation extensively and posting a few questions i was unable to find a proper solution.There is a boolean parameter you can pass when you are instantiating the adbapi.connectionPool class; however it never seemed to work and i kept getting the error irregardless. However, what i am guessing the reconnect boolean represents is the destruction of the connection object when SQL disconnect does occur.
adbapi.ConnectionPool("MySQLdb", cp_reconnect=True, host="", user="", passwd="", db="")
I have not tested this but i will re-post some results when i do or if anyone else has please share.
When i was developing the script i was using twisted 8.2.0 (i havent touched twisted in a while) and back then the framework had no such explicit keep alive method, so i developed a ping/keepalive extension employing event driven paradigm twisted builds upon in conjunction with direct MySQLdb module ping() method (see code comment).
As i was typing this response; however, i did look around the current twisted documentation i was still unable to find an explicit keep-alive method or parameter. My guess is because twisted itself does not have database connectivity libraries/classes. It uses the methods available to python and provides an indirect layer of interfacing with those modules; with some exposure for direct calls to the database library being used. This is accomplished by using the adbapi.runWithConnection method.
here is the module i wrote under twisted 8.2.0 and python 2.6; you can set the intervals between pings. what the script does is, every 20 minutes it pings the database and if it fails, it attempts to reconnect back to it every 60 seconds. I must warn that the script does NOT handle sudden/dropped connection; that you can handle through addErrback whenever you run a query through twisted, atleast thats how i did it. I have noticed that whenever database connection drops, you can only find out if it has when you are executing a query and the event raises an errback, and then at that point you deal with it. Basically, if i dont run a query for 10 minutes, and my database disconnects me, my application will not respond in real time. the application will realize the connection has been dropped when it runs the query that follows; so the database could have disconnected us 1 minute after the first query, 5, 9, etc....
I guess this sort of goes back to the original idea that i have stated, twisted utilizes python's own libraries or 3rd party libraries for database connectivity and because of that, some things are handled a bit differently.
from twisted.enterprise import adbapi
from twisted.internet import reactor, defer, task
class sqlClass:
def __init__(self, db_pointer):
self.dbpool=db_pointer
self.dbping = task.LoopingCall(self.dbping)
self.dbping.start(1200) #20 minutes = 1200 seconds; i found out that if MySQL socket is idled for 20 minutes or longer, MySQL itself disconnects the session for security reasons; i do believe you can change that in the configuration of the database server itself but it may not be recommended.
self.reconnect=False
print "database ping initiated"
def dbping(self):
def ping(conn):
conn.ping() #what happens here is that twisted allows us to access methods from the MySQLdb module that python posesses; i chose to use the native command instead of sending null commands to the database.
pingdb=self.dbpool.runWithConnection(ping)
pingdb.addCallback(self.dbactive)
pingdb.addErrback(self.dbout)
print "pinging database"
def dbactive(self, data):
if data==None and self.reconnect==True:
self.dbping.stop()
self.reconnect=False
self.dbping.start(1200) #20 minutes = 1200 seconds
print "Reconnected to database!"
elif data==None:
print "database is active"
def dbout(self, deferr):
#print deferr
if self.reconnect==False:
self.dbreconnect()
elif self.reconnect==True:
print "Unable to reconnect to database"
print "unable to ping MySQL database!"
def dbreconnect(self, *data):
self.dbping.stop()
self.reconnect=True
#self.dbping = task.LoopingCall(self.dbping)
self.dbping.start(60) #60
if __name__ == "__main__":
db = sqlClass(adbapi.ConnectionPool("MySQLdb", cp_reconnect=True, host="", user="", passwd="", db=""))
reactor.callLater(2, db.dbping)
reactor.run()
let me know how it works out for you :)
I'm using django with apache and mod_wsgi and PostgreSQL (all on same host), and I need to handle a lot of simple dynamic page requests (hundreds per second). I faced with problem that the bottleneck is that a django don't have persistent database connection and reconnects on each requests (that takes near 5ms).
While doing a benchmark I got that with persistent connection I can handle near 500 r/s while without I get only 50 r/s.
Anyone have any advice? How can I modify Django to use a persistent connection or speed up the connection from Python to DB?
Django 1.6 has added persistent connections support (link to doc for latest stable Django ):
Persistent connections avoid the overhead of re-establishing a
connection to the database in each request. They’re controlled by the
CONN_MAX_AGE parameter which defines the maximum lifetime of a
connection. It can be set independently for each database.
Try PgBouncer - a lightweight connection pooler for PostgreSQL.
Features:
Several levels of brutality when rotating connections:
Session pooling
Transaction pooling
Statement pooling
Low memory requirements (2k per connection by default).
In Django trunk, edit django/db/__init__.py and comment out the line:
signals.request_finished.connect(close_connection)
This signal handler causes it to disconnect from the database after every request. I don't know what all of the side-effects of doing this will be, but it doesn't make any sense to start a new connection after every request; it destroys performance, as you've noticed.
I'm using this now, but I havn't done a full set of tests to see if anything breaks.
I don't know why everyone thinks this needs a new backend or a special connection pooler or other complex solutions. This seems very simple, though I don't doubt there are some obscure gotchas that made them do this in the first place--which should be dealt with more sensibly; 5ms overhead for every request is quite a lot for a high-performance service, as you've noticed. (It takes me 150ms--I havn't figured out why yet.)
Edit: another necessary change is in django/middleware/transaction.py; remove the two transaction.is_dirty() tests and always call commit() or rollback(). Otherwise, it won't commit a transaction if it only read from the database, which will leave locks open that should be closed.
I created a small Django patch that implements connection pooling of MySQL and PostgreSQL via sqlalchemy pooling.
This works perfectly on production of http://grandcapital.net/ for a long period of time.
The patch was written after googling the topic a bit.
Disclaimer: I have not tried this.
I believe you need to implement a custom database back end. There are a few examples on the web that shows how to implement a database back end with connection pooling.
Using a connection pool would probably be a good solution for you case, as the network connections are kept open when connections are returned to the pool.
This post accomplishes this by patching Django (one of the comments points out that it is better to implement a custom back end outside of the core django code)
This post is an implementation of a custom db back end
Both posts use MySQL - perhaps you are able to use similar techniques with Postgresql.
Edit:
The Django Book mentions Postgresql connection pooling, using pgpool (tutorial).
Someone posted a patch for the psycopg2 backend that implements connection pooling. I suggest creating a copy of the existing back end in your own project and patching that one.
This is a package for django connection pool:
django-db-connection-pool
pip install django-db-connection-pool
You can provide additional options to pass to SQLAlchemy's pool creation, key's name is POOL_OPTIONS:
DATABASES = {
'default': {
...
'POOL_OPTIONS' : {
'POOL_SIZE': 10,
'MAX_OVERFLOW': 10
}
...
}
}
I made some small custom psycopg2 backend that implements persistent connection using global variable.
With this I was able to improve the amout of requests per second from 350 to 1600 (on very simple page with few selects)
Just save it in the file called base.py in any directory (e.g. postgresql_psycopg2_persistent) and set in settings
DATABASE_ENGINE to projectname.postgresql_psycopg2_persistent
NOTE!!! the code is not threadsafe - you can't use it with python threads because of unexpectable results, in case of mod_wsgi please use prefork daemon mode with threads=1
# Custom DB backend postgresql_psycopg2 based
# implements persistent database connection using global variable
from django.db.backends.postgresql_psycopg2.base import DatabaseError, DatabaseWrapper as BaseDatabaseWrapper, \
IntegrityError
from psycopg2 import OperationalError
connection = None
class DatabaseWrapper(BaseDatabaseWrapper):
def _cursor(self, *args, **kwargs):
global connection
if connection is not None and self.connection is None:
try: # Check if connection is alive
connection.cursor().execute('SELECT 1')
except OperationalError: # The connection is not working, need reconnect
connection = None
else:
self.connection = connection
cursor = super(DatabaseWrapper, self)._cursor(*args, **kwargs)
if connection is None and self.connection is not None:
connection = self.connection
return cursor
def close(self):
if self.connection is not None:
self.connection.commit()
self.connection = None
Or here is a thread safe one, but python threads don't use multiple cores, so you won't get such performance boost as with previous one. You can use this one with multi process one too.
# Custom DB backend postgresql_psycopg2 based
# implements persistent database connection using thread local storage
from threading import local
from django.db.backends.postgresql_psycopg2.base import DatabaseError, \
DatabaseWrapper as BaseDatabaseWrapper, IntegrityError
from psycopg2 import OperationalError
threadlocal = local()
class DatabaseWrapper(BaseDatabaseWrapper):
def _cursor(self, *args, **kwargs):
if hasattr(threadlocal, 'connection') and threadlocal.connection is \
not None and self.connection is None:
try: # Check if connection is alive
threadlocal.connection.cursor().execute('SELECT 1')
except OperationalError: # The connection is not working, need reconnect
threadlocal.connection = None
else:
self.connection = threadlocal.connection
cursor = super(DatabaseWrapper, self)._cursor(*args, **kwargs)
if (not hasattr(threadlocal, 'connection') or threadlocal.connection \
is None) and self.connection is not None:
threadlocal.connection = self.connection
return cursor
def close(self):
if self.connection is not None:
self.connection.commit()
self.connection = None