===========================
Host Maintenance Strategy
===========================

Synopsis
--------

**display name**: ``Host Maintenance Strategy``

**goal**: ``cluster_maintaining``

    .. watcher-term:: watcher.decision_engine.strategy.strategies.host_maintenance.HostMaintenance


Metrics
*******

None

Cluster data model
******************

Default Watcher's Compute cluster data model:

    .. watcher-term:: watcher.decision_engine.model.collector.nova.NovaClusterDataModelCollector

Actions
*******

Default Watcher's actions:

    .. list-table::
       :widths: 30 30
       :header-rows: 1

       * - action
         - description
       * - ``migration``
         - .. watcher-term:: watcher.applier.actions.migration.Migrate
       * - ``change_nova_service_state``
         - .. watcher-term:: watcher.applier.actions.change_nova_service_state.ChangeNovaServiceState

Planner
*******

Default Watcher's planner:

    .. watcher-term:: watcher.decision_engine.planner.weight.WeightPlanner

Configuration
-------------

Strategy parameters are:

==================== ====== ================================ ==================
parameter            type   description                      required/optional
==================== ====== ================================ ==================
``maintenance_node`` String The name of the compute node     Required
                            which needs maintenance.
``backup_node``      String The name of the compute node     Optional
                            which will backup the
                            maintenance node.
==================== ====== ================================ ==================

Efficacy Indicator
------------------

None

Algorithm
---------

For more information on the Host Maintenance Strategy please refer
to: https://specs.openstack.org/openstack/watcher-specs/specs/queens/approved/cluster-maintenance-strategy.html

How to use it ?
---------------

Run an audit using Host Maintenance strategy.
Executing the actions will move the servers from compute01 host
to a host determined by the Nova scheduler service.

.. code-block:: shell

    $ openstack optimize audit create \
      -g cluster_maintaining -s host_maintenance \
      -p maintenance_node=compute01

Run an audit using Host Maintenance strategy with a backup node specified.
Executing the actions will move the servers from compute01 host
to compute02 host.

.. code-block:: shell

    $ openstack optimize audit create \
      -g cluster_maintaining -s host_maintenance \
      -p maintenance_node=compute01 \
      -p backup_node=compute02

Note that after executing this strategy, the *maintenance_node* will be
marked as disabled, with the reason set to ``watcher_maintaining``.
To enable the node again:

.. code-block:: shell

   $ openstack compute service set --enable compute01

External Links
--------------

None.