Uznick/django-multidb-router

Round-robin master/slave db router for Django.

multidb provides two Django database routers useful in master-slave deployments.

MasterSlaveRouter

With multidb.MasterSlaveRouter all read queries will go to a slave database; all inserts, updates, and deletes will go to the default database.

First, define SLAVE_DATABASES in your settings. It should be a list of database aliases that can be found in DATABASES:

DATABASES = {
    'default': {...},
    'shadow-1': {...},
    'shadow-2': {...},
}
SLAVE_DATABASES = ['shadow-1', 'shadow-2']

Then put multidb.MasterSlaveRouter into DATABASE_ROUTERS:

DATABASE_ROUTERS = ('multidb.MasterSlaveRouter',)

The slave databases will be chosen in round-robin fashion.

If you want to get a connection to a slave in your app, use multidb.get_slave:

from django.db import connections
import multidb

connection = connections[multidb.get_slave()]

PinningMasterSlaveRouter

In some applications, the lag between the master receiving a write and its replication to the slaves is enough to cause inconsistency for the end user. For example, imagine a scenario with 1 second of replication lag. If a user makes a forum post (to the master) and then is redirected to a fully-rendered view of it (from a slave) 500ms later, the view will fail. If this is a problem in your application, consider using multidb.PinningMasterSlaveRouter. This router works in combination with multidb.middleware.PinningRouterMiddleware to assure that, after writing to the default database, future reads from the same user agent are directed to the default database for a configurable length of time.

Caveats

PinningRouterMiddleware identifies database writes primarily by request type, assuming that requests with HTTP methods that are not GET, TRACE, HEAD, or OPTIONS are writes. You can indicate that any view writes to the database by using the multidb.db_write decorator. This will cause the same result as if the request were, e.g., a POST.

You can also manually set response._db_write = True to indicate that a write occurred. This will not result in using the default database in this request, but only in the next request.

Configuration

To use PinningMasterSlaveRouter, put it into DATABASE_ROUTERS in your settings:

DATABASE_ROUTERS = ('multidb.PinningMasterSlaveRouter',)

Then, install the middleware. It must be listed before any other middleware which performs database writes:

MIDDLEWARE_CLASSES = (
    'multidb.middleware.PinningRouterMiddleware',
    ...more middleware here...
)

PinningRouterMiddleware attaches a cookie to any user agent who has just written. The cookie should be set to expire at a time longer than your replication lag. By default, its value is a conservative 15 seconds, but it can be adjusted like so:

MULTIDB_PINNING_SECONDS = 5

If you need to change the name of the cookie, use the MULTIDB_PINNING_COOKIE setting:

MULTIDB_PINNING_COOKIE = 'multidb_pin_writes'

use_master

multidb.pinning.use_master is both a context manager and a decorator for wrapping code to use the master database. You can use it as a context manager:

from multidb.pinning import use_master

with use_master:
    touch_the_database()
touch_another_database()

or as a decorator:

from multidb.pinning import use_master

@use_master
def func(*args, **kw):
    """Touches the master database."""

Running the Tests

To run the tests, you'll need to install the development requirements:

pip install -r requirements.txt
./run.sh test