Refresh control in Ubuntu Core
Snaps in Ubuntu Core update automatically, and by default, the snapd daemon checks for updates 4 times a day. Each update check is called a refresh .
However, some projects can benefit from holding a deployed snap at a tested revision until any new release is separately validated, and this requires refresh control.
A snap’s revision is an automatic number assigned by the Snap Store to give each snap build a unique identity within its channel. Refresh control allows specific snaps to be kept at a specified revision, even when a new or different revision of a snap is released.
Refresh control can be accomplished by using a gating snap to control which revisions of one or more gated snaps to install:
- the publisher of the gating snap validates which revisions of the one or more gated snaps to install.
- the gating snap lists the snaps that are to be controlled. Any system that has the gating snap installed is controlled.
- gated snaps are kept at their stated revisions. If any gated snaps are not currently at a validated revision, the next refresh ensures they’re moved to one, assuming the validated revision is available on the channel that the system follows.
A gated snap can also require verification from more than one gating snaps, but this is discouraged to help avoid confusion. The best practice is to use a single gating snap for each gated snap.
Gating is especially useful when a device manufacturer needs to mediate releases of its base snap, allowing time for testing and validation of new revisions on their device models and compatibility with their software. This is separate from the ability for device managers to control the schedule of refreshes and revision control via a snap proxy.
The mechanism of defining refresh control behaviour involves:
- setting the list of controlled snaps (by snap ID) in the gating snap. After the Canonical Store team does this initial setup, the gating snap’s snap-declaration assertion contains a refresh-control section that lists the gated snap IDs (see below)
- The gating snap publisher signing and issuing validation assertions that include:
- the name of the gating snap
- the name and revision the gated snap is held at
- This is done by the gating snap Publisher
Establishing the relationship between a gating and gated snap is required only once and can only be performed by a user with permission from the team operating the store controlling the gating snap.
Setting up the signing key
Gating involves pushing validation assertions signed by the publisher, indicating their authorisation to restrict updates of snaps. To do this, several GPG keys must be generated and published to the store. If a store key already exists, it can be used. If not, one must be generated and registered.
snapcraft create-keyto create a key. (by default it’s named “default” but a key name can be given to create a custom one so e.g. each snap is signed with its own key). Do not forget the passphrase used to secure the key or misplace the key files.
$ snapcraft create-key my-key Passphrase: Confirm passphrase: $ snapcraft list-keys Name SHA3-384 fingerprint - my-key mfomTtPB1cE3IFs51NtdnFoPlQwJxFgxOMU_q0mPkH7M2gKB-4m28d99XrVjA53B (not registered)
snapcraft register-key <the-key-name>to register it with the assertion server.
$ snapcraft register-key my-key Enter your Ubuntu One e-mail address and password. If you do not have an Ubuntu One account, you can create one at https://dashboard.snapcraft.io/openid/login Email: firstname.lastname@example.org Password: Second-factor auth: 111111 Registering key ... Done. The key "my-key" (mfomTtPB1cE3IFs51NtdnFoPlQwJxFgxOMU_q0mPkH7M2gKB-4m28d99XrVjA53B) may be used to sign your assertions.
Example gating workflows
At this point, the publisher can issue validations asserting which revision of the gated snap is allowed to be installed when the gating snap is installed on a device. For example, specify that if the roadmr-gating snap is installed, the roadmr-gated snap can only be installed or refreshed to revision 2.
$ snapcraft validate roadmr-gating roadmr-gated=2 --key-name my-key Getting details for roadmr-gated Signing validations assertion for roadmr-gated=2
This generates the assertion and pushes it to the assertions service. A copy of the assertion will also be left on the current directory.
type: validation authority-id: Q8HSISyvcdrniz52oa8Qjh6HlZwv0wiT series: 16 snap-id: gs2epiCF5LPioNKnajAOTiBhOogG26RN approved-snap-id: xZkR9MIMbusYipfJ80DGWf1nPKkD77hk approved-snap-revision: 2 revoked: false timestamp: 2018-05-30T21:00:04.092765Z sign-key-sha3-384: mfomTtPB1cE3IFs51NtdnFoPlQwJxFgxOMU_q0mPkH7M2gKB-4m28d99XrVjA53B
To verify this was pushed up to assertions service:
$ snap known --remote validation series=16 snap-id=gs2epiCF5LPioNKnajAOTiBhOogG26RN approved-snap-id=xZkR9MIMbusYipfJ80DGWf1nPKkD77hk approved-snap-revision=2 type: validation authority-id: Q8HSISyvcdrniz52oa8Qjh6HlZwv0wiT series: 16 snap-id: gs2epiCF5LPioNKnajAOTiBhOogG26RN approved-snap-id: xZkR9MIMbusYipfJ80DGWf1nPKkD77hk approved-snap-revision: 2 revoked: false timestamp: 2018-05-30T21:00:04.092765Z sign-key-sha3-384: mTVMTtPB1cE3IFs51NtdnFoPlQwJxFgxOMU_q0mPkH7M2gKB-4m28d99XrVjA53B ....
As described earlier, this means that, if the gating snap is installed on the system, the gated snap can only be installed or refreshed to the approved revision (2 in this case). If the gating snap is not installed on the system, the gated snap is not restricted and will be installable or updatable to whichever revision is available in the requested channel. The revision of the gating snap is not relevant here; what’s relevant is the latest validation assertion which tells which revision of roadmr-gated is allowed.
- To control the revision of a gated snap, the gating snap Publisher signs and issues a validation assertion that names the gating snap and the gated snap name and revision.
- To sign the validation assertion, the Publisher must name/use a snapcraft key (snapcraft create-key KEYNAME) that has been registered (snapcraft register-key KEYNAME). See below.
- When signing a validation assertion where the gating snap is published in a Brand Store, the
SNAPCRAFT_UBUNTU_STOREenvironment variable must be defined to equal the Brand Store’s store ID.
The snapcraft gated command can tell you which snaps are gated by a particular snap:
$ snapcraft gated roadmr-gating Name Revision Required Approved roadmr-gated 2 True 2018-05-30T21:00:04Z
Here we can see roadmr-gating, if installed, will constrain roadmr-gated to revision 2 (or nothing at all, if rev2 is not installable).
At this point one can install the gating snap:
$ snap install roadmr-gating roadmr-gating 2018-05-30-01 from 'snapdeveloper' installed $ snap list roadmr-gating Name Version Rev Tracking Developer Notes roadmr-gating 2018-05-30-01 1 stable snapdeveloper -
Note that trying to install the gated snap fails:
$ snap install roadmr-gated error: snap "roadmr-gated" not found
Even though it is indeed available on stable (but at revision 1, not the validated one, which would be 2):
$ snap info roadmr-gated name: roadmr-gated publisher: snapdeveloper snap-id: xZkR9MIMbusYipfJ80DGWf1nPKkD77hk channels: stable: 2018-05-30-02 (1) 4kB -
Once revision 2 is published…
$ snap info roadmr-gated name: roadmr-gated publisher: snapdeveloper snap-id: xZkR9MIMbusYipfJ80DGWf1nPKkD77hk channels: stable: 2018-05-30-03 (2) 4kB -
…installing can proceed.
$ snap install roadmr-gated roadmr-gated 2018-05-30-03 from 'snapdeveloper' installed $ snap list roadmr-gating roadmr-gated Name Version Rev Tracking Developer Notes roadmr-gated 2018-05-30-03 2 stable snapdeveloper - roadmr-gating 2018-05-30-01 1 stable snapdeveloper -
If at this point revision 3 of the gated snap is published…
$ snap info roadmr-gated name: roadmr-gated publisher: snapdeveloper snap-id: xZkR9MIMbusYipfJ80DGWf1nPKkD77hk tracking: stable refreshed: 2018-05-30T21:08:53Z installed: 2018-05-30-03 (2) 4kB - channels: stable: 2018-05-30-04 (3) 4kB -
…snap refresh will do nothing because even though v3 is available, it’s not validated.
$ snap refresh All snaps up to date. $ snap list roadmr-gating roadmr-gated Name Version Rev Tracking Developer Notes roadmr-gated 2018-05-30-03 2 stable snapdeveloper - roadmr-gating 2018-05-30-01 1 stable snapdeveloper -
Validation is independent of snap release/publishing. For example, we can validate the existing gated snap v3 with the existing gating snap at revision 1 (meaning no update of gating snap is needed):
$ snapcraft validate roadmr-gating roadmr-gated=3 --key-name my-key Getting details for roadmr-gated Signing validations assertion for roadmr-gated=3
At this point a refresh will take gated-1 to rev 3 which was validated.
$ snap refresh roadmr-gated 2018-05-30-04 from 'snapdeveloper' refreshed $ snap list roadmr-gating roadmr-gated Name Version Rev Tracking Developer Notes roadmr-gated 2018-05-30-04 3 stable snapdeveloper - roadmr-gating 2018-05-30-01 1 stable snapdeveloper -
Gating multiple snaps
A validation can be issued to restrict the revisions of more than one gated snap (assuming the “publisher setup” was done to allow the gating snap to control the indicated gated snaps):
$ snapcraft validate roadmr-gating roadmr-gated=2 facundo-gated=5 Getting details for roadmr-gated Getting details for facundo-gated Signing validations assertion for roadmr-gated=2 Signing validations assertion for facundo-gated=5
In this example, if roadmr-gating is installed on a device, roadmr-gated will only install or refresh to revision 2, and facundo-gated will only install or refresh to revision 5.
- At the moment, validations can only be issued to gate snaps which have a release on the stable channel at the moment of creating the validation. (There’s a snapcraft bug for this). This is because
snapcraft validateuses the details API to get the details from the stable channel only, and it doesn’t allow specifying another channel to get the gated snap’s details. Once the validation is in place, validations are channel-agnostic because they operate at the revision level.
$ snapcraft status gated-1 Track Arch Channel Version Revision latest amd64 stable - - candidate - - beta - - edge 2018-05-30-04 3 $ snapcraft validate gating-1 gated-1=3 Getting details for gated-1 Snap 'gated-1' was not found in the 'stable' channel.