Ticket 18657: Fix inconsistent DB names in router example.
This rewrites the entire example to use the same DB names throughout, and also is hopefully a bit more sensibly described. Additionally, the missing import of the random module for choosing a read slave is included in the example now.
This commit is contained in:
James Bennett
parent
d7853c55ed
commit
5d1f09f450
|
|
@ -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'
|
||||
return None
|
||||
"""
|
||||
Attempts to write auth models go to auth_db.
|
||||
"""
|
||||
if model._meta.app_label == 'auth':
|
||||
return 'auth_db'
|
||||
return Non
|
||||
|
||||
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':
|
||||
"""
|
||||
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
|
||||
|
|
|
|||
Loading…
Reference in New Issue