Re: [sqlalchemy] Hang when >= 15 MS SQL Server requests have been made
On Monday, December 30, 2019 at 9:07:45 AM UTC-6, Mike Bayer wrote: > > > On Sun, Dec 29, 2019, at 11:54 PM, Brian Paterni wrote: > > On Sunday, December 29, 2019 at 1:17:24 AM UTC-6, Mike Bayer wrote: > > > I can't run the test app however 15 seems like your connection pool is set > up at its default size of 5 connections + 10 overflow, all connections are > being checked out, and none are being returned. > > > Hm, is the issue with running the app possibly something I could help with? > > > sure, if you can turn it into a single file, runnable MCVE with zero > depedendencies other than SQLAlchemy, a single MSSQL Python driver (please > note that MS's ODBC driver, while necessary, is not a Python driver by > itself), and in this case eventlet, I can run that. However I think > you likely should be able to reproduce your issue not using SQLAlchemy at > all and simply using pyodbc directly assuming that's the driver you are > using. > > > http://stackoverflow.com/help/mcve > > I've created: https://github.com/bpaterni/simple-blocking-eventlet Which should be a stripped down version of the flask-app I'd posted before. You are correct in that the problem persists when using only pyodbc, and as a result I've gone ahead and created an issue with that project in order to try and get at the source of this problem: https://github.com/mkleehammer/pyodbc/issues/694 > > Apparently it *does* (or should) support async, as it is mentioned several > times in the RELEASE_NOTES shipped with the driver. I'm not sure if it's > does so implicitly or explicitly though. > > > unfortunately things are not that simple. PostgreSQL for example > supports a non-blocking API. However, you can't just use psycopg2 out of > the box and expect it to work, psycopg2 offers an explicit API for this > that has to be adapted, which you can see here: > http://initd.org/psycopg/docs/advanced.html#green-support in order for > that API to work with eventlet, you need to use a special eventlet > adaptation form here: https://pypi.org/project/psycogreen/ > > I believe psycogreen (or it's intended behavior) has already been integrated into eventlet: https://github.com/eventlet/eventlet/blob/master/eventlet/support/psycopg2_patcher.py which is probably the reason postgresql has been implicitly working as expected this whole time. I agree that some additional hoops may need to be jumped in order for MSSQL to work as expected, but this hang on >= 15 busy connections is strange. Hopefully it is something that can be bandaid'ed in pyodbc until some kind of genuine async interface can be added to the project... -- SQLAlchemy - The Python SQL Toolkit and Object Relational Mapper http://www.sqlalchemy.org/ To post example code, please provide an MCVE: Minimal, Complete, and Verifiable Example. See http://stackoverflow.com/help/mcve for a full description. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy/e5d47afa-dbfa-4d99-b5e9-47fe6ab0690f%40googlegroups.com.
Re: [sqlalchemy] Hang when >= 15 MS SQL Server requests have been made
On Sunday, December 29, 2019 at 1:17:24 AM UTC-6, Mike Bayer wrote: > > > I can't run the test app however 15 seems like your connection pool is set > up at its default size of 5 connections + 10 overflow, all connections are > being checked out, and none are being returned. > Hm, is the issue with running the app possibly something I could help with? I agree the magic 15 figure seems to be related to connection pool exhaustion. The funny thing is that the app still hangs on the 15th connection even if pool_size+overflow are expanded > 15, but not if the pool_size+overflow < 15 (?!) > > while I strongly recommend against using eventlet with Python DBAPI > drivers or SQLAlchemy, when using eventlet or gevent with SQLAlchemy you > need to ensure that a full monkeypatch of "thereading" / "socket" and > everything is performed before anything else is imported. SQLAlchemy's > pool makes use of a port of the Queue class which makes use of threading > mutexes all of which will wreck an eventlet application that did not > correctly monkeypatch these. > > I'm also not familiar with any driver for MSSQL that supports implicit or > explicit async. SQLAlchemy only works with PyODBC or pymssql neither of > which have async support that I'm aware of, what driver are you using ? > The test app is careful to initiate eventlet monkey patching before any additional logic/imports (except for the `import os` required to check if an envvar is set. The problem persists even if the envvar comparison is taken out and eventlet monkeypatching becomes the absolute first action of the test app). https://github.com/bpaterni/flask-app-simple-blocking-eventlet/blob/d279ef9b6210122ebd9c7bd17fc7e8f4346ca74d/app.py#L3 The driver I'm using to connect to SQL Server is the official ODBC driver from Microsoft (mssql+pyodbc): https://docs.microsoft.com/en-us/sql/connect/odbc/microsoft-odbc-driver-for-sql-server?view=sql-server-ver15 Apparently it *does* (or should) support async, as it is mentioned several times in the RELEASE_NOTES shipped with the driver. I'm not sure if it's does so implicitly or explicitly though. -- SQLAlchemy - The Python SQL Toolkit and Object Relational Mapper http://www.sqlalchemy.org/ To post example code, please provide an MCVE: Minimal, Complete, and Verifiable Example. See http://stackoverflow.com/help/mcve for a full description. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy/37931757-664c-4d42-babe-e59a3e4fd177%40googlegroups.com.
[sqlalchemy] Re: Hang when >= 15 MS SQL Server requests have been made
Plus, if it's any help. the *does* seem resolve itself after ~2 hours.
That, or it can be side-stepped by sending a SIGINT signal (ctrl-c) to the
flask app when it is hung. the SIGINT seems to kill the 15th (hung) request
and allows the app to continue processing other requests successfully.
On Saturday, December 28, 2019 at 10:12:13 PM UTC-6, Brian Paterni wrote:
>
> Hi,
>
> I seemingly have a problem with flask/socketio/eventlet/sqlalchemy + MSSQL
> when >= 15 parallel requests have been made. I've built a test app:
>
> https://github.com/bpaterni/flask-app-simple-blocking-eventlet
>
> that can be used to reproduce the problem. It will hang if >= 15 parallel
> request have been made to the '/api/busy/mssql' endpoint.
>
> I'm not sure if the root cause of the problem is based in the SQL Server
> ODBC Driver, sqlalchemy, or eventlet, but I've already paid the microsoft
> support tax only to be told that there's insufficient evidence to indicate
> the ODBC driver is at fault. So I thought I would post the issue here to
> see if anybody would be able to help in pinpointing the code that is at
> fault with this problem.
>
> Once the test app above is running and has a valid SQL server to query,
> you should be able to reproduce the hang with
>
> seq 15 | parallel -j0 "curl -s localhost:5000/api/busy/mssql && echo {}"
>
> The hang seems lo occur consistently on the 15th request. This happens
> even when connection pool_size/max_overflow are adjusted away from their
> respective default values which leads me to believe that exhausting the
> connection pool is not the cause of the problem. Though there may be some
> other reason behind the scenes for the hang occurring at the 15th
> connection(?)
>
> Thanks very much for any help that can be provided in resolving this issue!
> :)
>
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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[sqlalchemy] Hang when >= 15 MS SQL Server requests have been made
Hi,
I seemingly have a problem with flask/socketio/eventlet/sqlalchemy + MSSQL
when >= 15 parallel requests have been made. I've built a test app:
https://github.com/bpaterni/flask-app-simple-blocking-eventlet
that can be used to reproduce the problem. It will hang if >= 15 parallel
request have been made to the '/api/busy/mssql' endpoint.
I'm not sure if the root cause of the problem is based in the SQL Server
ODBC Driver, sqlalchemy, or eventlet, but I've already paid the microsoft
support tax only to be told that there's insufficient evidence to indicate
the ODBC driver is at fault. So I thought I would post the issue here to
see if anybody would be able to help in pinpointing the code that is at
fault with this problem.
Once the test app above is running and has a valid SQL server to query, you
should be able to reproduce the hang with
seq 15 | parallel -j0 "curl -s localhost:5000/api/busy/mssql && echo {}"
The hang seems lo occur consistently on the 15th request. This happens even
when connection pool_size/max_overflow are adjusted away from their
respective default values which leads me to believe that exhausting the
connection pool is not the cause of the problem. Though there may be some
other reason behind the scenes for the hang occurring at the 15th
connection(?)
Thanks very much for any help that can be provided in resolving this issue!
:)
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
---
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