Add read replica (#115)

* Use a custom NestedSensorSerializer since sensor_type isn't being used at any place

* Update dev postgres to version 13.7 which is what we are using in production

* Add a read replica for the primary database

* Add a Router for determining which database to be used for which database operation.

* Use NestedSensorSerializer from feinstaub

* Update database environment variable naming

* Use the default database for user authentication queries

* Adds a Postgresql data volume

Author: esirK

docker-compose.yml

@@ -12,18 +12,21 @@ services:
       - RABBITMQ_USERNAME=sensorsafrica
       - RABBITMQ_PASSWORD=sensorsafrica
   postgres:
-    image: postgres:9.6
+    image: postgres:13.7
     ports:
       - "54321:5432"
     environment:
       - POSTGRES_USER=sensorsafrica
       - POSTGRES_PASSWORD=sensorsafrica
       - POSTGRES_DB=sensorsafrica
+    volumes:
+      - postgres_data:/var/lib/postgresql/data/
   api:
     build:
       context: .
     environment:
       SENSORSAFRICA_DATABASE_URL: postgres://sensorsafrica:sensorsafrica@postgres:5432/sensorsafrica
+      SENSORSAFRICA_READ_DATABASE_URL: postgres://sensorsafrica:sensorsafrica@postgres:5432/read_replica
       SENSORSAFRICA_RABBITMQ_URL: amqp://sensorsafrica:sensorsafrica@rabbitmq//
       SENSORSAFRICA_FLOWER_ADMIN_USERNAME: admin
       SENSORSAFRICA_FLOWER_ADMIN_PASSWORD: password
@@ -38,3 +41,6 @@ services:
     ports:
       - "8000:8000"
       - "5555:5555"
+
+volumes:
+  postgres_data:

sensorsafrica/router.py

@@ -0,0 +1,20 @@
+class ReplicaRouter:
+    route_app_labels = {'auth', 'sessions',}
+
+    def db_for_read(self, model, **hints):
+        if model._meta.app_label in self.route_app_labels:
+            return 'default'
+        return "read_replica"
+    
+    def db_for_write(self, model, **hints):
+        return "default"
+
+    def allow_relation(self, obj1, obj2, **hints):
+        """
+        Relations between objects are allowed if both objects are
+        in the primary/replica pool.
+        """
+        db_set = {'default', 'read_replica'}
+        if obj1._state.db in db_set and obj2._state.db in db_set:
+            return True
+        return None

sensorsafrica/settings.py

@@ -109,8 +109,16 @@
     "SENSORSAFRICA_DATABASE_URL",
     "postgres://sensorsafrica:sensorsafrica@localhost:5432/sensorsafrica",
 )
-DATABASES = {"default": dj_database_url.parse(DATABASE_URL)}
+READ_DATABASE_URL = os.getenv(
+    "SENSORSAFRICA_READ_DATABASE_URL", DATABASE_URL
+)
+
+DATABASES = {
+                "default": dj_database_url.parse(DATABASE_URL),
+                "read_replica": dj_database_url.parse(READ_DATABASE_URL),
+            }
 
+DATABASE_ROUTERS = ["sensorsafrica.router.ReplicaRouter", ]
 
 # Password validation
 # https://docs.djangoproject.com/en/1.10/ref/settings/#auth-password-validators