Django modify DATABASE_HOST at runtime

Question

I am trying to switch between 2 mysql servers at runtime. I do not need to maintain both connections alive all the time.

This is what I am doing

from django.conf import settings
from django.db import connection
from django.contrib.auth.models import User

connection.close()
setattr(settings, 'DATABASE_HOST', 'mysql1.com')
list1 = User.objects.all()

connection.close()
setattr(settings, 'DATABASE_HOST', 'mysql2.com')
list2 = User.objects.all()

I have the following settings.py:

DATABASE_HOST = '' # localhost
DATABASE_NAME = test
...

The database name is the same on all servers and only the content of each tables differ.

I should get list1 != list2 as the users are different on both servers.

The issue is that I always get the list of users from the default database defined in settings.py (which is running on localhost) instead of the one from mysql 1 server and then from mysql 2 server.

Any idea what I am doing wrong here?

Laurent

Answer 1

My guess, from the information, would be a potential error in your set DATABASE_HOST lines (in yor pseudo code above). read: "setattr(settings..."

Other than that, I'm not sure how you've configured your database to switch based on your criteria, as you've not explained this. If you are doing it by model, it may be worth considering how Django knows this, or even using external connections (manually loading the database driver and running commands by hand prior to the render stage), and using the main.

I'd query the whole approach, but mostly because I'm not sure how you're actually differentiating the two databases, or why. Could you provide a bit more information on how you're doing this? I assume the variables you're pulling in dot-points 2 and 5 above are different. I don't need the values, I'm just making sure you've not used the old code duplication and forgotten to edit it (we've all been there).

Note: I'd post this as a comment if I could, but I think the solution may be in how you're pulling the variables. Finally, you could try adding the database name (just the server IP or whatever) to the output, if you're in 'dev'/debug (offline/non-production) mode, to check if it's actually making it to the second server.

Answer 2

For reference, the Django documentation explicitly states you shouldn't do this -- Altering settings at runetime.

There is a lot of talk within the Django community about the ORM supporting multiple connections/databases at once. There's a lot of good reference info out there on it. Check out this blog post: Easy Multi-Database Support for Django and this Django wiki page Multiple Database Support.

In the blog post, Eric Florenzano does something like this in his settings.py file:

DATABASES = dict(
    primary = dict(
        DATABASE_NAME=DATABASE_NAME,
        # ...
    ),
    secondary = dict(
        DATABASE_NAME='secondary.db',
        # ...
    ),
)