Add a StickyMasterSlaveRouter that pins a request to the master after…

… a write.

multidb/__init__.py

@@ -31,6 +31,8 @@
 import random
 
 from django.conf import settings
+from django.utils.thread_support import currentThread
+from django.core.signals import post_save, post_delete, request_started
 
 
 DEFAULT_DB_ALIAS = 'default'
@@ -53,6 +55,29 @@ def get_slave():
     return slaves.next()
 
 
+_requests = {}
+
+
+def get_written():
+    return _requests.get(currentThread())
+
+
+def set_written():
+    _requests[currentThread()] = True
+
+
+def clear_written():
+    try:
+        del _requests[currentThread()]
+    except KeyError:
+        pass
+
+
+request_started.connections(clear_written)
+post_save.connect(set_written)
+post_delete.connect(set_written)
+
+
 class MasterSlaveRouter(object):
     """Router that sends all reads to a slave, all writes to default."""
 
@@ -71,3 +96,16 @@ def allow_relation(self, obj1, obj2, **hints):
     def allow_syncdb(self, db, model):
         """Only allow syncdb on the master."""
         return db == DEFAULT_DB_ALIAS
+
+
+class StickyMasterSlaveRouter(MasterSlaveRouter):
+    """Sends all reads to a slave unless there has already been a write,
+    then sends reads to the master for the remainder of the request."""
+
+    def db_for_read(self, model, **hints):
+        """Send reads to slaves in round-robin, unless there has been a
+        write."""
+
+        if get_written():
+            return DEFAULT_DB_ALIAS
+        return get_slave()