The Snap layer for Juju enables layered charms to more easily deal with snap packages in a simple and efficient manner. It provides consistent configuration for users, allowing them the choice of pulling snaps from the main snap store, or uploading them as Juju resources for deploys in environments with limited network access.

To have the Snap layer install snaps automatically, declare the snaps in layer.yaml:

includes:
  - layer:basic
  - layer:snap
options:
  snap:
    telegraf:
      channel: stable
      devmode: false
      jailmode: false
      dangerous: false
      classic: false
      revision: null
      connect:
        - ["telegraf:system-observe", ":system-observe"]
        - ["telegraf:log-observe", ":log-observe"]

In addition, for Juju 2.0 you should declare Juju resource slots for the snaps. This allows operators to have snaps distributed from their Juju controller node rather than the Snap Store, and is necessary for when your charm is deployed in network restricted environments.

resources:
  telegraf:
    type: file
    filename: telegraf.snap
    description: Telegraf snap

⛔ Charms that need to support Juju 1.25 or earlier cannot declare the resource entry in metadata.yaml and can only support snap installs and updates from the Snap Store.

With the Juju resource defined, the operator may deploy your charm using locally provided snaps instead of the Snap Store:

juju deploy --resource telegraf=telegraf_0_19.snap cs:telegraf

If your charm needs to control installation, update and removal of snaps itself then do not declare the snaps in layer.yaml. Instead, use the API provided by the charms.layer.snap Python package.

In the example layer.yaml above, each snap to install is declared as an entry in the snap layer options mapping. Each of these entries is itself a mapping, with a number of optional keys. Most of the keys correspond to snap install command line options.

• channel (str) - The channel to use instead of stable. Defaults to stable. Ignored if the snap is being installed from a Juju resource.
• classic (bool) - Acknowledge that the snap uses classic confinement and will have full system access.
• devmode (bool) - Install with non-enforcing security.
• jailmode (bool) - Override a snap's request for non-enforcing security
• dangerous (bool) - Install the snap even if it is unverified and could be dangerous. Implicitly set if the snap is being installed from a Juju resource.
• revision (str) - Install an explicit revision of the snap. Ignored if the snap is being installed from a Juju resource.

The other key is connect, which declares the snap connect commands to run to connect the snap's plugs to suitable slots. Each entry is a two element list, with the first item being the plug name and the second the target snap and slot name. The connections are made after all snaps have been installed, so you do not need to worry about installation order.

By default, the snapd daemon will query the Snap Store four (4) times per day to process updates for installed snaps. This layer provides a charm configuration option for authors and operators to control this refresh frequency.

NOTE: this is a global configuration option and will affect the refresh time for all snaps installed on a system.

NOTE: requires snapd 2.31 or higher.

Examples:

## refresh snaps every tuesday
juju config telegraf snapd_refresh="tue"

## refresh snaps at 11pm on the last (5th) friday of the month
juju config telegraf snapd_refresh="fri5,23:00"

## use the system default refresh timer
juju config telegraf snapd_refresh=""

Currently, the snapd refresh timer may be delayed up to one (1) month. This can be configured using the max option:

## delay snap refreshes as long as possible
juju config telegraf snapd_refresh="max"

For more information on the possible values for snapd_refresh, see the refresh.timer section in the system options documentation.

If you have defined your snaps in layer.yaml for automatic installation and updates, there is nothing further to do.

If your charm need to control installation, update and removal of snaps itself, the following methods are available via the charms.layer.snap package::

• install(snapname, **args). Install the snap from the corresponding Juju resource (using --dangerous implicitly). If the resource is not available, download and install from the Snap Store using the provided keyword arguments.

• refresh(snapname, **args). Update the snap. If the snap was installed from a local resource then the resource is checked for updates and the snap updated if the snap or arguments have changed. If the snap was installed from the Snap Store, snap refresh is run to update the snap.

• create_cohort_snapshot(snapname). Creates a new cohort snapshot and returns the associated key. A cohort snapshot allows snaps on different machines to coordinate their refreshes by sharing the cohort snapshot key.

• join_cohort_snapshot(snapname, cohort_key). Joins a cohort snapshot. When in a cohort snapshot, new snap revisions will not be automatically applied; instead, the leader should periodically check for an available refresh, then create a new cohort snapshot and distribute the key in a controlled fashion to roll out updates.

• is_refresh_available(snapname). Check whether the given snap can be updated. Also available as an automatically managed flag, of the form snap.refresh-available.{snapname}.

• remove(snapname). The snap is removed.

Keyword arguments correspond to the layer.yaml options and snap command line options. See the snap command line documentation for authorative details on what these options do:

• channel (str)
• classic (boolean)
• devmode (boolean)
• jailmode (boolean)
• dangerous (boolean)
• revision (str)

The Charm Store does not yet understand that most resources should be optional and requires them to be uploaded before publication. The Snap layer supports the common workaround for this, requiring you to upload an empty (0 bytes in size) as a stand in for the resource.

charm push $JUJU_REPOSITORY/builds/mycharm cs:~me/mycharm
charm attach cs:~me/mycharm-0 mysnap=empty.snap
charm release cs:~me/mycharm-1 --channel=beta --resource mysnap-0
juju deploy cs:~me/mycharm --channel=beta

⌚ This should no longer be required once 🐛 Issue 103 is dealt with.

This layer is maintained on Launchpad by Stuart Bishop (stuart.bishop@canonical.com).

Code is available using git at git+ssh://git.launchpad.net/layer-snap.

Bug reports can be made at https://bugs.launchpad.net/layer-snap.

Queries and comments can be made on the Juju mailing list, Juju IRC channels, or at https://answers.launchpad.net/layer-snap.