Add Support for Resource Chains

Problem description

The resource registry enables template extension and customization through the ability to map a particular template/implementation to a resource type name. TripleO uses this pattern extensively to provide integration points at various steps in the stack deployment that can be used to add 3rd party integrations, such as service and driver configuration.

The problem is, only one template can be specified for each of these hooks. This spec is attempting to alleviate that by introducing a new type that will aggregate one or more templates into a nested stack.

Proposed change

The proposed change is to introduce a ResourceChain type, similar in behavior to the existing ResourceGroup. For example:

    type: OS::Heat::ResourceChain
      resources: <list>  # resource types to add to the chain's nested stack
      concurrent: <boolean>  # optional; default is false
        # properties to pass each template in the chain

Internally, Heat will create a nested stack comprised of each template specified in the chain’s configuration.

By default, each resource will be treated as having a dependency on the resource before it in the list. If the concurrent property is set to true, no dependencies between the created resources will be established.

A failure in a resource creation will abort the creation of the remaining resources in the chain (and obviously flag the chain resource as failed). This will allow users to control the order in which the resources in the chain will be created. If it is set to false, the resources in the chain will be created concurrently.

The resource_properties parameter is similar to the properties section of the resource_def used in a resource group and will be passed to each of the resources in the chain’s stack.

Resources in the chain may be accessed by index, where the index corresponds to the template’s location in the resources property.

The primary drawback is that it moves the template selection from the resource registry out to the parameters section. There is the potential to confuse users attempting to extend a template if some of the substitutions are done in the registry while others are done through parameters. This will also complicate the capabilities detection since we’ll have to take into account templates specified not only in the registry, but through a parameter as well.

To apply this to the TripleO use cases, below is an example of how resource chains may be used to configure which resources are created at a particular step in the deployment:

  type: json
    step1: [controller/loadbalancer.yaml]
    step2: [controller/db.yaml, controller/rabbit.yaml]
    step3: [controller/keystone.yaml, controller/glance-api.yaml ...]


  type: OS::Heat::ResourceChain
      resources: {get_param: [ControllerDeploymentSteps, step1]}
        servers: {get_param: servers}

  type: OS::Heat::ResourceChain
      resources: {get_param: [ControllerDeploymentSteps, step2]}
        servers: {get_param: servers}





Primary assignee:


Target Milestone for completion:

Work Items

  • Add a resource plugin for ResourceChain
  • Add the corresponding functional tests