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