A Control Loop Type is defined in a TOSCA service template. A TOSCA Service Template has two parts: a definition part in the service template itself, which contains the definitions of concepts that can be used to define the types of concepts that can appear on a Toplogy Template and a Topology Template that defines a topology. See the Oasis Open TOSCA web page for more details on TOSCA.

Unsurprisingly, to define a Control Loop Type in TOSCA, of Control Loop related concepts that we can use in all control loops exist. They are described in Section 1. Section 2 describes how properties are managed. Properties are the configuration parameters that are provided to Control Loops and the Control Loop Elements they use. Section 3 describes how to define a Control Loop using the predefined Control Loop concepts.

1 Standard TOSCA Service Template Concepts for Control Loops

These concepts are the base concepts available to users who write definitions for control loops in TOSCA. TOSCA control loop definitions are written using these concepts.

1.1 Fundamental TOSCA Concepts for Control Loops

The following TOSCA concepts are the fundamental concepts in a TOSCA Service Template for defining control loops.

The TOSCA concepts above may be declared in the TOSCA Service Template of a control loop. If the concepts already exist in the Design Time Catalogue or the Runtime Inventory, they may be omitted from a TOSCA service template that defines a control loop type.

The start_phase is a value indicating the start phase in which this control loop element will be started, the first start phase is zero. Control Loop Elements are started in their start_phase order and stopped in reverse start phase order. Control Loop Elements with the same start phase are started and stopped simultaneously

The Yaml file that holds the  Definition of TOSCA fundamental Control Loop Types is available in Github and is the canonical definition of the Control Loop concepts.

1.2 TOSCA Concepts for Control Loop Elements delivered by ONAP

1.2.1 Policy Control Loop Element

The Policy Participant runs Policy Control Loop Elements. Each Policy Control Loop Element manages the deployment of the policy specified in the Policy Control Loop Element definition. The Yaml file that holds the  Policy Control Loop Element Type definition is available in Github and is the canonical definition of the Policy Control Loop Element type. For a description of the Policy Control Loop Element and Policy Participant, please see The CLAMP Policy Framework Participant page.

1.2.2 HTTP Control Loop Element

The HTTP Participant runs HTTP Control Loop Elements. Each HTTP Control Loop Element manages REST communication towards a REST endpoint using the REST calls a user has specified in the configuration of the HTTP Control Loop Element. The Yaml file that holds the  HTTP Control Loop Element Type definition is available in Github and is the canonical definition of the HTTP Control Loop Element type. For a description of the HTTP Control Loop Element and HTTP Participant, please see The CLAMP HTTP Participant page.

1.2.3 Kubernetes Control Loop Element

The Kubernetes Participant runs Kubernetes Control Loop Elements. Each Kubernetes Control Loop Element manages a Kubernetes microservice using Helm. The user defines the Helm chart for the Kubernetes microservice as well as other properties that the microservice requires in order to execute. The Yaml file that holds the Kubernetes Control Loop Element Type defintion is available in Github and is the canonical definition of the Kubernetes Control Loop Element type. For a description of the Kubernetes Control Loop Element and Kubernetes Participant, please see The CLAMP Kubernetes Participant page.

2 Common and Instance Specific Properties

Properties are used to define the configuration for Control Loops and Control Loop Elements. At design time, the types, constraints, and descriptions of the properties are specified. The values for properties are specified in the CLAMP GUI at runtime. TOSCA provides support for defining properties, see Section 3.6.10: TOSCA Property Definition in the TOSCA documentation.

2.1 Terminology for Properties

Property: Metadata defined in TOSCA that is associated with a Control Loop, a Control Loop Element, or a Participant.

TOSCA Property Type: The TOSCA definition of the type of a property. A property can have a generic type such as string or integer or can have a user defined TOSCA data type.

TOSCA Property Value: The value of a Property Type. Property values are assigned at run time in CLAMP.

Common Property Type: Property Types that apply to all instances of a Control Loop Type.

Common Property Value: The value of a Property Type. It is assigned at run time once for all instances of a Control Loop Type.

Instance Specific Property Type: Property Types that apply to an individual instance of a Control Loop Type.

Instance Specific Property Value: The value of a Property Type that applies to an individual instance of a Control Loop Type. The value is assigned at run time for each control loop instance.

Control Loop Properties can be common or instance specific. See Section 2 of TOSCA Defined Control Loops: Architecture and Design for a detailed description of the usage of common and instance specific properties.

2.2 Common Properties

Common properties apply to all instances of a control loop. Common properties are identified by a special metadata flag in Control Loop and Control Loop Element definitions. For example, the startPhase parameter on any Control Loop Element has the same value for any instance of that control loop element, so it is defined as shown below in the Definition of TOSCA fundamental Control Loop Types yaml file.

The "common: true" value in the metadata of the startPhase property identifies that property as being a common property. This property will be set on the CLAMP GUI during control loop commissioning.

2.3 Instance Specific Properties

Instance Specific  properties apply to individual instances of a Control Loop and/or Control Loop Element and must be set individually for Control Loop and Control Loop Element instance. Properties are instance specific by default, but can be identified by a special metadata flag in Control Loop and Control Loop Element definitions. For example, the chart parameter on a Kubernetes Control Loop Element has a different value for every instance of a Kubernetes Control Loop Element, so it can be defined as shown below in the  Kubernetes Control Loop Type definition yaml file.

The "common: false" value in the metadata of the chart property identifies that property as being an instance specific property. This property will be set on the CLAMP GUI during control loop instantiation.

3. Writing a Control Loop Type Definition

The TOSCA definition of a control loop contains a TOSCA Node Template for the control loop itself, which contains TOSCA Node Templates for each Control Loop Element that makes up the Control Loop.

To create a control loop, a user creates a TOSCA Topology Template. In the Topology Template, the user creates a TOSCA Node Template for each Control Loop Element that will be in the Control Loop Definition. Finally, the user creates the Node Template that defines the Control Loop itself, and references the Control Loop Element definitions that make up the Control Loop Definition.

3.1 The Gentle Guidance Control Loop

The best way to explain how to create a Control Loop Definition is by example.

The example Gentle Guidance control loop is illustrated in the diagram above. The domain logic for the control loop is implemented in a microservice running in Kubernetes, a policy, and some configuration that is passed to the microservice over a REST endpoint. We want to manage the life cycle of the domain logic for our Gentle Guidance control loop using our TOSCA based Control Loop Life Cycle Management approach. To do this we create four Control Loop Element definitions, one for the Kubernetes microservice, one for the policy and one or the REST configuration.

3.2 The TOSCA Control Loop Definition

We use a TOSCA Topology Template to specify a Control Loop definition and the definitions of its Control Loop Elements. Optionally, we can specify default parameter values in the TOSCA Topology Template. The actual values of Control Loop common and instance specific parameters are set at run time in the CLAMP GUI.

In the case of the Gentle Guidance control loop, we define a Control Loop Element Node Template for each part of the domain logic we are managing. We then define the Control Loop Node Template for the control loop itself.

The code fragment below shows the TOSCA Topology Template for the Gentle Guidance domain when no parameters are defined. Please refer to the Yaml file in Github for the definitive Yaml specification. 

The Gentle Guidance Control Loop Definition: No default parameter values

The code fragment below shows the TOSCA Topology Template for the Gentle Guidance domain when the default values of  parameters are defined. Please refer to the Yaml file in Github for the definitive Yaml specification.

The Gentle Guidance Control Loop Definition: With default parameter values

4. Creating Custom Control Loop Elements

Any organization can include their own component in the framework and use the framework and have the Policy Framework CLAMP manage the lifecycle of domain logic in their component as part of a Control Loop. To do this, a participant for the component must be developed that allows Control Loop Elements for that component to be run. To develop a participant, the participant must comply with the CLAMP Participants framework and in particular comply with The CLAMP Automation Composition Participant Protocol. The organization must also specify a new Control Loop Element type definition in TOSCA similar to those supplied in ONAP and described in Section 1.2. This Control Loop Element type tells the CLAMP Control Loop Lifecycle management that the Control Loop Element exists and can be included in control loops. It also specifies the properties that can be specified for the Control Loop Element.

An organization can supply the code for the Participant (for example as a Java jar file) and a TOSCA artifact with the Control Loop Element definition and it can be added to the platform. In future releases, support will be provided to include participants and their Control Loop Element definitions as packaged plugins that can be installed on the platform.