Add database router for read-only replica

labour_project/dbrouter.py

@@ -0,0 +1,15 @@
+import random
+
+class HotStandbyRouter:
+    def db_for_read(self, model, **hints):
+        return random.choice(["default", "replica"])
+
+    def db_for_write(self, model, **hints):
+        return "default"
+
+    def allow_relation(self, obj1, obj2, **hints):
+        # XXX do we need this?
+        db_set = {"default", "replica"}
+        if obj1._state.db in db_set and obj2._state.db in db_set:
+            return True
+        return None

labour_project/settings.py

@@ -97,6 +97,18 @@
     }
 }
 
+if config.get('MAPIT_DB_RO_HOST'):
+    DATABASES['replica'] = {
+        'ENGINE': 'django.contrib.gis.db.backends.postgis',
+        'NAME': config.get('MAPIT_DB_NAME', 'mapit'),
+        'USER': config.get('MAPIT_DB_USER', 'mapit'),
+        'PASSWORD': config.get('MAPIT_DB_PASS', ''),
+        'HOST': config.get('MAPIT_DB_RO_HOST', ''),
+        'PORT': config.get('MAPIT_DB_RO_PORT', ''),
+    }
+    DATABASE_ROUTERS = [ "labour_project.dbrouter.HotStandbyRouter" ]
+
+
 # Make this unique, and don't share it with anybody.
 SECRET_KEY = config.get('DJANGO_SECRET_KEY', '')