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


OS::Heat::ResourceGroup provides a way to create several instances of identically configured nested resource; resources within the group are identifiable with with their index.
OS::Heat::ResourceGroup can only change number of resources in the group by changing the count property in the Heat template and perform a stack update of on the root stack.
OS::Heat::AutoScalingGroup/Policy provides for a resource group that can change in size(scale) by receiving signals controlling the size change.
There are some cloud applications that need a scalable resource group with the added ability to specify the exact resources to be add or removed from the group.

Release Note



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.




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