Spec Lite: Lazy update stores information

Change-Id: I2c5a227fc15e6ac958ea700a176ac719b977e2b7

specs/train/approved/glance/spec-lite-lazy-update-stores-info.rst

@@ -0,0 +1,53 @@
+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=========================================
+Spec Lite: Lazy update stores information
+=========================================
+
+:project: glance
+
+:problem: In Rocky multiple backend support is added as experimental feature.
+          The location object will have the 'store_id' (Store name) stored as
+          a metadata attribute which helps to identify in which store the
+          image is located. Glance API service has information about all
+          configured stores in a global map.
+
+          After enabling/upgrading glance-api to use multiple stores feature,
+          existing images will not have a stores information associated with
+          them. As a result if existing image is requested for download then
+          it will search that image in all configured stores which will cause
+          a performance overhead.
+
+          All configured stores are stored in memory, if operator change the
+          store name in configuration file and restart the glance-api service
+          there is no way to update the new name to existing images which has
+          store information associated with them.
+
+:solution: Add a decorator to get image call which will retrieve the location
+           object from image and then from location URL it will fetch the store
+           information which is stored in the location map. Once location url
+           is matched with the url from the global map, it will check if
+           location metadata has existing store associated with it and then
+           store name will be added/updated to the location metadata.
+
+           If user/operator uses GET call to show a single image or list
+           all the images the particular decorator will update the store
+           information to single or all images available depending upon
+           the request. There will be slight impact on performance on list
+           call as it will revisit the location call for each of the image
+           in the list.
+
+
+:alternatives: None, carry on using current mechanism.
+
+impacts: DocImpact
+
+:timeline: Include in Train release.
+
+:link: None
+
+:assignee: abhishekk

specs/train/approved/index.rst

@@ -6,5 +6,11 @@ Train Approved Specifications
    :glob:
    :maxdepth: 1
 
-TODO: fill this in until a new approved space is added.
+Train approved specs for Glance:
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+
+   glance/*