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, we must first define a number of Control Loop related concepts that we can use in all control loops.

Add a high level section explaining how the TOSCA file is formatted and give general help and instructions for a user.

A Control Loop Definition is made up of several components, those which represent applications, those which represent dynamic config schemas, and the actual node_templates which makes up the loop itself.

Applications can be a DCAE microservice, an operational policy, or any other application as long as it can be modeled, and the targeted ecosystem to has a participant client waiting for the event distributions from CLAMP via DMaaP Message Router.

Dynamic config on the other hand can be a monitoring policy, or any other resource that provides config to parts of the loop, can be updated after the run time phase has started and is supported by the components hosting the applications in the control loop.

A Control Loop Component that can be part of a control loop, it defines the components that partake in a control loop, and are implemented at run time by participants. The control loop component definition is truly dynamic and, as long as the participant that the control loop component definition relates to understands its definition, it can be anything. However, we have designed a base control loop component attribute that's generic and that can act as a good starting point.

The loop definition is explicit in the node_templates within the topology_template, a Control Loop node template is specified and any node template specified in the Control Loop node template is part of the control loop managed by CLAMP.

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 <INSERT LINK HERE WHEN IT IS MERGED> 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 <INSERT LINK HERE WHEN IT IS MERGED> 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 <INSERT LINK HERE WHEN IT IS MERGED> 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.

1.2.4 CDS Control Loop Element

The CDS Participant runs CDS Control Loop Elements. Each CDS Control Loop Element manages the deployment of the CDS blueprint specified in the CDS Control Loop Element definition. The Yaml file that holds the <INSERT LINK HERE WHEN IT IS MERGED> is available in Github and is the canonical definition of the CDS Control Loop Element type. For a description of the CDS Control Loop Element and CDS Participant, please see The CLAMP CDS Participant page.

1.2.5 DCAE Participant

The DCAE Participant runs DCAE Control Loop Elements. Each DCAE Control Loop Element manages a DCAE microservice on DCAE. The user defines the DCAE blueprint for the DCAE microservice as well as other properties that the microservice requires in order to execute. The Yaml file that holds the <INSERT LINK HERE WHEN IT IS MERGED> is available in Github and is the canonical definition of the DCAE Control Loop Element type. For a description of the DCAE Control Loop Element and DCAE Participant, please see The CLAMP DCAE 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 fir 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.

startPhase:
  type: integer
  required: false
  constraints:
  - greater-or-equal: 0
  description: 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
  metadata:
    common: true

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 <INSERT LINK HERE WHEN IT IS MERGED> yaml file.

# Definition that omits the common flag metadata
chart:
  type: org.onap.datatypes.policy.clamp.controlloop.kubernetesControlLoopElement.Chart
  typeVersion: 1.0.0
  description: The helm chart for the microservice
  required: true

# Definition that specifies the common flag metadata
chart:
  type: org.onap.datatypes.policy.clamp.controlloop.kubernetesControlLoopElement.Chart
  typeVersion: 1.0.0
  description: The helm chart for the microservice
  required: true
  metadata:
    common: false

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.

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

3.1 The Gentle Guidance Control Loop

The Gentle Guidance control loop is illustrated in the diagram below.

4. Creating Custom Control Loop Elements

Defining Control Loops in TOSCA for CLAMP