Defaults CMS and LMS draft docstore read_preference to `PRIMARY`,

and the LMS split docstore read_preference to `SECONDARY_PREFERRED`.

CHANGELOG.md

@@ -11,16 +11,19 @@
 - Role: edxapp
   - Added `EDXAPP_MONGO_REPLICA_SET`, which is required to use
     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.
+    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.
+    This setting causes the `EDXAPP_*_READ_PREFERENCE` settings below to be used.
   - 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.
+    `SECONDARY_PREFERED` to distribute the read workload across the replica set
+    for replicated docstores and contentstores.
+  - Added `EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE` with a default value of
+    `EDXAPP_MONGO_READ_PREFERENCE`.
+  - Added `EDXAPP_CMS_DOC_STORE_READ_PREFERENCE` with a default value of `PRIMARY`.
+  - Added `EDXAPP_LMS_DRAFT_DOC_STORE_CONFIG` with a default value of
+    `EDXAPP_CMS_DOC_STORE_READ_PREFERENCE`, to enforce consistency between
+    Studio and the LMS Preview modes.
 
 - 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

@@ -74,8 +74,11 @@ EDXAPP_MONGO_USER: 'edxapp'
 EDXAPP_MONGO_DB_NAME: 'edxapp'
 EDXAPP_MONGO_USE_SSL: False
 EDXAPP_MONGO_REPLICA_SET: ''
+# Used iff EDXAPP_MONGO_REPLICA_SET is provided.
 EDXAPP_MONGO_READ_PREFERENCE: 'SECONDARY_PREFERRED'
-EDXAPP_LMS_DRAFT_DOC_STORE_READ_PREFERENCE: 'PRIMARY'
+EDXAPP_CMS_DOC_STORE_READ_PREFERENCE: 'PRIMARY'
+EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE: '{{ EDXAPP_MONGO_READ_PREFERENCE }}'
+EDXAPP_LMS_DRAFT_DOC_STORE_READ_PREFERENCE: '{{ EDXAPP_CMS_DOC_STORE_READ_PREFERENCE }}'
 EDXAPP_MONGO_ADDITIONAL_OPTIONS: &edxapp_mongo_additional_options
   replicaSet: "{{ EDXAPP_MONGO_REPLICA_SET }}"
   read_preference: "{{ EDXAPP_MONGO_READ_PREFERENCE }}"
@@ -871,16 +874,20 @@ edxapp_generic_doc_store_config: &edxapp_generic_default_docstore
   # Not setting waitQueueTimeoutMS and waitQueueMultiple since pymongo defaults to nobody being allowed to wait
   <<: *edxapp_mongo_additional_options
 
-
 EDXAPP_LMS_DRAFT_DOC_STORE_CONFIG:
   <<: *edxapp_generic_default_docstore
+  # We use the CMS read_preference here because the draft docstore's view of
+  # the modulestore should mirror Studio's, so that instructors can check their
+  # changes in Preview mode.
   read_preference: "{{ EDXAPP_LMS_DRAFT_DOC_STORE_READ_PREFERENCE }}"
 
 EDXAPP_LMS_SPLIT_DOC_STORE_CONFIG:
   <<: *edxapp_generic_default_docstore
+  read_preference: "{{ EDXAPP_LMS_SPLIT_DOC_STORE_READ_PREFERENCE }}"
 
 EDXAPP_CMS_DOC_STORE_CONFIG:
   <<: *edxapp_generic_default_docstore
+  read_preference: "{{ EDXAPP_CMS_DOC_STORE_READ_PREFERENCE }}"
 
 edxapp_databases:
 # edxapp's edxapp-migrate scripts and the edxapp_migrate play