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+.. This work is licensed under a Creative Commons Attribution 4.0 International License.
+.. http://creativecommons.org/licenses/by/4.0
+
+======================
+Get valid server state
+======================
+
+Related Blueprints:
+===================
+
+https://blueprints.launchpad.net/nova/+spec/get-valid-server-state
+
+Problem description
+===================
+
+Previously when the owner of a VM has queried his VMs, he has not received
+enough state information, states have not changed fast enough in the VIM and
+they have not been accurate in some scenarios. With this change this gap is now
+closed.
+
+A typical case is that, in case of a fault of a host, the user of a high
+availability service running on top of that host, needs to make an immediate
+switch over from the faulty host to an active standby host. Now, if the compute
+host is forced down [1] as a result of that fault, the user has to be notified
+about this state change such that the user can react accordingly. Similarly,
+a change of the host state to "maintenance" should also be notified to the
+users.
+
+What is changed
+===============
+
+A new ``host_status`` parameter is added to the ``/servers/{server_id}`` and
+``/servers/detail`` endpoints in microversion 2.16. By this new parameter
+user can get additional state information about the host.
+
+``host_status`` possible values where next value in list can override the
+previous:
+
+- ``UP`` if nova-compute is up.
+- ``UNKNOWN`` if nova-compute status was not reported by servicegroup driver
+  within configured time period. Default is within 60 seconds,
+  but can be changed with ``service_down_time`` in nova.conf.
+- ``DOWN`` if nova-compute was forced down.
+- ``MAINTENANCE`` if nova-compute was disabled. MAINTENANCE in API directly
+  means nova-compute service is disabled. Different wording is used to avoid
+  the impression that the whole host is down, as only scheduling of new VMs
+  is disabled.
+- Empty string indicates there is no host for server.
+
+``host_status`` is returned in the response in case the policy permits. By
+default the policy is for admin only in Nova policy.json::
+
+  "os_compute_api:servers:show:host_status": "rule:admin_api"
+
+For an NFV use case this has to also be enabled for the owner of the VM::
+
+  "os_compute_api:servers:show:host_status": "rule:admin_or_owner"
+
+REST API examples:
+==================
+
+Case where nova-compute is enabled and reporting normally::
+
+    GET /v2.1/{tenant_id}/servers/{server_id}
+
+    200 OK
+    {
+      "server": {
+        "host_status": "UP",
+        ...
+      }
+    }
+
+Case where nova-compute is enabled, but not reporting normally::
+
+    GET /v2.1/{tenant_id}/servers/{server_id}
+
+    200 OK
+    {
+      "server": {
+        "host_status": "UNKNOWN",
+        ...
+      }
+    }
+
+Case where nova-compute is enabled, but forced_down::
+
+    GET /v2.1/{tenant_id}/servers/{server_id}
+
+    200 OK
+    {
+      "server": {
+        "host_status": "DOWN",
+        ...
+      }
+    }
+
+Case where nova-compute is disabled::
+
+    GET /v2.1/{tenant_id}/servers/{server_id}
+
+    200 OK
+    {
+      "server": {
+        "host_status": "MAINTENANCE",
+        ...
+      }
+    }
+
+Host Status is also visible in python-novaclient::
+
+  +-------+------+--------+------------+-------------+----------+-------------+
+  | ID    | Name | Status | Task State | Power State | Networks | Host Status |
+  +-------+------+--------+------------+-------------+----------+-------------+
+  | 9a... | vm1  | ACTIVE | -          | RUNNING     | xnet=... | UP          |
+  +-------+------+--------+------------+-------------+----------+-------------+
+
+Links:
+======
+
+[1] Manual for OpenStack NOVA API for marking host down
+http://artifacts.opnfv.org/doctor/docs/manuals/mark-host-down_manual.html
+
+[2] OpenStack compute manual page
+http://developer.openstack.org/api-ref-compute-v2.1.html#compute-v2.1