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          |
+-------+------+--------+------------+-------------+----------+-------------+