Add mongo read preference OPS-2440

CHANGELOG.md

+- Role: edxapp
+  - Added `EDXAPP_MONGO_READ_PREFERENCE`,
+    `EDXAPP_MONGO_MODULESTORE_READ_PREFERENCE` and
+    `EDXAPP_MONGO_CONTENTSTORE_READ_PREFERENCE` to control MongoDB read
+    preference. If unset `EDXAPP_MONGO_MODULESTORE_READ_PREFERENCE` and
+    `EDXAPP_MONGO_CONTENTSTORE_READ_PREFERENCE` take the value of
+    `EDXAPP_MONGO_READ_PREFERENCE`, which is SECONDARY_PREFERED by
+    default.
+
 - Role: server_utils
   - Install "vim", not "vim-tiny".
 

playbooks/roles/edxapp/defaults/main.yml

@@ -71,6 +71,9 @@ EDXAPP_MONGO_PORT: 27017
 EDXAPP_MONGO_USER: 'edxapp'
 EDXAPP_MONGO_DB_NAME: 'edxapp'
 EDXAPP_MONGO_USE_SSL: False
+EDXAPP_MONGO_READ_PREFERENCE: 'SECONDARY_PREFERRED'
+EDXAPP_MONGO_MODULESTORE_READ_PREFERENCE: "{{ EDXAPP_MONGO_READ_PREFERENCE }}"
+EDXAPP_MONGO_CONTENTSTORE_READ_PREFERENCE: "{{ EDXAPP_MONGO_READ_PREFERENCE }}"
 
 EDXAPP_MYSQL_DB_NAME: 'edxapp'
 EDXAPP_MYSQL_USER: 'edxapp001'
@@ -851,6 +854,7 @@ edxapp_generic_doc_store_config: &edxapp_generic_default_docstore
   user: "{{ EDXAPP_MONGO_USER }}"
   collection:  'modulestore'
   ssl: "{{ EDXAPP_MONGO_USE_SSL }}"
+  read_preference: "{{ EDXAPP_MONGO_MODULESTORE_READ_PREFERENCE }}"
   # https://api.mongodb.com/python/2.9.1/api/pymongo/mongo_client.html#module-pymongo.mongo_client
   socketTimeoutMS: 3000 # default is never timeout while the connection is open, this means it needs to explicitly close raising pymongo.errors.NetworkTimeout
   connectTimeoutMS: 2000 # default is 20000, I believe raises pymongo.errors.ConnectionFailure
@@ -937,6 +941,7 @@ edxapp_generic_auth_config:  &edxapp_generic_auth
       port: "{{ EDXAPP_MONGO_PORT }}"
       user: "{{ EDXAPP_MONGO_USER }}"
       ssl: "{{ EDXAPP_MONGO_USE_SSL }}"
+      read_preference: "{{ EDXAPP_MONGO_CONTENTSTORE_READ_PREFERENCE }}"
     ADDITIONAL_OPTIONS: "{{ EDXAPP_CONTENTSTORE_ADDITIONAL_OPTS }}"
     DOC_STORE_CONFIG: *edxapp_generic_default_docstore
   DATABASES: "{{ edxapp_databases }}"