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+Copyright 2017 Open Platform for NFV Project, Inc. and its contributors
+
+Open Platform for NFV Project Documentation License
+===================================================
+Any documentation developed by the "Open Platform for NFV Project"
+is licensed under a Creative Commons Attribution 4.0 International License.
+You should have received a copy of the license along with this. If not,
+see <http://creativecommons.org/licenses/by/4.0/>.
+
+Unless required by applicable law or agreed to in writing, documentation
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
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+.. This work is licensed under a Creative Commons Attribution 4.0 International License.
+.. http://creativecommons.org/licenses/by/4.0
+
+
+Maintenance use case
+""""""""""""""""""""
+
+* A consumer of the NFVI wants to interact with NFVI maintenance, upgrade,
+  scaling and to have graceful retirement. Receiving notifications over these
+  NFVI events and responding to those within given time window, consumer can
+  guarantee zero downtime to his service.
+
+The maintenance use case adds the Doctor platform an `admin tool` and an
+`app manager` component. Overview of maintenance components can be seen in
+:numref:`figure-p2`.
+
+.. figure:: ./images/Maintenance-design.png
+    :name: figure-p2
+    :width: 100%
+
+    Doctor platform components in maintenance use case
+
+In maintenance use case, `app manager` (VNFM) will subscribe to maintenance
+notifications triggered by project specific alarms through AODH. This is the way
+it gets to know different NFVI maintenance, upgrade and scaling operations that
+effect to its instances. The `app manager` can do actions depicted in `green
+color` or tell `admin tool` to do admin actions depicted in `orange color`
+
+Any infrastructure component like `Inspector` can subscribe to maintenance
+notifications triggered by host specific alarms through AODH. Subscribing to the
+notifications needs admin privileges and can tell when a host is out of use as
+in maintenance and when it is taken back to production.
+
+Maintenance test case
+"""""""""""""""""""""
+
+Maintenance test case is currently running in our Apex CI and executed by tox.
+This is because the special limitation mentioned below and also the fact we
+currently have only sample implementation as a proof of concept and we also
+support unofficial OpenStack project Fenix. Environment variable
+TEST_CASE='maintenance' needs to be used when executing "doctor_tests/main.py"
+and ADMIN_TOOL_TYPE='fenix' if want to test with Fenix instead of sample
+implementation. Test case workflow can be seen in :numref:`figure-p3`.
+
+.. figure:: ./images/Maintenance-workflow.png
+    :name: figure-p3
+    :width: 100%
+
+    Maintenance test case workflow
+
+In test case all compute capacity will be consumed with project (VNF) instances.
+For redundant services on instances and an empty compute needed for maintenance,
+test case will need at least 3 compute nodes in system. There will be 2
+instances on each compute, so minimum number of VCPUs is also 2. Depending on
+how many compute nodes there is application will always have 2 redundant
+instances (ACT-STDBY) on different compute nodes and rest of the compute
+capacity will be filled with non-redundant instances.
+
+For each project specific maintenance message there is a time window for
+`app manager` to make any needed action. This will guarantee zero
+down time for his service. All replies back are done by calling `admin tool` API
+given in the message.
+
+The following steps are executed:
+
+Infrastructure admin will call `admin tool` API to trigger maintenance for
+compute hosts having instances belonging to a VNF.
+
+Project specific `MAINTENANCE` notification is triggered to tell `app manager`
+that his instances are going to hit by infrastructure maintenance at a specific
+point in time. `app manager` will call `admin tool` API to answer back
+`ACK_MAINTENANCE`.
+
+When the time comes to start the actual maintenance workflow in `admin tool`,
+a `DOWN_SCALE` notification is triggered as there is no empty compute node for
+maintenance (or compute upgrade). Project receives corresponding alarm and scales
+down instances and call `admin tool` API to answer back `ACK_DOWN_SCALE`.
+
+As it might happen instances are not scaled down (removed) from a single
+compute node, `admin tool` might need to figure out what compute node should be
+made empty first and send `PREPARE_MAINTENANCE` to project telling which instance
+needs to be migrated to have the needed empty compute. `app manager` makes sure
+he is ready to migrate instance and call `admin tool` API to answer back
+`ACK_PREPARE_MAINTENANCE`. `admin tool` will make the migration and answer
+`ADMIN_ACTION_DONE`, so `app manager` knows instance can be again used.
+
+:numref:`figure-p3` has next a light blue section of actions to be done for each
+compute. However as we now have one empty compute, we will maintain/upgrade that
+first. So on first round, we can straight put compute in maintenance and send
+admin level host specific `IN_MAINTENANCE` message. This is caught by `Inspector`
+to know host is down for maintenance. `Inspector` can now disable any automatic
+fault management actions for the host as it can be down for a purpose. After
+`admin tool` has completed maintenance/upgrade `MAINTENANCE_COMPLETE` message
+is sent to tell host is back in production.
+
+Next rounds we always have instances on compute, so we need to have
+`PLANNED_MAINTANANCE` message to tell that those instances are now going to hit
+by maintenance. When `app manager` now receives this message, he knows instances
+to be moved away from compute will now move to already maintained/upgraded host.
+In test case no upgrade is done on application side to upgrade instances
+according to new infrastructure capabilities, but this could be done here as
+this information is also passed in the message. This might be just upgrading
+some RPMs, but also totally re-instantiating instance with a new flavor. Now if
+application runs an active side of a redundant instance on this compute,
+a switch over will be done. After `app manager` is ready he will call
+`admin tool` API to answer back `ACK_PLANNED_MAINTENANCE`. In test case the
+answer is `migrate`, so `admin tool` will migrate instances and reply
+`ADMIN_ACTION_DONE` and then `app manager` knows instances can be again used.
+Then we are ready to make the actual maintenance as previously trough
+`IN_MAINTENANCE` and `MAINTENANCE_COMPLETE` steps.
+
+After all computes are maintained, `admin tool` can send `MAINTENANCE_COMPLETE`
+to tell maintenance/upgrade is now complete. For `app manager` this means he
+can scale back to full capacity.
+
+There is currently sample implementation on VNFM and test case. In
+infrastructure side there is sample implementation of 'admin_tool' and
+there is also support for the OpenStack Fenix that extends the use case to
+support 'ETSI FEAT03' for VNFM interaction and to optimize the whole
+infrastructure mainteannce and upgrade.