I have a question about Django, unixODBC, FreeTDS, Apache2, mod_wsgi, that is somewhat similar to this question asked before on SO.
I have a Django website built to run on the latest Django, that is, 1.2.3. It uses managed models for the most part, in that, save for the session information, nothing is written to the DB by Django, only read from it.
The DB in question is hosted on a Windows machine on MSSQL 2005.
The Django project is hosted on a Linux machine. It is served over mod_wsgi on Apache2. The DB connection is through the infamous FreeTDS and unixODBC duo. Latest versions of both FreeTDS and unixODBC are running. On the Python end, pyodbc, and django-pyodbc are used for the DB stuff.
The project is deployed on two Linux live servers, with identical setups, that are behind a load balancer. There is one DB server that both connect to.
On the live, production servers, there are some intermittent issues. On the pages that query the DB to get records to display, sometimes, and I mean sometimes, an exception is thrown, with the error:
('IM001', '[IM001] [unixODBC][Driver Manager]Driver does not support this function (0) (SQLColAttribute)')
One of the strange bits about this conundrum is that whenever you get this error, simply hitting refresh on the web browser clears it and renders the page with the records fetched from the DB.
The generated SQL, from the debug output when the error occurs, is very simple:
SELECT COUNT(*) FROM [TABLE] WHERE ([TABLE].[category] = 5 AND [TABLE].[newRelease] = 1 )
On the Django development server, this doesn't show up, of course. On the development machine, with mod_wsgi running in daemon mode, I came across this error a number of times. I tested by deploying on mod_python, where I didn't see any errors. I then tested with mod_wsgi in embedded mode, where, as well, I didn't see any errors. I think changed the setup on the live server to use mod_wsgi in embedded mode, but the errors still crop up sometimes.
I am really stuck with this. I am not sure what to try to get closer to figuring out what's really causing the problem. I am not sure whether I have provided all helpful information. If I haven't, please do point it out, and I will update the question.
I don't believe there is anything in the code that is wrong and causing this. It wouldn't work most or all of the time, if it were the case. I've tried clearing .pyc files, restarting Apache, etc, on the live servers, but to no avail.
Any, any help will be very much appreciated.
Thanks.
UPDATE: I tacked the "never_cache" decorator to most of the view functions, hoping that perhaps whatever little model-level caching that Django does could be turned off. But that didn't really do anything on the live server. I am really out of ideas now.
UPDATE #2: I threw in some logging inside sql_sever/pyodbc/base.py (django-pyodbc) around the code where the exception is generated. And I've got more SQL queries for which, apparently, the elusive error was generated:
sql = SELECT * FROM (SELECT [TABLE].[id], [TABLE].[productID], [TABLE].[title], [TABLE].[price], [TABLE].[rrp], [TABLE].[saving], [TABLE].[hmvPoints], [TABLE].[availability], [TABLE].[shipping], [TABLE].[rating], [TABLE].[thumbnail], [TABLE].[details], [TABLE].[images], [TABLE].[certImage], [TABLE].[trackListing], [TABLE].[category], [TABLE].[subCategory], [TABLE].[genreId], [TABLE].[bestSeller], [TABLE].[preOrder], [TABLE].[newRelease], (ROW_NUMBER() OVER (ORDER BY [TABLE].[id] ASC)) AS [rn] FROM [TABLE] WHERE [TABLE].[productID] = ? ) AS X WHERE X.rn BETWEEN 1 AND 21
params = (799742,)
exception = ('IM001', '[IM001] [unixODBC][Driver Manager]Driver does not support this function (0) (SQLColAttribute)')
sql = SELECT * FROM (SELECT (1) AS [a], (ROW_NUMBER() OVER (ORDER BY RAND() )) AS [rn] FROM [django_session] WHERE [django_session].[session_key] = ? ) AS X WHERE X.rn BETWEEN 1 AND 1
params = ('e4b669b40d10c336d62c8435198bf1db',)
exception = ('IM001', '[IM001] [unixODBC][Driver Manager]Driver does not support this function (0) (SQLColAttribute)')