All charms: migration to channels

Charmed OpenStack deployments must eventually migrate from legacy non-channel charms to charms that use a channel.

Important

See Charm delivery for an overview of how OpenStack charms are distributed.

Background

All charms are now served from the Charmhub, regardless of which prefix (cs: or ch:) is used to deploy charms. Furthermore, when a channel is not requested at deploy time, the latest/stable channel in the Charmhub is sourced, which points to the 21.10 stable release of OpenStack Charms (or the 21.06 stable release of Trilio Charms).

All maintenance for stable charms occurs on the various explicitly-named channels (i.e. not based on the latest track). These are the channels that the charms must be migrated to.

Warning

The OpenStack Charms project strongly advises against the use of the latest track due to its implicit nature. In doing so, a future charm upgrade may result in a charm version that does not support your current OpenStack release.

Determine current versions

A charm’s channel is selected based on its corresponding service’s software version. Use the information in the below two sub-sections to determine the version running for each application in your deployment.

OpenStack service charms

For OpenStack service charms, to get the running OpenStack version you can inspect the value assigned to the openstack-origin charm configuration option.

For example, if the juju config keystone openstack-origin command outputs ‘focal-xena’ then the running OpenStack version is Xena.

It is expected that all OpenStack service charms will report the same OpenStack version.

All other charms

For all other charms, utilise the juju status command.

Examples:

  • rabbitmq-server:

    App              Version  Status  Scale  Charm            Store       Channel  Rev  OS      Message
rabbitmq-server  3.8.2    active      1  rabbitmq-server  charmstore  stable   117  ubuntu  Unit is ready

    RabbitMQ is running version 3.8.2.

  • ceph-osd:

    App           Version  Status  Scale  Charm         Store       Channel  Rev  OS      Message
ceph-osd      16.2.6   active      3  ceph-osd      charmstore  stable   315  ubuntu  Unit is ready (2 OSD)

    Ceph is running version 16.2.6 (Pacific).

    Since the Ceph channels are based on code names, as a convenience, a mapping of versions to code names is provided:

    Version

    Code name

    12.2.13

    Luminous

    13.2.9

    Mimic

    14.2.22

    Nautilus

    15.2.14

    Octopus

    16.2.6

    Pacific

Select the channels

Charm delivery includes a list of all the tracks available to the OpenStack charms.

Examples:

  • if Ceph Octopus is running, then any Ceph charm that supports the octopus/stable channel should use that channel

  • if OVN 20.03 is running, then any OVN charm that supports the 20.03/stable channel should use that channel

  • if RabbitMQ 3.8.2 is running, then the rabbitmq-server charm should use the 3.8/stable channel

Based on this information, select the appropriate channel for each charm in your deployment.

Upgrade Juju

Upgrade every Juju component of the given deployment to Juju 2.9. This includes the Juju client, the controller model, and the workload model. See the Juju documentation for guidance.

Perform the migration

The migration consists of replacing all charms with new but software-equivalent charms. Technically, this is not an upgrade but a form of crossgrade.

Note

There is no need to upgrade the current charms to their latest stable revision prior to the migration.

The charm of a currently-deployed application is migrated according to the following syntax:

juju refresh --switch ch:<charm> --channel=<channel> <application>

For example, if the selected channel for the rabbitmq-server charm is 3.8/stable then:

juju refresh --switch ch:rabbitmq-server --channel=3.8/stable rabbitmq-server

The application argument represents the application as it appears in the model. That is, it may be a named application (e.g. ‘mysql’ and not ‘mysql-innodb-cluster’).

Change operator behaviour

Once all of your deployment’s charms have been migrated to channels it is important to:

  • stop using the cs: prefix when referencing charms, whether in bundles or on the command line. Use the ch: prefix instead. Note that Juju 2.9 uses the ch: prefix by default on the command line.

  • always specify a channel when deploying a charm (e.g. juju deploy --channel=pacific/stable ceph-radosgw)