Heat/Blueprints/ScalableResourceGroup

< Heat Launchpad Entry: HeatSpec:ScalableResourceGroup
Created: 26 July 2015
Contributors: Ahmed El-Khouly

Summary

OS::Heat::ResourceGroup provides a way to create several instances of identically configured nested resource; resources within the group can be accessed with their index.
OS::Heat::ResourceGroup can only scale by changing the count property followed by a stack update of the root stack.
OS::Heat::AutoScalingGroup/Policy also provide a way to create several instances identically configured nested resources with the added ability of changing size(scale) when the accompanying AutoScalingPolicy receives a signal. Auto-scaling policy defines characteristics of the scale operation (size of adjustment, size of adjustment, cool down period).

Some cloud application needs a resource group with the following features:
(1) Dynamically change group members by specifying a resource(resource index) to be added(removed) to(from) the group.
(2) Dynamically scale up or down.

These features can be realized by doing nested stack update (without the need for root stack update).

Release Note

TBD

Rationale

Some cloud applications need a new resource group with the following characteristics:
1) Create an initial number of identically configured nested resources.
2) Identify resources within the group using the index with the ability to specify a variable to be replaced by the index.
3) Add/Remove specific resource within the group by specifying the indices of involved resources.
4) Grow/shrink the group size (with a certain increment) when user defined triggers are met.

User Stories

1) Recreate a (failed) resource by adding/deleting the corresponding nested resource in the group.
2) Create new resources in increments when certain triggers are met.
3) Remove a number resources according to a predefined criteria(increasing or decreasing order of age or index).
4) Specify which resources to remove due to low load periods.

Assumptions

None.

Design

Define a new heat resource type: OS::HEAT::ScalableResourceGroup with the following properties:

   resource_def : Map
   Resource definition for the resources in the group. 
   Required property.
   Can be updated without replacement.

   index_var : String
   A variable that this resource will use to replace with the current index of a given resource in the group. 
   Updates cause replacement.
   Optional property, defaults to “%index%”.

   MAX_SIZE: Integer
   Maximum number of resources in the group.
   Optional Property
   Updates can cause creation/deletion of some resources according to de-grow policy
   INIT_SIZE: Integer
   Initial number of resources in the group at creation time.
   Optional property.
   Default value is 1.

   MIN_SIZE: Integer
   Min number of resources in the group.
   Optional property.
   Updates can cause creation/deletion of some resources according to de-grow policy

  DEGROW_Policy:String
  Controls which resources to remove when during de-grow.
  Optional property.
  Possible Values: "Index" or "Age"
  Default: "Index"

  DEGROW_Order:String
  Optional property.
  Possible Values: "Increasing" or "Decreasing"
  Default: "Decreasing"

ScalableResourceGroup class inherits from ResourceGroup:

   - Provide handle_signal implementation to receive signals to add/delete and grow/degrow signals.
   - Override create/validate/update/delete handlers.