OS Series Upgrades

Upgrades of the operating system’s series, or version, are difficult to automate in a general fashion, so most of the work is done manually by the operator and the role that the charm plays is somewhat limited. However, the charm does need to ensure that during the upgrade, all of the application services on the unit are disabled and stopped so that nothing runs while the operator is making changes that could break the application, even if the machine is rebooted one or more times.

When the operator is about to initiate an OS upgrade, they will run:

juju upgrade-series <machine> prepare <target-series>

The framework will then set the upgrade.series.in-progress flag, which will give the charm one and only one chance to disable and stop its application services in preparation for the upgrade. Once that flag is set and the charm’s handlers have had a chance to respond, Juju will no longer run any charm code for the duration of the upgrade.

Once the operator has completed the upgrade, they will run:

juju upgrade-series <machine> complete

Juju will once again enable the charm code to run, and the framework will re-bootstrap the charm environment to ensure that it is setup properly for the new OS series. it will then remove the upgrade.series.in-progress flag. At this point, the charm should check the new OS series and perform any necessary migration the application may require to run on the new OS (unless that was to be performed manually by the operator). Finally, the charm should re-enable and start its application services.

Note that it is likely that the charm will need an additional self-managed flag to track whether the application services were disabled. The handlers might look something along the lines of:

def disable_application():

def enable_application():