only cache discussion forum info for 5 minutes before flushing. This is needed for Studio backed courses that can add new discussion items at any point in time. Also we need to also set metadata fields (which are basically the same as what gets passed in the XML) as the forums expect those values as metadata

common/lib/xmodule/xmodule/templates/discussion/default.yaml

@@ -1,6 +1,9 @@
 ---
 metadata:
     display_name: Discussion Tag
+    for: Topic-Level Student-Visible Label 
+    id: 6002x_group_discussion_by_this 
+    discussion_category: Week 1
 data: |
   <discussion for="Topic-Level Student-Visible Label" id="6002x_group_discussion_by_this" discussion_category="Week 1" />
 children: []

lms/djangoapps/django_comment_client/utils.py

@@ -2,6 +2,7 @@ from collections import defaultdict
 import logging
 import time
 import urllib
+from datetime import datetime
 
 from django.contrib.auth.models import User
 from django.core.urlresolvers import reverse
@@ -63,22 +64,19 @@ def get_discussion_id_map(course):
         return a dict of the form {category: modules}
     """
     global _DISCUSSIONINFO
-    if not _DISCUSSIONINFO[course.id]:
-        initialize_discussion_info(course)
+    initialize_discussion_info(course)
     return _DISCUSSIONINFO[course.id]['id_map']
 
 def get_discussion_title(course, discussion_id):
     global _DISCUSSIONINFO
-    if not _DISCUSSIONINFO[course.id]:
-        initialize_discussion_info(course)
+    initialize_discussion_info(course)
     title = _DISCUSSIONINFO[course.id]['id_map'].get(discussion_id, {}).get('title', '(no title)')
     return title
 
 def get_discussion_category_map(course):
 
     global _DISCUSSIONINFO
-    if not _DISCUSSIONINFO[course.id]:
-        initialize_discussion_info(course)
+    initialize_discussion_info(course)
     return filter_unstarted_categories(_DISCUSSIONINFO[course.id]['category_map'])
 
 def filter_unstarted_categories(category_map):
@@ -131,8 +129,18 @@ def sort_map_entries(category_map):
 def initialize_discussion_info(course):
 
     global _DISCUSSIONINFO
+
+    # only cache in-memory discussion information for 10 minutes
+    # this is because we need a short-term hack fix for
+    # mongo-backed courseware whereby new discussion modules can be added
+    # without LMS service restart
+
     if _DISCUSSIONINFO[course.id]:
-        return
+        timestamp = _DISCUSSIONINFO[course.id].get('timestamp', datetime.now())
+        age = datetime.now() - timestamp
+        # expire every 5 minutes
+        if age.seconds < 300:
+            return
 
     course_id = course.id
 
@@ -209,6 +217,7 @@ def initialize_discussion_info(course):
 
     _DISCUSSIONINFO[course.id]['id_map'] = discussion_id_map
     _DISCUSSIONINFO[course.id]['category_map'] = category_map
+    _DISCUSSIONINFO[course.id]['timestamp'] = datetime.now()
 
 class JsonResponse(HttpResponse):
     def __init__(self, data=None):