Replaces EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE with EDXAPP_MONGO_READ_PREFERENCE

to indicate that this is the default read_preference for all mongo
replicasets.

Adds EDXAPP_LMS_DRAFT_DOC_STORE_READ_PREFERENCE, so it can differ from
the EDXAPP_MONGO_READ_PREFERENCE and default to PRIMARY.

Updates the CHANGELOG to indicate the variable changes, and to better
clarify their purpose and usage.

CHANGELOG.md

@@ -9,16 +9,18 @@
     your configuration to set `EDXAPP_CELERY_BROKER_TRANSPORT` explicitly.
 
 - 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.
-    Note that setting `EDXAPP_MONGO_REPLICA_SET` requires `EDXAPP_MONGO_HOSTS` to be a comma-delimited string listing
-    the host names, not an array.
+    pymongo.MongoReplicaSetClient in PyMongo 2.9.1.  This should be set to the
+    name of your replica set.  Note that setting `EDXAPP_MONGO_REPLICA_SET`
+    requires `EDXAPP_MONGO_HOSTS` to be a comma-delimited string listing the
+    host names, not an array.
+  - Added `EDXAPP_MONGO_READ_PREFERENCE` with a default value of
+    SECONDARY_PREFERED to distribute read workload across the replica set for
+    replicated docstores and contentstores.
+    This setting is ignored if `EDXAPP_MONGO_REPLICA_SET` is not set.
+  - Added `EDXAPP_LMS_DRAFT_DOC_STORE_READ_PREFERENCE` with a default value of
+    PRIMARY to prevent race conditions when reading from the draft replica set.
+    This setting is ignored if `EDXAPP_MONGO_REPLICA_SET` is not set.
 
 - Role: nginx
   - Modified `lms.j2` , `cms.j2` , `credentials.j2` , `edx_notes_api.j2` and `insights.j2` to enable HTTP Strict Transport Security

playbooks/roles/edxapp/defaults/main.yml

@@ -74,10 +74,11 @@ 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_MONGO_READ_PREFERENCE: 'SECONDARY_PREFERRED'
+EDXAPP_LMS_DRAFT_DOC_STORE_READ_PREFERENCE: 'PRIMARY'
 EDXAPP_MONGO_ADDITIONAL_OPTIONS: &edxapp_mongo_additional_options
   replicaSet: "{{ EDXAPP_MONGO_REPLICA_SET }}"
-  read_preference: "{{ EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE }}"
+  read_preference: "{{ EDXAPP_MONGO_READ_PREFERENCE }}"
 EDXAPP_CONTENTSTORE_ADDITIONAL_OPTS:
   default: *edxapp_mongo_additional_options
   trashcan: *edxapp_mongo_additional_options
@@ -873,6 +874,7 @@ edxapp_generic_doc_store_config: &edxapp_generic_default_docstore
 
 EDXAPP_LMS_DRAFT_DOC_STORE_CONFIG:
   <<: *edxapp_generic_default_docstore
+  read_preference: "{{ EDXAPP_LMS_DRAFT_DOC_STORE_READ_PREFERENCE }}"
 
 EDXAPP_LMS_SPLIT_DOC_STORE_CONFIG:
   <<: *edxapp_generic_default_docstore