Allow specification of ReadPreference via DOC_STORE_CONFIG.

Use a MongoReplicaSetClient for Split modulestore, but only
if replicaSet config is set.

common/lib/xmodule/xmodule/contentstore/mongo.py

@@ -26,6 +26,10 @@ class MongoContentStore(ContentStore):
         :param collection: ignores but provided for consistency w/ other doc_store_config patterns
         """
         logging.debug('Using MongoDB for static content serving at host={0} port={1} db={2}'.format(host, port, db))
+
+        # Remove the replicaSet parameter.
+        kwargs.pop('replicaSet', None)
+
         _db = pymongo.database.Database(
             pymongo.MongoClient(
                 host=host,

common/lib/xmodule/xmodule/modulestore/django.py

@@ -20,6 +20,8 @@ if not settings.configured:
 from django.core.cache import get_cache, InvalidCacheBackendError
 import django.dispatch
 import django.utils
+
+from pymongo import ReadPreference
 from xmodule.contentstore.django import contentstore
 from xmodule.modulestore.draft_and_published import BranchSettingMixin
 from xmodule.modulestore.mixed import MixedModuleStore
@@ -156,6 +158,9 @@ def create_modulestore_instance(
     else:
         xb_user_service = None
 
+    if 'read_preference' in doc_store_config:
+        doc_store_config['read_preference'] = getattr(ReadPreference, doc_store_config['read_preference'])
+
     return class_(
         contentstore=content_store,
         metadata_inheritance_cache_subsystem=metadata_inheritance_cache,

common/lib/xmodule/xmodule/modulestore/mongo/base.py

@@ -552,6 +552,9 @@ class MongoModuleStore(ModuleStoreDraftAndPublished, ModuleStoreWriteBase, Mongo
             """
             Create & open the connection, authenticate, and provide pointers to the collection
             """
+            # Remove the replicaSet parameter.
+            kwargs.pop('replicaSet', None)
+
             self.database = MongoProxy(
                 pymongo.database.Database(
                     pymongo.MongoClient(

common/lib/xmodule/xmodule/modulestore/split_mongo/mongo_connection.py

@@ -81,16 +81,19 @@ class MongoConnection(object):
         """
         Create & open the connection, authenticate, and provide pointers to the collections
         """
+        if kwargs.get('replicaSet') is None:
+            kwargs.pop('replicaSet', None)
+            mongo_class = pymongo.MongoClient
+        else:
+            mongo_class = pymongo.MongoReplicaSetClient
+        _client = mongo_class(
+            host=host,
+            port=port,
+            tz_aware=tz_aware,
+            **kwargs
+        )
         self.database = MongoProxy(
-            pymongo.database.Database(
-                pymongo.MongoClient(
-                    host=host,
-                    port=port,
-                    tz_aware=tz_aware,
-                    **kwargs
-                ),
-                db
-            ),
+            pymongo.database.Database(_client, db),
             wait_time=retry_wait_time
         )