Deferred service events¶
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:
OpenStack upgrades (charm-managed)
package upgrades (non-charm-managed)
charm configuration option changes
Deferred restarts are supported on a per-charm basis. This support will be mentioned in a charm’s README along with any charm-specific deferred restart information.
Here is the current list of deferred restart-aware charms:
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
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.
neutron-openvswitch/1 unit is affected by a deferred service
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
juju run-action --wait neutron-openvswitch/1 restart-services deferred-only=True
deferred-only ensures that only the necessary services are
restarted (for a charm that manages multiple services).
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).
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
juju run-action --wait neutron-openvswitch/1 run-deferred-hooks