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+Overview
+=====================================================================
+
+:abstract: This document describes a set of new optional
+   capabilities where the OpenStack Cloud messages into the Guest
+   VMs in order to provide improved Availability of the hosted VMs.
+   The initial set of new capabilities include: enabling the
+   detection of and recovery from internal VM faults and providing
+   a simple out-of-band messaging service to prevent scenarios such
+   as split brain.
+
+
+.. sectnum::
+
+.. contents:: Table of Contents
+
+
+
+Introduction
+=====================================================================
+
+   This document provides an overview and rationale for a
+   set of new capabilities where the OpenStack Cloud messages
+   into the Guest VMs in order to provide improved Availability
+   of the hosted VMs.
+
+   The initial set of new capabilities specifically include:
+
+        - VM Heartbeating and Health Checking
+        - VM Peer State Notification and Messaging
+
+   All of these capabilities leverage Host-to-Guest Messaging
+   Interfaces / APIs which are built on a messaging service between the
+   OpenStack Host and the Guest VM that uses a simple low-bandwidth
+   datagram messaging capability in the hypervisor and therefore has no
+   requirements on OpenStack Networking, and is available very early
+   after spawning the VM.
+
+   For each capability, the document outlines the interaction with
+   the Guest VM, any key technologies involved, the integration into
+   the larger OpenStack and OPNFV Architectures (e.g. interactions
+   with VNFM), specific OPNFV HA Team deliverables, and the use cases
+   for how availability of the hosted VM is improved.
+
+
+
+
+Messaging Layer
+========================================================================
+
+   The Host-to-Guest messaging APIs used by the services discussed
+   in this document use a JSON-formatted application messaging layer
+   on top of a ‘virtio serial device�between QEMU on the OpenStack Host
+   and the Guest VM.  JSON formatting provides a simple, humanly readable
+   messaging format which can be easily parsed and formatted using any
+   high level programming language being used in the Guest VM (e.g. C/C++,
+   Python, Java, etc.).  Use of the ‘virtio serial device�provides a
+   simple, direct communication channel between host and guest which is
+   independent of the Guest’s L2/L3 networking.
+
+   The upper layer JSON messaging format is actually structured as a
+   hierarchical JSON format containing a Base JSON Message Layer and an
+   Application JSON Message Layer:
+
+        - the Base Layer provides the ability to multiplex different groups
+          of message types on top of a single ‘virtio serial device�
+          e.g.
+
+           + heartbeating and healthchecks,
+           + server group messaging,
+
+          and
+
+        - the Application Layer provides the specific message types and
+          fields of a particular group of message types.
+
+
+
+VM Heartbeating and Health Checking
+============================================================================
+
+   Normally OpenStack monitoring of the health of a Guest VM is limited
+   to a black-box approach of simply monitoring the presence of the
+   QEMU/KVM PID containing the VM, and/or by enabling libvirt's emulated
+   hardware watchdog.
+
+   VM Heartbeating and Health Checking provides a heartbeat service to enhance
+   the monitoring of the health of guest application(s) within a VM running
+   under the OpenStack Cloud.  Loss of heartbeat or a failed health check status
+   will result in a fault event being reported to OPNFV's DOCTOR infrastructure
+   for alarm identification, impact analysis and reporting.  This would then enable
+   VNF Managers (VNFMs) listening to OPNFV's DOCTOR External Alarm Reporting through
+   Telemetry's AODH, to initiate any required fault recovery actions.
+
+   .. image:: OPNFV_HA_Guest_APIs-Overview_HLD-Guest_Heartbeat-FIGURE-1.png
+
+   Or, in the context of the OPNFV DOCTOR's Fault Management Architecture:
+
+   .. image:: OPNFV_HA_Guest_APIs-Overview_HLD-Guest_Heartbeat-FIGURE-1b.png
+
+   The VM Heartbeating and Health Checking functionality is enabled on
+   a VM through a new flavor extraspec indicating that the VM supports
+   and wants to enable Guest Heartbeating.  An extension to Nova Compute uses
+   this extraspec to setup the required 'virtio serial device' for Host-to-Guest
+   messaging, on the QEMU/KVM instance created for the VM.
+
+   A daemon within the Guest VM will register with the OpenStack Guest
+   Heartbeat Service on the compute node to initiate the heartbeating on itself
+   (i.e. the Guest VM).  The OpenStack Compute Node will start heartbeating the
+   Guest VM, and if the heartbeat fails, the OpenStack Compute Node will report
+   the VM Fault thru DOCTOR and ultimately VNFM will see this thru NOVA VM
+   State Change Notifications thru AODH.  I.e. VNFM wouild see the VM Heartbeat
+   Failure events in the same way it sees all other VM Faults, thru DOCTOR
+   initiated VM state changes.
+
+   Part of the Guest VM's registration process is the specification of the
+   heartbeat interval in msecs.  I.e. the registering Guest VM specifies the
+   heartbeating interval.
+
+   Guest heartbeat works on a challenge response model.  The OpenStack
+   Guest Heartbeat Service on the compute node will challenge the registered
+   Guest VM daemon with a message each interval.  The registered Guest VM daemon
+   must respond prior to the next interval with a message indicating good health.
+   If the OpenStack Host does not receive a valid response, or if the response
+   specifies that the VM is in ill health, then a fault event for the Guest VM
+   is reported to the OpenStack Guest Heartbeat Service on the controller node which
+   will report the event to OPNFV's DOCTOR (i.e. thru the Doctor SouthBound (SB)
+   APIs).
+
+   In summary, the Guest Heartbeating Messaging Specification is quite simple,
+   including the following PDUs: Init, Init-Ack, Challenge-Request,
+   Challenge-Response, Exit.  The Challenge-Response returning a healthy /
+   not-healthy boolean.
+
+   The registered Guest VM daemon's response to the challenge can be as simple
+   as just immediately responding with OK.  This alone allows for detection of
+   a failed or hung QEMU/KVM instance, or a failure of the OS within the VM to
+   schedule the registered Guest VM's daemon or failure to route basic IO within
+   the Guest VM.
+
+   However the registered Guest VM daemon's response to the challenge can be more
+   complex, running anything from a quick simple sanity check of the health of
+   applications running in the Guest VM, to a more thorough audit of the
+   application state and data.  In either case returning the status of the
+   health check enables the OpenStack host to detect and report the event in order
+   to initiate recovery from application level errors or failures within the Guest VM.
+
+   In summary, the deliverables of this activity would be:
+
+   - Host Deliverables:    (OpenStack and OPNFV blueprints and implementation)
+
+   + an OpenStack Nova or libvirt extension to interpret the new flavor extraspec and
+     if present setup the required 'virtio serial device' for Host-to-Guest
+     heartbeat / health-check messaging, on the QEMU/KVM instance created
+     for the VM,
+   + an OPNFV Base Host-to-Guest Msging Layer Agent for multiplexing of Application
+     Layer messaging over the 'virtio serial device' to the VM,
+   + an OPNFV Heartbeat / Health-Check Compute Agent for local heartbeating of VM
+     and reporting of failures to the OpenStack Controller,
+   + an OPNFV Heartbeat / Health-check Server on the OpenStack Controller for
+     receiving VM failure notifications and reporting these to Vitrage thru
+     Vitrage's Data Source API,
+
+   - Guest Deliverables:
+
+   + a Heartbeat / Health-Check Message Specification covering
+
+      - Heartbeat / Health-Check Application Layer JSON Protocol,
+      - Base Host-to-Guest JSON Protocol,
+      - Details on the use of the underlying 'virtio serial device',
+
+   + a Reference Implementation of the Guest-side support of
+     Heartbeat / Health-check containing the peer protocol layers
+     within the Guest.
+
+      - will provide code and compile instructions,
+      - Guest will compile based on its specific OS.
+
+   NOTE that the described VM Heartbeating and Healthchecking functionality provides
+   enhanced monitoring over and above libvirt's emulated hardware watchdog.  VM
+   Heartbeating and Healthchecking can detect a wider range of issues than simply
+   lack of cpu time scheduling for a lower priority process feeding the hardware
+   watchdog.  VM Heartbeating and Healthchecking can ensure that specific key processes
+   within the application are not blocked, kernel resources for basic IO within
+   the Guest VM are available, and/or ensure the application-specific health of the VM
+   is good.
+
+   This proposal has been reviewed with both the OPNFV's Doctor and Management
+   and Orchestration teams, and general agreement was that the proposal integrated
+   / inter-worked correctly with the OPNFV DOCTOR's Vitrage, Congress and the overall
+   OPNFV fault reporting architecture.
+
+
+
+VM Peer State Notification and Messaging
+===================================================================================
+
+   Server Group State Notification and Messaging is a service to provide
+   simple low-bandwidth datagram messaging and notifications for servers that
+   are part of the same server group.  This messaging channel is available
+   regardless of whether IP networking is functional within the server, and
+   it requires no knowledge within the server about the other members of the group.
+
+   NOTE: A Server Group here is the OpenStack Nova Server Group concept where VMs
+   are grouped together for purposes of scheduling.  E.g. A specific Server Group
+   instance can specify whether the VMs within the group should be scheduled to
+   run on the same compute host or different compute hosts.  A 'peer' VM in the
+   context of this section refers to a VM within the same Nova Server Group.
+
+   This Server Group Messaging service provides three types of messaging:
+
+        - Broadcast: this allows a server to send a datagram (size of up to 3050 bytes)
+          to all other servers within the server group.
+        - Notification: this provides servers with information about changes to the
+          (Nova) state of other servers within the server group.
+        - Status: this allows a server to query the current (Nova) state of all servers within
+          the server group (including itself).
+
+   A Server Group Messaging entity on both the controller node and the compute nodes
+   manage the routing of of VM-to-VM messages through the platform, leveraging Nova
+   to determine Server Group membership and compute node locations of VMs.  The Server
+   Group Messaging entity on the controller also listens to Nova VM state change notifications
+   and querys VM state data from Nova, in order to provide the VM query and notification
+   functionality of this service.
+
+   .. image:: OPNFV_HA_Guest_APIs-Overview_HLD-Peer_Messaging-FIGURE-2.png
+
+   This service is not intended for high bandwidth or low-latency operations.  It
+   is best-effort, not reliable.  Applications should do end-to-end acks and
+   retries if they care about reliability.
+
+   This service provides building block type capabilities for the Guest VMs that
+   contribute to higher availability of the VMs in the Guest VM Server Group.  Notifications
+   of VM Status changes potentially provide a faster and more accurate notification
+   of failed peer VMs than traditional peer VM monitoring over Tenant Networks.  While
+   the Broadcast Messaging mechanism provides an out-of-band messaging mechanism to
+   monitor and control a peer VM under fault conditions; e.g. providing the ability to
+   avoid potential split brain scenarios between 1:1 VMs when faults in Tenant
+   Networking occur.
+
+   In summary, the deliverables for Server Group Messaging would be:
+
+   - Host Deliverables:
+
+   + a Nova or libvirt extension to interpret the new flavor extraspec and
+     if present setup the required 'virtio serial device' for Host-to-Guest
+     Server Group Messaging, on the QEMU/KVM instance created
+     for the VM,
+   + [ leveraging the Base Host-to-Guest Msging Layer Agent from previous section ],
+   + a Server Group Messaging Compute Agent for implementing the Application Layer
+     Server Group Messaging JSON Protocol with the VM, and forwarding the
+     messages to/from the Server Group Messaging Server on the Controller,
+   + a Server Group Messaging Server on the Controller for routing broadcast
+     messages to the proper Computes and VMs, as well as listening for Nova
+     VM State Change Notifications and forwarding these to applicable Computes
+     and VMs,
+
+   - Guest Deliverables:
+
+   + a Server Group Messaging Message Specification covering
+
+      - Server Group Messaging Application Layer JSON Protocol,
+      - [ leveraging Base Host-to-Guest JSON Protocol from previous section ],
+      - [ leveraging Details on the use of the underlying 'virtio serial device' from previous section ],
+
+   + a Reference Implementation of the Guest-side support of
+     Server Group Messaging containing the peer protocol layers
+     and Guest Application hooks within the Guest.
+
+   This proposal has been reviewed with both the OPNFV's Doctor and Management
+   and Orchestration teams, and general agreement was that the proposal did not
+   conflict with the OPNFV Doctor Architecture, and provided, at the very least,
+   an alternative messaging and state-change-notification mechanism for hosted
+   VMs in various HA use cases.
+
+
+
+Conclusion
+======================================================================================
+
+   The Reach-thru Guest Monitoring and Services described in this document
+   leverage Host-to-Guest messaging to provide a number of extended capabilities
+   that improve the Availability of the hosted VMs.  These new capabilities
+   enable detection of and recovery from internal VM faults and provides a simple
+   out-of-band messaging service to prevent scenarios such as split brain.
+
+   The next steps in progressing this proposal will be to submit blueprints to
+   the appropriate OpenStack working groups;  Vitrage for VM Heartbeating and
+   Healthchecking and Nova for VM Server Group Messaging.