A context cache for Resource objects

A context cache which memoizes the resources fetched by calls to
Resource.get_all_by_root_stack(..., cache=True)
which are recalled by subsequent calls to Resource.get_all_by_stack.

Because get_all_by_stack returns a collection instead of a single
resource, there is no way of taking advantage of the SQLAlchemy
identity map [1].

[1] http://docs.sqlalchemy.org/en/latest/orm/session_basics.html#is-the-session-a-cache

Change-Id: Ia5aae0c86a586041020e9798566c9e0af48c180d
Partial-Bug: #1578854

heat/engine/service.py

@@ -1879,6 +1879,11 @@ class EngineService(service.Service):
             # so sqlalchemy filters can't be used.
             res_type = filters.pop('type', None)
 
+        if depth > 0:
+            # populate context with resources from all nested depths
+            resource_objects.Resource.get_all_by_root_stack(
+                cnxt, stack.id, filters, cache=True)
+
         def filter_type(res_iter):
             for res in res_iter:
                 if res_type not in res.type():

heat/objects/resource.py

@@ -15,6 +15,8 @@
 
 """Resource object."""
 
+import collections
+
 from oslo_config import cfg
 from oslo_serialization import jsonutils
 from oslo_versionedobjects import base
@@ -42,6 +44,19 @@ def retry_on_conflict(func):
     return wrapper(func)
 
 
+class ResourceCache(object):
+
+    def __init__(self):
+        self.delete_all()
+
+    def delete_all(self):
+        self.by_stack_id_name = collections.defaultdict(dict)
+
+    def set_by_stack_id(self, resources):
+        for res in six.itervalues(resources):
+            self.by_stack_id_name[res.stack_id][res.name] = res
+
+
 class Resource(
     heat_base.HeatObject,
     base.VersionedObjectDictCompat,
@@ -136,6 +151,10 @@ class Resource(
 
     @classmethod
     def get_all_by_stack(cls, context, stack_id, filters=None):
+        cache = context.cache(ResourceCache)
+        resources = cache.by_stack_id_name.get(stack_id)
+        if resources:
+            return dict(resources)
         resources_db = db_api.resource_get_all_by_stack(context, stack_id,
                                                         filters)
         return cls._resources_to_dict(context, resources_db)
@@ -165,12 +184,15 @@ class Resource(
         return dict(resources)
 
     @classmethod
-    def get_all_by_root_stack(cls, context, stack_id, filters):
+    def get_all_by_root_stack(cls, context, stack_id, filters, cache=False):
         resources_db = db_api.resource_get_all_by_root_stack(
             context,
             stack_id,
             filters)
-        return cls._resources_to_dict(context, resources_db)
+        all = cls._resources_to_dict(context, resources_db)
+        if cache:
+            context.cache(ResourceCache).set_by_stack_id(all)
+        return all
 
     @classmethod
     def purge_deleted(cls, context, stack_id):