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+.. This work is licensed under a Creative Commons Attribution 4.0 International License.
+
+.. _clamp-controlloop_architecture-label:
+
+TOSCA Defined Control Loops: Architecture and Design
+####################################################
+
+
+.. contents::
+    :depth: 4
+
+The idea of using control loops to automatically (or autonomously) perform network management
+has been the subject of much research in the Network Management research community, see
+:download:`this paper <files/ControlLoops.pdf>` for some background. However, it is only with
+the advent of ONAP that we have a platform that supports control loops for network management.
+Before ONAP, Control Loops have been implemented by hard-coding components together and hard
+coding logic into components. ONAP has taken a step forward towards automatic implementation
+of Control Loops by allowing parameterization of Control Loops that work on the premise that
+the Control Loops use a set of analytic, policy, and control components connected together in
+set ways.
+
+The goal of the work is to extend and enhance the current ONAP Control Loop support to provide
+a complete open-source framework for Control Loops. This will enhance the current support to
+provide TOSCA based Control Loop definition and development, commissioning and run-time management.
+The participants that comprise a Control Loop and the metadata needed to link the participants
+together to create a Control Loop are specified in a standardized way using the `OASIS TOSCA
+modelling language <http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/>`_. The TOSCA
+description is then used to commission, instantiate, and manage the Control Loops in the run
+time system.
+
+.. image:: images/01-controlloop-overview.png
+
+1 Terminology
+=============
+
+This section describes the terminology used in the system.
+
+1.1 Control Loop Terminology
+----------------------------
+
+**Control Loop Type:** A definition of a Control Loop in the TOSCA language. This definition describes
+a certain type of a control loop. The life cycle of instances of a Control Loop Type are managed
+by CLAMP.
+
+**Control Loop Instance:** An instance of a Control Loop Type. The life cycle of a Control Loop
+Instance is managed by CLAMP. A Control Loop Instance is a set of executing elements on which
+Life Cycle Management (LCM) is executed collectively. For example, a set of microservices may be
+spawned and executed together to deliver a service. This collection of services is a control loop.
+
+**Control Loop Element Type:** A definition of a Control Loop Element in the TOSCA language. This
+definition describes a certain type of Control Loop Element for a control loop in a Control
+Loop Type.
+
+**Control Loop Element Instance:** A single entity executing on a participant, with its Life Cycle
+being managed as part of the overall control loop. For example, a single microservice that is
+executing as one microservice in a service.
+
+**CLAMP Control Loop Runtime:** The CLAMP server that holds Control Loop Type definitions and manages
+the life cycle of Control Loop Instances and their Control Loop Elements in cooperation with
+participants.
+
+
+1.2 Participant Terminology
+---------------------------
+
+**Participant Type:** Definition of a type of system or framework that can take part in control
+loops and a definition of the capabilities of that participant type. A participant advertises
+its type to the CLAMP Control Loop Runtime.
+
+**Participant:** A system or framework that takes part in control loops by executing Control Loop
+Elements in cooperation with the CLAMP Control Loop Runtime. A participant chooses to partake
+in control loops, to manage Control Loop Elements for CLAMP, and to receive, send and act on
+LCM messages for the CLAMP runtime.
+
+1.3 Terminology for Properties
+------------------------------
+
+**Common Properties:** Properties that apply to all Control Loop Instances of a certain Control
+Loop Type and are specified when a Control Loop Type is commissioned.
+
+**Instance Specific Properties:** Properties that must be specified for each Control Loop Instance
+and are specified when a Control Loop Instance is Initialized.
+
+1.4 Concepts and their relationships
+------------------------------------
+
+The UML diagram below shows the concepts described in the terminology sections above and how
+they are interrelated.
+
+.. image:: images/02-controlloop-concepts.png
+
+The Control Loop Definition concepts describe the types of things that are in the system. These
+concepts are defined at design time and are passed to the runtime in a TOSCA document. The
+concepts in the Control Loop Runtime are created by the runtime part of the system using the
+definitions created at design time.
+
+.. _controlloop-capabilities:
+
+2 Capabilities
+==============
+
+We consider the capabilities of Control Loops at Design Time and Run Time.
+
+At Design Time, three capabilities are supported:
+
+#. **Control Loop Element Definition Specification.** This capability allows users to define Control
+   Loop Element Types and the metadata that can be used on and configured on a Control Loop Element
+   Type. Users also define the Participant Type that will run the Control Loop Element when it is
+   taking part in in a control loop. The post condition of an execution of this capability is that
+   metadata for a Control Loop Element Type is defined in the Control Loop Design Time Catalogue.
+
+#. **Control Loop Element Definition Onboarding.** This capability allows external users and systems
+   (such as SDC or DCAE-MOD) to define the metadata that can be used on and configured on a Control
+   Loop Element Type and to define the Participant Type that will run the Control Loop Element when
+   it is taking part in in a control loop. The post condition of an execution of this capability
+   is that metadata for a Control Loop Element Type is defined in the Control Loop Design Time
+   Catalogue.
+
+#. **Control Loop Type Definition.** This capability allows users and other systems to create Control
+   Loop Type definitions by specifying a set of Control Loop Element Definitions from those that
+   are available in the Control Loop Design Time Catalogue. These Control Loop Elements will
+   work together to form Control Loops. In an execution of this capability, a user specifies the
+   metadata for the Control Loop and specifies the set of Control Loop Elements and their Participant
+   Types. The user also selects the correct metadata sets for each participant in the Control Loop
+   Type and defines the overall Control Loop Type metadata. The user also specifies the Common
+   Property Types that apply to all instances of a control loop type and the Instance Specific
+   Property Types that apply to individual instances of a Control Loop Type. The post condition for
+   an execution of this capability is a Control Loop definition in TOSCA stored in the Control Loop
+   Design Time Catalogue.
+
+.. note::
+    Once a Control Loop Definition is commissioned to the Control Loop Runtime and has been
+    stored in the Run Time Inventory, it cannot be further edited unless it is decommissioned.
+
+
+At Run Time, the following participant related capabilities are supported:
+
+#. **System Pre-Configuration.** This capability allows participants to register and deregister
+   with CLAMP. Participants explicitly register with CLAMP when they start. Control Loop Priming
+   is performed on each participant once it registers. The post condition for an execution of this
+   capability is that a participant becomes available (registration) or is no longer available
+   (deregistration) for participation in a control loop.
+
+#. **Control Loop Priming on Participants.** A participant is primed to support a Control Loop Type.
+   Priming a participant means that the definition of a control loop and the values of Common
+   Property Types that apply to all instances of a control loop type on a participant are sent
+   to a participant. The participant can then take whatever actions it need to do to support
+   the control loop type in question. Control Loop Priming takes place at participant
+   registration and at Control Loop Commissioning. The post condition for an execution of this
+   capability is that all participants in this control loop type are commissioned, that is they
+   are prepared to run instances of their Control Loop Element types.
+
+
+At Run Time, the following Control Loop Life Cycle management capabilities are supported:
+
+#. **Control Loop Commissioning:** This capability allows version controlled Control Loop Type
+   definitions to be taken from the Control Loop Design Time Catalogue and be placed in the
+   Commissioned Control Loop Inventory. It also allows the values of Common Property Types
+   that apply to all instances of a Control Loop Type to be set. Further, the Control Loop
+   Type is primed on all concerned participants. The post condition for an execution of this
+   capability is that the Control Loop Type definition is in the Commissioned Control Loop
+   Inventory and the Control Loop Type is primed on concerned participants.
+
+#. **Control Loop Instance Life Cycle Management:** This capability allows a Control Loop
+   Instance to have its life cycle managed.
+
+   #. **Control Loop Instance Creation:** This capability allows a Control Loop Instance to be
+      created. The Control Loop Type definition is read from the Commissioned Control Loop
+      Inventory and values are assigned to the Instance Specific Property Types defined for
+      instances of the Control Loop Type in the same manner as the existing CLAMP client does.
+      A Control Loop Instance that has been created but has not yet been instantiated on
+      participants is in state UNINITIALIZED. In this state, the Instance Specific Property Type
+      values can be revised and updated as often as the user requires. The post condition for an
+      execution of this capability is that the Control Loop instance is created in the
+      Instantiated Control Loop Inventory but has not been instantiated on Participants.
+
+   #. **Control Loop Instance Update on Participants:** Once the user is happy with the property
+      values, the Control Loop Instance is updated on participants and the Control Loop Elements
+      for this Control Loop Instance are initialized or updated by participants using the control
+      loop metadata. The post condition for an execution of this capability is that the Control
+      Loop instance is updated on Participants.
+
+   #. **Control Loop State Change:** The user can now order the participants to change the state
+      of the Control Loop Instance. If the Control Loop is set to state RUNNING, each participant
+      begins accepting and processing control loop events and the Control Loop Instance is set
+      to state RUNNING in the Instantiated Control Loop inventory. The post condition for an
+      execution of this capability is that the Control Loop instance state is changed on
+      participants.
+
+   #. **Control Loop Instance Monitoring:** This capability allows Control Loop Instances to be
+      monitored. Users can check the status of Participants, Control Loop Instances, and Control
+      Loop Elements. Participants report their overall status and the status of Control Loop
+      Elements they are running periodically to CLAMP. Clamp aggregates these status reports
+      into an aggregated Control Loop Instance status record, which is available for monitoring.
+      The post condition for an execution of this capability is that Control Loop Instances are
+      being monitored.
+
+   #. **Control Loop Instance Supervision:** This capability allows Control Loop Instances to be
+      supervised. The CLAMP runtime expects participants to report on Control Loop Elements
+      periodically. The CLAMP runtime checks that periodic reports are received and that each
+      Control Loop Element is in the state it should be in. If reports are missed or if a
+      Control Loop Element is in an incorrect state, remedial action is taken and notifications
+      are issued. The post condition for an execution of this capability is that Control Loop
+      Instances are being supervised by the CLAMP runtime.
+
+   #. **Control Loop Instance Removal from Participants:** A user can order the removal of a Control
+      Loop Instance from participants. The post condition for an execution of this capability is
+      that the Control Loop instance is removed from Participants.
+
+   #. **Control Loop Instance Deletion:** A user can order the removal of a Control Loop Instance
+      from the CLAMP runtime. Control Loop Instances that are instantiated on participants cannot
+      be removed from the CLAMP runtime. The post condition for an execution of this capability
+      is that the Control Loop instance is removed from Instantiated Control Loop Inventory.
+
+#. **Control Loop Decommissioning:** This capability allows version controlled Control Loop Type
+   definitions to be removed from the Commissioned Control Loop Inventory. A Control Loop
+   Definition that has instances in the Instantiated Control Loop Inventory cannot be removed.
+   The post condition for an execution of this capability is that the Control Loop Type
+   definition removed from the Commissioned Control Loop Inventory.
+
+.. note::
+    The system dialogues for run time capabilities are described in detail on the
+    :ref:`System Level Dialogues <system-level-label>` page.
+
+.. _controlloop-instance-states:
+
+2.1 Control Loop Instance States
+--------------------------------
+
+When a control loop definition has been commissioned, instances of the control loop can be
+created, updated, and deleted. The system manages the lifecycle of control loops and control
+loop elements following the state transition diagram below.
+
+.. image:: images/03-controlloop-instance-states.png
+
+3 Overall Target Architecture
+=============================
+
+The diagram below shows an overview of the architecture of TOSCA based Control Loop
+Management in CLAMP.
+
+.. image:: images/04-overview.png
+
+Following the ONAP Reference Architecture, the architecture has a Design Time part and
+a Runtime part.
+
+The Design Time part of the architecture allows a user to specify metadata for participants.
+It also allows users to compose control loops. The Design Time Catalogue contains the metadata
+primitives and control loop definition primitives for composition of control loops. As shown
+in the figure above, the Design Time component provides a system where Control Loops can be
+designed and defined in metadata. This means that a Control Loop can have any arbitrary
+structure and the Control Loop developers can use whatever analytic, policy, or control
+participants they like to implement their Control Loop. At composition time, the user
+parameterises the Control Loop and stores it in the design time catalogue. This catalogue
+contains the primitive metadata for any participants that can be used to compose a Control
+Loop. A Control Loop SDK is used to compose a Control Loop by aggregating the metadata for
+the participants chosen to be used in a Control Loop and by constructing the references between
+the participants. The architecture of the Control Loop Design Time part will be elaborated in
+future releases.
+
+Composed Control Loops are commissioned on the run time part of the system, where they are
+stored in the Commissioned Control Loop inventory and are available for instantiation. The
+Commissioning component provides a CRUD REST interface for Control Loop Types, and implements
+CRUD of Control Loop Types. Commissioning also implements validation and persistence of incoming
+Control Loop Types. It also guarantees the integrity of updates and deletions of Control Loop
+Types, such as performing updates in accordance with semantic versioning rules and ensuring that
+deletions are not allowed on Control Loop Types that have instances defined.
+
+The Instantiation component manages the Life Cycle Management of Control Loop Instances and
+their Control Loop Elements. It publishes a REST interface that is used to create Control Loop
+Instances and set values for Common and Instance Specific properties. This REST interface is
+public and is used by the CLAMP GUI. It may also be used by any other client via the public
+REST interface. the REST interface also allows the state of Control Loop Instances to be changed.
+A user can change the state of Control Loop Instances as described in the state transition
+diagram shown in section 2 above. The Instantiation component issues update and state change
+messages via DMaaP to participants so that they can update and manage the state of the Control
+Loop Elements they are responsible for. The Instantiation component also implements persistence
+of Control Loop Instances, control loop elements, and their state changes.
+
+The Monitoring component reads updates sent by participants. Participants report on the
+state of their Control Loop Elements periodically and in response to a message they have
+received from the Instantiation component. The Monitoring component reads the contents of
+the participant messages and persists their state updates and statistics records. It also
+publishes a REST interface that publishes the current state of all Participants, Control
+Loop Instances and their Control Loop Elements, as well as publishing Participant and
+Control Loop statistics.
+
+The Supervision component is responsible for checking that Control Loop Instances are correctly
+instantiated and are in the correct state (UNINITIALIZED/READY/RUNNING). It also handles
+timeouts and on state changes to Control Loop Instances, and retries and rolls back state
+changes where state changes failed.
+
+A Participant is an executing component that partakes in control loops. More explicitly, a
+Participant is something that implements the Participant Instantiation and Participant
+Monitoring messaging protocol over DMaaP for Life Cycle management of Control Loop Elements.
+A Participant runs Control Loop Elements and manages and reports on their life cycle
+following the instructions it gets from the CLAMP runtime in messages delivered over DMaaP.
+
+In the figure above, five participants are shown. A Configuration Persistence Participant
+manages Control Loop Elements that interact with the `ONAP Configuration Persistence Service
+<https://docs.onap.org/projects/onap-cps/en/latest/overview.html>`_
+to store common data. The DCAE Participant runs Control Loop Elements that manage DCAE
+microservices. The Kubernetes Participant hosts the Control Loop Elements that are managing
+the life cycle of microservices in control loops that are in a Kubernetes ecosystem. The
+Policy Participant handles the Control Loop Elements that interact with the Policy Framework
+to manage policies for control loops. A Controller Participant such as the CDS Participant
+runs Control Loop Elements that load metadata and configure controllers so that they can
+partake in control loops. Any third party Existing System Participant can be developed to
+run Control Loop Elements that interact with any existing system (such as an operator's
+analytic, machine learning, or artificial intelligence system) so that those systems can
+partake in control loops.
+
+4. Other Considerations
+=======================
+
+.. _management-cl-instance-configs:
+
+4.1 Management of Control Loop Instance Configurations
+------------------------------------------------------
+
+In order to keep management of versions of the configuration of control loop instances
+straightforward and easy to implement, the following version management scheme using
+semantic versioning is implemented. Each configuration of a Control Loop Instance and
+configuration of a Control Loop Element has a semantic version with 3 digits indicating
+the **major.minor.patch** number of the version.
+
+.. note::
+    A **configuration** means a full set of parameter values for a Control Loop Instance.
+
+.. image:: images/05-upgrade-states.png
+
+Change constraints:
+
+#. A Control Loop or Control Loop Element in state **RUNNING** can be changed to a higher patch
+   level or rolled back to a lower patch level. This means that hot changes that do not
+   impact the structure of a Control Loop or its elements can be executed.
+
+#. A Control Loop or Control Loop Element in state **PASSIVE** can be changed to a higher
+   minor/patch level or rolled back to a lower minor/patch level. This means that structural
+   changes to Control Loop Elements that do not impact the Control Loop as a whole can be
+   executed by taking the control loop to state **PASSIVE**.
+
+#. A Control Loop or Control Loop Element in state **UNINITIALIZED** can be changed to a higher
+   major/minor/patch level or rolled back to a lower major/minor/patch level. This means
+   that where the structure of the entire control loop is changed, the control loop must
+   be uninitialized and reinitialized.
+
+#. If a Control Loop Element has a **minor** version change, then its Control Loop Instance
+   must have at least a **minor** version change.
+
+#. If a Control Loop Element has a **major** version change, then its Control Loop Instance
+   must have a **major** version change.
+
+4.2 Scalability
+---------------
+
+The system is designed to be inherently scalable. The CLAMP runtime is stateless, all state
+is preserved in the Instantiated Control Loop inventory in the database. When the user
+requests an operation such as an instantiation, activation, passivation, or an uninitialization
+on a Control Loop Instance, the CLAMP runtime broadcasts the request to participants over
+DMaaP and saves details of the request to the database. The CLAMP runtime does not directly
+wait for responses to requests.
+
+When a request is broadcast on DMaaP, the request is asynchronously picked up by participants
+of the types required for the Control Loop Instance and those participants manage the life
+cycle of its control loop elements. Periodically, each participant reports back on the status
+of operations it has picked up for the Control Loop Elements it controls, together with
+statistics on the Control Loop Elements over DMaaP. On reception of these participant messages,
+the CLAMP runtime stores this information to its database.
+
+The participant to use on a control loop can be selected from the registered participants
+in either of two ways:
+
+**Runtime-side Selection:** The CLAMP runtime selects a suitable participant from the list of
+participants and sends the participant ID that should be used in the Participant Update message.
+In this case, the CLAMP runtime decides on which participant will run the Control Loop Element
+based on a suitable algorithm. Algorithms could be round robin based or load based.
+
+**Participant-side Selection:** The CLAMP runtime sends a list of Participant IDs that may be used
+in the Participant Update message. In this case, the candidate participants decide among
+themselves which participant should host the Control Loop Element.
+
+This approach makes it easy to scale Control Loop life cycle management. As Control Loop
+Instance counts increase, more than one CLAMP runtime can be deployed and REST/supervision
+operations on Control Loop Instances can run in parallel. The number of participants can
+scale because an asynchronous broadcast mechanism is used for runtime-participant communication
+and there is no direct connection or communication channel between participants and CLAMP
+runtime servers. Participant state, Control Loop Instance state, and Control Loop Element
+state is held in the database, so any CLAMP runtime server can handle operations for any
+participant. Because many participants of a particular type can be deployed and participant
+instances can load balance control loop element instances for different Control Loop Instances
+of many types across themselves using a mechanism such as a Kubernetes cluster.
+
+
+4.3 Sandboxing and API Gateway Support
+--------------------------------------
+
+At runtime, interaction between ONAP platform services and application microservices are
+relatively unconstrained, so interactions between Control Loop Elements for a given Control
+Loop Instance remain relatively unconstrained. A
+`proposal to support access-controlled access to and between ONAP services
+<https://wiki.onap.org/pages/viewpage.action?pageId=103417456>`_
+will improve this. This can be complemented by intercepting and controlling services
+accesses between Control Loop Elements for Control Loop Instances for some/all Control
+Loop types.
+
+API gateways such as `Kong <https://konghq.com/kong/>`_ have emerged as a useful technology
+for exposing and controlling service endpoint access for applications and services. When a
+Control Loop Type is onboarded, or when Control Loop Instances are created in the Participants,
+CLAMP can configure service endpoints between Control Loop Elements to redirect through an
+API Gateway.
+
+Authentication and access-control rules can then be dynamically configured at the API gateway
+to support constrained access between Control Loop Elements and Control Loop Instances.
+
+The diagram below shows the approach for configuring API Gateway access at Control Loop
+Instance and Control Loop Element level.
+
+.. image:: images/06-api-gateway-sandbox.png
+
+At design time, the Control Loop type definition specifies the type of API gateway configuration
+that should be supported at Control Loop and Control Loop Element levels.
+
+At runtime, the CLAMP can configure the API gateway to enable (or deny) interactions between
+Control Loop Instances and individually for each Control Loop Element. All service-level
+interactions in/out of a Control Loop Element, except that to/from the API Gateway, can be
+blocked by networking policies, thus sandboxing a Control Loop Element and an entire Control
+Loop Instance if desired. Therefore, a Control Loop Element will only have access to the APIs
+that are configured and enabled for the Control Loop Element/Instance in the API gateway.
+
+For some Control Loop Element Types the Participant can assist with service endpoint
+reconfiguration, service request/response redirection to/from the API Gateway, or
+annotation of requests/responses.
+
+Once the Control Loop instance is instantiated on participants, the participants configure
+the API gateway with the Control Loop Instance level configuration and with the specific
+configuration for their Control Loop Element.
+
+Monitoring and logging of the use of the API gateway may also be provided. Information and
+statistics on API gateway use can be read from the API gateway and passed back in monitoring
+messages to the CLAMP runtime.
+
+Additional isolation and execution-environment sandboxing can be supported depending on the
+Control Loop Element Type. For example: ONAP policies for given Control Loop Instances/Types
+can be executed in a dedicated PDP engine instances; DCAE or K8S-hosted services can executed
+in isolated namespaces or in dedicated workers/clusters; etc..
+
+
+5 APIs and Protocols
+====================
+
+The APIs and Protocols used by CLAMP for Control Loops are described on the pages below:
+
+#. :ref:`System Level Dialogues <system-level-label>`
+#. :ref:`The CLAMP Control Loop Participant Protocol <controlloop-participant-protocol-label>`
+#. :ref:`REST APIs for CLAMP Control Loops <controlloop-rest-apis-label>`
+
+
+6 Design and Implementation
+===========================
+
+The design and implementation of TOSCA Control Loops in CLAMP is described for each executable entity on the pages below:
+
+#. :ref:`The CLAMP Control Loop Runtime Server <clamp-controlloop-runtime>`
+#. :ref:`CLAMP Control Loop Participants <clamp-controlloop-participants>`
+#. :ref:`Managing Control Loops using The CLAMP GUI <clamp-gui-controlloop>`
+
+End of Document