Add database routing configuration for production

Non-priority production containers will use the master for
writes and a read-replica database for reads. This commit adds
functionality for getting the replica's information from
environment variables and sets up a database routing scheme.

config/settings/container/production.py

 # Settings for a production GraceDB instance running in a container
+import copy
+from django.core.exceptions import ImproperlyConfigured
 from .base import *
 
 DEBUG = False
@@ -15,12 +17,46 @@ if (isinstance(is_priority_server, str) and
     is_priority_server.lower() in ['true', 't']):
     PRIORITY_SERVER = True
 
-# If priority server, add custom permissions for API
+# If priority server, do some things
 if PRIORITY_SERVER:
+    # Add custom permissions for API
     default_perms = list(REST_FRAMEWORK['DEFAULT_PERMISSION_CLASSES'])
     default_perms = ['api.permissions.IsPriorityUser'] + default_perms
     REST_FRAMEWORK['DEFAULT_PERMISSION_CLASSES'] = tuple(default_perms)
 
+    # Don't do anything to databases. Priority servers use the master
+    # for both read and write operations
+else:
+    # If not a priority server, we use the read-only replica database
+    # for reads and master for writes.
+
+    # Get information from environment variables
+    replica_db_name = os.environ.get('DJANGO_REPLICA_DB_NAME', None)
+    if replica_db_name is None:
+        raise ImproperlyConfigured('Could not get replica database name from '
+            'envvars.')
+    replica_db_password = os.environ.get('DJANGO_REPLICA_DB_PASSWORD', None)
+    if replica_db_password is None:
+        raise ImproperlyConfigured('Could not get replica database password '
+            'from envvars.')
+
+    # Set up dict and add to DATABASES setting
+    read_replica = {
+        'NAME': replica_db_name,
+        'ENGINE': 'django.db.backends.mysql',
+        'USER': os.environ.get('DJANGO_REPLICA_DB_USER', 'gracedb'),
+        'PASSWORD': replica_db_password,
+        'HOST': os.environ.get('DJANGO_REPLICA_DB_HOST', ''),
+        'PORT': os.environ.get('DJANGO_REPLICA_DB_PORT', ''),
+        'OPTIONS': {
+            'init_command': 'SET storage_engine=MyISAM',
+        },
+    }
+    DATABASES['read_replica'] = read_replica
+
+    # Set up database router
+    DATABASE_ROUTERS = ['core.db.routers.NonPriorityRouter',]
+
 # Safety check on debug mode for production
 if (DEBUG == True):
     raise RuntimeError("Turn off debug mode for production")

gracedb/core/db/routers.py

+
+
+class NonPriorityRouter(object):
+    """For non-priority production workers in a swarm container deployment"""
+
+    def db_for_read(self, model, **hints):
+        return 'read_replica'
+
+    def db_for_write(self, model, **hints):
+        return 'default'