Add read preference for module store OPS-2440

CHANGELOG.md

+- Role: edxapp
+  - Added `EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE` with a default value of
+    SECONDARY_PREFERED to distribute read workload across the replica set.
+  - Changed `EDXAPP_MONGO_HOSTS` to be a comma seperated string, which is
+    required by pymongo.MongoReplicaSetClient for multiple hosts instead of an
+    array.
+  - Added `EDXAPP_MONGO_REPLICA_SET`, which is required to use
+    pymongo.MongoReplicaSetClient in PyMongo 2.9.1, whis is required to use the
+    read_preference setting. This should be set to the name of your replica set.
+
 - Role: nginx
   - Modified `lms.j2` , `cms.j2` , `credentials.j2` , `edx_notes_api.j2` and `insights.j2` to enable HTTP Strict Transport Security
   - Added `NGINX_HSTS_MAX_AGE` to make HSTS header `max_age` value configurable and used in templates
- 
+
 - Role: server_utils
   - Install "vim", not "vim-tiny".
 

playbooks/roles/edxapp/defaults/main.yml

@@ -65,12 +65,16 @@ EDXAPP_XQUEUE_DJANGO_AUTH:
   password: 'password'
 EDXAPP_XQUEUE_URL: 'http://localhost:18040'
 
-EDXAPP_MONGO_HOSTS: ['localhost']
+# EDXAPP_MONGO_HOSTS must be a comma seperated list of hosts/ips for
+# compatibility with pymongo.MongoReplicaSetClient in PyMongo 2.9.1
+EDXAPP_MONGO_HOSTS: 'localhost'
 EDXAPP_MONGO_PASSWORD: 'password'
 EDXAPP_MONGO_PORT: 27017
 EDXAPP_MONGO_USER: 'edxapp'
 EDXAPP_MONGO_DB_NAME: 'edxapp'
 EDXAPP_MONGO_USE_SSL: False
+EDXAPP_MONGO_REPLICA_SET: ''
+EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE: 'SECONDARY_PREFERRED'
 
 EDXAPP_MYSQL_DB_NAME: 'edxapp'
 EDXAPP_MYSQL_USER: 'edxapp001'
@@ -863,6 +867,8 @@ EDXAPP_LMS_DRAFT_DOC_STORE_CONFIG:
 
 EDXAPP_LMS_SPLIT_DOC_STORE_CONFIG:
   <<: *edxapp_generic_default_docstore
+  replicaSet: "{{ EDXAPP_MONGO_REPLICA_SET }}"
+  read_preference: "{{ EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE }}"
 
 EDXAPP_CMS_DOC_STORE_CONFIG:
   <<: *edxapp_generic_default_docstore