Heat/Blueprints/alarm-triggers-update


 * Launchpad Entry: HeatSpec:update-on-alarm-action
 * Created: 08 Feb 2013
 * Contributors:

Summary
To make templates more powerful (like what the loadbalancer resource does internally) we need to automatically trigger a stackupdate after a cloudwatch alarm action (one that causes changes to the stack).

The idea is that anything that might cause a change in a Ref or GetAttr will cause an update and therefore you could run hook scripts on your instance.

This is similar to juju signals (I believe).

Rationale
Give the user power, har har har (Tim Allen anyone?)

User stories
Implement an HA multi-inst stack.
 * db on one instance
 * webserver on another
 * if the db-inst gets restarted, the webserver needs to know about it (reload it's config)

this is a case of using the OS::Heat::Restarter resource

Implement your own loadbalancer
 * autoscaling group
 * cloudwatch to step up/down the number of instances
 * the loadbalancer needs to reload it's config with the new list of servers

this is a case of using the autoscaling grow action.

Design

 * 1) After any non-user action that might cause a change the parsed-stack we call update stack.
 * 2) update stack will post new metadata that the instances can pull and run their hook scripts

UI Changes
No changes needed to the template.

Migration
None

Test/Demo Plan
This need not be added or completed until the specification is nearing beta.

BoF agenda and discussion
Use this section to take notes during the BoF; if you keep it in the approved spec, use it for summarizing what was discussed and note any options that were rejected.