Deferred service events

Overview

Operational or maintenance procedures applied to a cloud often lead to the restarting of various OpenStack services and/or the calling of certain charm hooks. Although normal, such events can be undesirable due to the service interruptions they can cause.

The deferred service events feature allows the operator the choose to prevent these service restarts and hook calls from occurring. These deferred events can then be resolved by the operator at a more opportune time.

Situations in which these service events are prone to take place include:

  • charm upgrades

  • OpenStack upgrades (charm-managed)

  • package upgrades (non-charm-managed)

  • charm configuration option changes

Charms

Deferred service events are supported on a per-charm basis.

Here is the current list of deferred event-aware charms:

  • neutron-gateway

  • neutron-openvswitch

  • ovn-central

  • ovn-chassis

  • ovn-dedicated-chassis

  • rabbitmq-server

Enabling and disabling deferred service events

Deferred service events are disabled by default for all charms. To enable them for a charm:

juju config <charm-name> enable-auto-restarts=False

Important

The enable-auto-restarts option can only be set post-deployment.

To disable deferred service events for a charm:

juju config <charm-name> enable-auto-restarts=True

Identifying and resolving deferred service events

The existence of a deferred service event is exposed in the output of the juju status command. The following two sections provide an example of how to identify and resolve each type.

Service restarts

Here the neutron-openvswitch/1 unit is affected by a deferred service restart:

App                  Version  Status  Scale  Charm                Store       Channel  Rev  OS      Message
neutron-openvswitch  16.3.0   active      2  neutron-openvswitch  charmstore           433  ubuntu  Unit is ready
nova-compute         21.1.2   active      2  nova-compute         charmstore           537  ubuntu  Unit is ready.

Unit                      Workload  Agent  Machine  Public address  Ports  Message
nova-compute/0*           active    idle   6        172.20.0.13            Unit is ready.
  neutron-openvswitch/1   active    idle            172.20.0.13            Unit is ready. Services queued for restart: openvswitch-switch
nova-compute/1            active    idle   7        172.20.0.4             Unit is ready
  neutron-openvswitch/0*  active    idle            172.20.0.4             Unit is ready

To see more detail the show-deferred-events action is used:

juju run-action --wait neutron-openvswitch/1 show-deferred-events

unit-neutron-openvswitch-1:
  UnitId: neutron-openvswitch/1
  id: "67"
  results:
    Stdout: |
      none
    output: |
      hooks: []
      restarts:
      - 1618896650 openvswitch-switch                       Package update
  status: completed
  timing:
    completed: 2021-04-20 05:52:39 +0000 UTC
    enqueued: 2021-04-20 05:52:32 +0000 UTC
    started: 2021-04-20 05:52:33 +0000 UTC

In this example, the message “Package update” is displayed. This signifies that the package management software of the host is responsible for the service restart request.

Resolving deferred service restarts

To resolve a deferred service restart on a unit run the restart-services action:

juju run-action --wait neutron-openvswitch/1 restart-services deferred-only=True

The argument deferred-only ensures that only the necessary services are restarted (for a charm that manages multiple services).

Note

Alternatively, the service can be restarted manually on the unit. The status message will be removed in due course by the charm (i.e. during the next update-status hook execution - a maximum delay of five minutes).

Hook calls

Here the neutron-openvswitch/1 unit is affected by a deferred hook call:

App                  Version  Status  Scale  Charm                Store       Channel  Rev  OS      Message
neutron-openvswitch  16.3.0   active      2  neutron-openvswitch  charmstore           433  ubuntu  Unit is ready. Hooks skipped due to disabled auto restarts: config-changed
nova-compute         21.1.2   active      2  nova-compute         charmstore           537  ubuntu  Unit is ready

Unit                      Workload  Agent  Machine  Public address  Ports  Message
nova-compute/0*           active    idle   6        172.20.0.13            Unit is ready
  neutron-openvswitch/1   active    idle            172.20.0.13            Unit is ready. Hooks skipped due to disabled auto restarts: config-changed

Resolving deferred hook calls

To resolve a deferred hook call on a unit run the run-deferred-hooks action:

juju run-action --wait neutron-openvswitch/1 run-deferred-hooks