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Merge pull request #363 from ubernostrum/multidb-examples-18657

Ticket 18657: Fix inconsistent DB names in router example.
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ubernostrum committed Sep 8, 2012
2 parents d7853c5 + 5c5226a commit c26dd4f154fe9d7aedeb28b0de4bb8cca85b55f9
Showing with 92 additions and 36 deletions.
  1. +92 −36 docs/topics/db/multi-db.txt
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@@ -201,73 +201,129 @@ An example
write to propagate to the slaves). It also doesn't consider the
interaction of transactions with the database utilization strategy.
So - what does this mean in practice? Say you want ``myapp`` to
exist on the ``other`` database, and you want all other models in a
master/slave relationship between the databases ``master``, ``slave1`` and
``slave2``. To implement this, you would need 2 routers::
So - what does this mean in practice? Let's consider another sample
configuration. This one will have several databases: one for the
``auth`` application, and all other apps using a master/slave setup
with two read slaves. Here are the settings specifying these
databases::
class MyAppRouter(object):
"""A router to control all database operations on models in
the myapp application"""
DATABASES = {
'auth_db': {
'NAME': 'auth_db',
'ENGINE': 'django.db.backends.mysql',
'USER': 'mysql_user',
'PASSWORD': 'swordfish',
},
'master': {
'NAME': 'master',
'ENGINE': 'django.db.backends.mysql',
'USER': 'mysql_user',
'PASSWORD': 'spam',
},
'slave1': {
'NAME': 'slave1',
'ENGINE': 'django.db.backends.mysql',
'USER': 'mysql_user',
'PASSWORD': 'eggs',
},
'slave2': {
'NAME': 'slave2',
'ENGINE': 'django.db.backends.mysql',
'USER': 'mysql_user',
'PASSWORD': 'bacon',
},
}
Now we'll need to handle routing. First we want a router that knows to
send queries for the ``auth`` app to ``auth_db``::
class AuthRouter(object):
"""
A router to control all database operations on models in the
auth application.
"""
def db_for_read(self, model, **hints):
"Point all operations on myapp models to 'other'"
if model._meta.app_label == 'myapp':
return 'other'
"""
Attempts to read auth models go to auth_db.
"""
if model._meta.app_label == 'auth':
return 'auth_db'
return None
def db_for_write(self, model, **hints):
"Point all operations on myapp models to 'other'"
if model._meta.app_label == 'myapp':
return 'other'
"""
Attempts to write auth models go to auth_db.
"""
if model._meta.app_label == 'auth':
return 'auth_db'
return None
def allow_relation(self, obj1, obj2, **hints):
"Allow any relation if a model in myapp is involved"
if obj1._meta.app_label == 'myapp' or obj2._meta.app_label == 'myapp':
return True
"""
Allow relations if a model in the auth app is involved.
"""
if obj1._meta.app_label == 'auth' or \
obj2._meta.app_label == 'auth':
return True
return None
def allow_syncdb(self, db, model):
"Make sure the myapp app only appears on the 'other' db"
if db == 'other':
return model._meta.app_label == 'myapp'
elif model._meta.app_label == 'myapp':
"""
Make sure the auth app only appears in the 'auth_db'
database.
"""
if db == 'auth_db':
return model._meta.app_label == 'auth'
elif model._meta.app_label == 'auth':
return False
return None
class MasterSlaveRouter(object):
"""A router that sets up a simple master/slave configuration"""
And we also want a router that sends all other apps to the
master/slave configuration, and randomly chooses a slave to read
from::
import random
class MasterSlaveRouter(object):
def db_for_read(self, model, **hints):
"Point all read operations to a random slave"
return random.choice(['slave1','slave2'])
"""
Reads go to a randomly-chosen slave.
"""
return random.choice(['slave1', 'slave2'])
def db_for_write(self, model, **hints):
"Point all write operations to the master"
"""
Writes always go to master.
"""
return 'master'
def allow_relation(self, obj1, obj2, **hints):
"Allow any relation between two objects in the db pool"
db_list = ('master','slave1','slave2')
if obj1._state.db in db_list and obj2._state.db in db_list:
"""
Relations between objects are allowed if both objects are
in the master/slave pool.
"""
db_list = ('master', 'slave1', 'slave2')
if obj1.state.db in db_list and obj2.state.db in db_list:
return True
return None
def allow_syncdb(self, db, model):
"Explicitly put all models on all databases."
"""
All non-auth models end up in this pool.
"""
return True
Then, in your settings file, add the following (substituting ``path.to.`` with
the actual python path to the module where you define the routers)::
Finally, in the settings file, we add the following (substituting
``path.to.`` with the actual python path to the module(s) where the
routers are defined)::
DATABASE_ROUTERS = ['path.to.MyAppRouter', 'path.to.MasterSlaveRouter']
DATABASE_ROUTERS = ['path.to.AuthRouter', 'path.to.MasterSlaveRouter']
The order in which routers are processed is significant. Routers will
be queried in the order the are listed in the
:setting:`DATABASE_ROUTERS` setting . In this example, the
``MyAppRouter`` is processed before the ``MasterSlaveRouter``, and as a
result, decisions concerning the models in ``myapp`` are processed
``AuthRouter`` is processed before the ``MasterSlaveRouter``, and as a
result, decisions concerning the models in ``auth`` are processed
before any other decision is made. If the :setting:`DATABASE_ROUTERS`
setting listed the two routers in the other order,
``MasterSlaveRouter.allow_syncdb()`` would be processed first. The
@@ -276,11 +332,11 @@ that all models would be available on all databases.
With this setup installed, lets run some Django code::
>>> # This retrieval will be performed on the 'credentials' database
>>> # This retrieval will be performed on the 'auth_db' database
>>> fred = User.objects.get(username='fred')
>>> fred.first_name = 'Frederick'
>>> # This save will also be directed to 'credentials'
>>> # This save will also be directed to 'auth_db'
>>> fred.save()
>>> # These retrieval will be randomly allocated to a slave database

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