Refactors GCE backend config updates.

appengine/gce-backend/README.md

 # GCE Backend
 
 A GCE backend for Machine Provider.
 
 GCE Backend is composed of a series of cron jobs. Each cron runs independently
 of the others and performs idempotent operations. Many of the jobs are only
 eventually consistent, converging on the desired state over multiple calls.
 
 
 ## Setting up
 
 *   Visit http://console.cloud.google.com and create a project. Replace
     `<appid>` below with your project id.
 *   Visit Google Cloud Console, IAM & Admin, click Add Member and add someone
     else so you can safely be hit by a bus.
 *   Upload the code with: `./tools/gae upl -x -A <appid>`
 *   Enable Pub/Sub and Compute Engine.
 *   In Pub/Sub, create a topic with the same name as
     pubsub.get\_machine\_provider\_topic(), and a pull subscription with the
     same name as pubsub.get\_machine\_provider\_subscription(). On the topic,
     authorize the Machine Provider's default service account as a publisher,
     e.g. machine-provider@appspot.gserviceaccount.com.
-*   Store the following three entities in the datastore:
-    *   config.Configuration: Most of this entity will be updated by the app,
-        but the config\_set field needs to be set to the name of the config set
-        to fetch from the config service.
-    *   components.config.common.ConfigSettings: Set service\_hostname to the
-        address of the config service, e.g. luci-config.appspot.com. All others
-        fields are unused.
+*   Use the ConfigApi to set the ConfigSettings datastore entity to the
+    address of the config service, e.g. luci-config.appspot.com.
+*   Store the following entity in the datastore:
     *   components.machine\_provider.utils.MachineProviderConfiguration: Set
         instance\_url to the URL for the Machine Provider service, e.g.
         https://machine-provider.appspot.com.
 *   TODO(smut): Simplify the above.
 
 
 TODO(smut): Move the following into doc/ as applicable.
 
 
 ## Config
 
 The configuration defines the desired instance templates, and the desired
 instance group managers created from those templates. Configured in the config
 service as templates.cfg and managers.cfg respectively.
 
 
 ## import-config
 
 Responsible for importing the current GCE configuration from the config service.
 When a new config is detected, the local copy of the config in the datastore is
-updated.
+updated. This task ensures that the valid manager and template configurations
+are updated synchronously.
 
 
 ## process-config
 
 Responsible for creating an entity structure corresponding to the config.
 
 A models.InstanceTemplate is created for each template defined in templates.cfg
 which has a globally unique base\_name. Any reuse of base\_name, even when the
 configured project is different, is considered to be another revision of the
 same instance template. A revision of an instance template is one set of fields
@@ skipping 89 matching lines @@
 
 ## delete-instance-templates
 
 Deletes GCE instance templates that aren't found in the config and have no
 instance group managers configured to use them.
 
 
 ## cleanup-entities
 
 Deletes entities that are no longer needed.