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 addressed with their resource 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 applications need a resource group type that can change size and with the added ability of removal or addition of specific resources.

Release Note

TBD

Rationale

Some cloud applications need a new resource group with the following features:
1) Create an initial number of identically configured nested resources.
2) Address single resource 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) Scale out(in) by adjusting 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) Add new resources to group when certain criteria met(ex. increase in workload).
3) Remove a number of resources according to a predefined criteria(increasing or decreasing order of age or index).
4) Specify certain resources for removal.

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%”.

   initial_size: Integer
   Initial number of resources in the group at creation time.
   Optional property, defaults to 1.

   max_size: Integer
   Maximum number of resources in the group.
   Optional Property, defaults to 1.
   Updates can cause creation/deletion of some resources according to scale_in_criteria/scale_in_order.

   min_size: Integer
   Minimum number of resources in the group.
   Optional property, defaults to 1.
   Updates can cause addition/removal of resources according to scale_in_criteria/scale_in_order.

  scale_in_criteria:String
  Specifies criteria for removal of resources form the group.
  Optional property, defaults to "Index".
  Possible Values: "Index" or "Age"
  Default: "Index"

  scale_in_order:String
  Specifies order for removal of resources form the group according to the specified criteria.
  Optional property, defaults to "Decreasing". 
  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.