move the instantiation of the metadata cache out of modulestore.py as it was causing a circular import dependency when running on AWS. Put instantiation into one_time_startup.py which I believe is run before any Django requests are handled

cms/one_time_startup.py

@@ -1,5 +1,13 @@
 from dogapi import dog_http_api, dog_stats_api
 from django.conf import settings
+from xmodule.modulestore.django import modulestore
+
+from django.core.cache import get_cache, InvalidCacheBackendError
+
+cache = get_cache('mongo_metadata_inheritance')
+for store_name in settings.MODULESTORE:
+    store = modulestore(store_name)
+    store.metadata_inheritance_cache = cache
 
 if hasattr(settings, 'DATADOG_API'):
     dog_http_api.api_key = settings.DATADOG_API

common/lib/xmodule/xmodule/modulestore/__init__.py

@@ -423,6 +423,7 @@ class ModuleStoreBase(ModuleStore):
         Set up the error-tracking logic.
         '''
         self._location_errors = {}    # location -> ErrorLog
+        self.metadata_inheritance_cache = None
 
     def _get_errorlog(self, location):
         """

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

@@ -8,8 +8,6 @@ from __future__ import absolute_import
 from importlib import import_module
 from os import environ
 
-from django.core.cache import get_cache, InvalidCacheBackendError
-
 from django.conf import settings
 
 _MODULESTORES = {}
@@ -35,10 +33,6 @@ def modulestore(name='default'):
         class_ = load_function(settings.MODULESTORE[name]['ENGINE'])
 
         options = {}
-        try:
-            options = {'metadata_inheritance_cache': get_cache('mongo_metadata_inheritance')}
-        except InvalidCacheBackendError:
-            pass
 
         options.update(settings.MODULESTORE[name]['OPTIONS'])
         for key in FUNCTION_KEYS:

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

@@ -215,7 +215,7 @@ class MongoModuleStore(ModuleStoreBase):
     def __init__(self, host, db, collection, fs_root, render_template,
                  port=27017, default_class=None,
                  error_tracker=null_error_tracker,
-                 user=None, password=None, metadata_inheritance_cache=None, **kwargs):
+                 user=None, password=None, **kwargs):
 
         ModuleStoreBase.__init__(self)
 
@@ -246,10 +246,6 @@ class MongoModuleStore(ModuleStoreBase):
         self.fs_root = path(fs_root)
         self.error_tracker = error_tracker
         self.render_template = render_template
-        if metadata_inheritance_cache is None:
-            logging.warning('metadata_inheritance_cache is None. Should be defined (unless running unit tests). Check config files....')
-
-        self.metadata_inheritance_cache = metadata_inheritance_cache
 
     def get_metadata_inheritance_tree(self, location):
         '''
@@ -322,6 +318,9 @@ class MongoModuleStore(ModuleStoreBase):
         tree = None
         if self.metadata_inheritance_cache is not None:
             tree = self.metadata_inheritance_cache.get(key_name)
+        else:
+            # This is to help guard against an accident prod runtime without a cache
+            logging.warning('Running MongoModuleStore without metadata_inheritance_cache. This should not happen in production!')
 
         if tree is None or force_refresh:
             tree = self.get_metadata_inheritance_tree(location)

common/lib/xmodule/xmodule/modulestore/xml.py

@@ -253,7 +253,7 @@ class XMLModuleStore(ModuleStoreBase):
     """
     An XML backed ModuleStore
     """
-    def __init__(self, data_dir, default_class=None, course_dirs=None, load_error_modules=True, metadata_inheritance_cache=None):
+    def __init__(self, data_dir, default_class=None, course_dirs=None, load_error_modules=True):
         """
         Initialize an XMLModuleStore from data_dir
 

lms/envs/dev.py

@@ -57,6 +57,13 @@ CACHES = {
         'KEY_PREFIX': 'general',
         'VERSION': 4,
         'KEY_FUNCTION': 'util.memcache.safe_key',
+    },
+
+    'mongo_metadata_inheritance': {
+        'BACKEND': 'django.core.cache.backends.filebased.FileBasedCache',
+        'LOCATION': '/var/tmp/mongo_metadata_inheritance',
+        'TIMEOUT': 300,
+        'KEY_FUNCTION': 'util.memcache.safe_key',
     }
 }
 

lms/one_time_startup.py

@@ -1,5 +1,14 @@
+import logging
 from dogapi import dog_http_api, dog_stats_api
 from django.conf import settings
+from xmodule.modulestore.django import modulestore
+
+from django.core.cache import get_cache, InvalidCacheBackendError
+
+cache = get_cache('mongo_metadata_inheritance')
+for store_name in settings.MODULESTORE:
+    store = modulestore(store_name)
+    store.metadata_inheritance_cache = cache
 
 if hasattr(settings, 'DATADOG_API'):
     dog_http_api.api_key = settings.DATADOG_API