django connection router

在多库情况下，django使用router指定sql将在哪个库执行，这也可以被用来做读写分离。

在django/db/__init__.py中有

router = ConnectionRouter(settings.DATABASE_ROUTERS)

ConnectionRouter定义在django/db/utils.py中：

class ConnectionRouter(object):
def __init__(self, routers):
self.routers = []
for r in routers:
#load and import class
self.routers.append(router)

def _router_func(action):
def _route_db(self, model, **hints):
chosen_db = None
for router in self.routers:
try:
method = getattr(router, action)
except AttributeError:
# If the router doesn't have a method, skip to the next one.
pass
else:
chosen_db = method(model, **hints)
if chosen_db:
return chosen_db
try:
return hints['instance']._state.db or DEFAULT_DB_ALIAS
except KeyError:
return DEFAULT_DB_ALIAS
return _route_db

db_for_read = _router_func('db_for_read')
db_for_write = _router_func('db_for_write')

def allow_relation(self, obj1, obj2, **hints):
for router in self.routers:
try:
method = router.allow_relation
except AttributeError:
# If the router doesn't have a method, skip to the next one.
pass
else:
allow = method(obj1, obj2, **hints)
if allow is not None:
return allow
return obj1._state.db == obj2._state.db

def allow_syncdb(self, db, model):
for router in self.routers:
try:
method = router.allow_syncdb
except AttributeError:
# If the router doesn't have a method, skip to the next one.
pass
else:
allow = method(db, model)
if allow is not None:
return allow
return True

所以要实现分数据库，只需要一个实现db_for_read, db_for_write等函数的类就可以了

django源码中没有给出实现得例子，但文档中有两个例子： https://docs.djangoproject.com/en/1.3/topics/db/multi-db/