A validation set is an assertion that lists specific snaps that are either required to be installed together or are permitted to be installed together on a device or system.
One or more validation sets can be used to ensure only specific snaps are installed, and optionally, only specific snaps at fixed revisions. They can help a set of interdependent snaps maintain their testing and certification integrity, as well as help orchestrate their updates. But they can equally be used to simplify dependency deployment and to help manage devices. In particular, if the model assertion for a device includes optional snaps, a validation set can be used to ensure specific collections of snaps are installed together on derivatives of the same devices.
Prerequisites
- A developer account is required, along with your developer id
- Snapd version 2.54 or newer
See below for further details on the following:
- Creating a validation set
- Listing a validation set
- Monitoring assertion validity
- Enforcing assertion validity
Creating a validation set
For devices running Ubuntu Core, a validation set can be declared as part of the model definition.
To create a validation set from the command line, use the snapcraft edit-validation-sets command:
snapcraft edit-validation-sets <account-id> <set-name> <sequence>
This command requires your developer account id, an arbitrary name for the validation set, and a sequence number. The sequence number starts at 1.
snapcraft edit-validation-sets xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f myset1 1
An additional --key-name argument can be used to specify a key other than the default.
A text editor will open containing a template for a validation set assertion that needs to be filled in by the developer issuing the assertion.
validation-set template
account-id: <account-id>
name: <set-name>
sequence: <sequence>
# The revision for this validation set
# revision: 0
snaps:
# - name: <name> # The name of the snap.
# id: <id> # The ID of the snap. Optional, defaults to the current ID for
# the provided name.
# presence: [required|optional|invalid] # Optional, defaults to required.
# revision: <n> # The revision of the snap. Optional.
The template validation set assertion needs to be populated with the details of the snaps you wish to include in the set. These are listed beneath the snaps: section, and each snap can use the following fields:
- account-id (required) the brand account associated with this validation set, as retrieved with
snapcraft whoami. - name your descriptive name for this validation set.
- sequence an integer that should be manually incremented with each updated validation set snap or snap revision. If an update fails to apply, snapd can revert only to a previous sequence.
- revision defaults to 0 if not included. An arbitrary number used to differentiate between different uploaded versions of the validation set.
snaps:
name(required): The name of the snap, as you find on the store or in snap search.id(optional): The unique snap-id of the snap (see snap info <snap name> ). Defaults to the snap-id of the named snap.presence(optional): Can be eitherrequired,optionalorinvalid.requiredsnaps need to be installed,optionalsnaps are permitted to be installed andinvalidsnaps explicitly must not be installed. Defaults to required.revision(optional): Specifies which revision of the snap needs to be installed.
account-id: xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f
name: myset1
# revision: 0
sequence: 1
snaps:
- name: hello-world
id: buPKUD3TKqCOgLEjjHx5kSiCpIs5cMuQ
presence: required
- name: test-snapd-base-bare
id: oXC9AkhtCxhlY80KZA3peZzWbnO4xPOT
presence: optional
- name: bare
id: EISPgh06mRh1vordZY9OZ34QHdd7OrdR
presence: optional
We strongly recommend incrementing the sequence number whenever a validation set used on production devices is modifiedāespecially when updating snap revisions. This enables snapd to revert to the previous validation set if applying the updated assertion fails. The sequence mechanism was introduced to enable safe rollbacks, because reverting to an earlier validation set revision of the same sequence is not supported.
Modifying a validation set without updating the sequence can violate the constraints of an enforced validation set, leading to an invalid state that cannot be automatically recovered. For example, accidentally specifying an approved revision of a snap meant for a different architecture will cause the refresh to fail, leaving the validation set in an invalid state.
We also recommend making a copy of the saved validation set assertion before closing the editor. Closing the editor will first check the integrity of the assertion before automatically uploading it to the store.
To modify the assertion at a later point, run the same snapcraft edit-validation-sets command with the same name but an incremented sequence number and/or revision.
Listing validation sets
Use the snapcraft list-validation-sets command to check which validation sets area available in the store:
$ snapcraft list-validation-sets
Account-ID Name Sequence Revision When
xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f myset1 1 0 2021-04-08
xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f testset1 2 0 2021-03-31
To list only validation-sets with a specific set name, use the additional --name argument:
$ snapcraft list-validation-sets --name myset1
Account-ID Name Sequence Revision When
xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f myset1 1 0 2021-04-08
An additional --sequence argument can be used to list validation sets with a specific sequence number:
$ snapcraft list-validation-sets --name myset1 --sequence 1
Account-ID Name Sequence Revision When
xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f myset1 1 0 2021-04-08
By default, only the latest validation sets are listed. To list every validation set available, add the --all argument.
Monitoring assertion validity
The snap validate --monitor command is used to enable monitoring of a validation assertion on the system; in this mode the constraints of the assertion are not enforced (e.g. snaps may get automatically refreshed to newer revisions that make the assertion invalid as shown in the next example):
snap validate --monitor xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f/testset1
The snap validate command, with no further arguments, checks whether the snaps: rules for all validation set assertions in the store are valid for the system:
$ snap validate
Validation Mode Seq Current Notes
xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f/myset1 monitor 1 valid
xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f/testset1 monitor 2 invalid
An assertion is invalid if snaps in the system do not satisfy the constraints of the assertion, such as if required snaps are missing or whether unwanted snaps are present. Multiple validation sets can be used, as shown above, as long as they donāt have conflicting constraints and that they can cover different sets of snaps.
A specific validation set can be checked with snap validate <account id>/<validation set name>, with an optional sequence point set by adding =<sequence> to the validation set name:
$ snap validate xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f/myset1=1
valid
A validation set assertion can be pinned by the system administrator at the given sequence number:
snap validate --monitor xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f/testset1=3
A pinned validation set is kept at the given sequence number, even if thereās a higher sequence number in the store. However, the validation will be updated to a newer version if one becomes available with the same sequence number.
Monitor mode validation requires a manual action (snap validate, as shown above), but nothing is enforced in the system. Only when enforce mode has been implemented will validation sets have an impact on the system and will prevent installing/removing snaps that violate an assertionās constraints.
Finally, to remove a validation set from the system, use the --forget argument:
snap validate --forget xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f/myset1
Enforcing assertion validity
When validation set validity is enforced, snapd will block any operations that would result in snap revisions violating the validation setās constraints and rendering it invalid.
A validation set can only be enforced if all required snaps are installed, and at the correct revision if specified. This is done by adding the --enforce argument to the āsnap validateā command:
snap validate --enforce xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f/myset1
This command will fetch the validation set assertion from the store if not available on the system, but will not refresh assertions, install missing snaps or remove invalid snaps. However, the --refresh --enforce arguments can be used to refresh the validation set assertion, install or refresh any snaps and revisions required for the assertion to become valid. Note that, if the assertion requires snaps to be removed, the --refresh --enforce request will not remove them and will instead quit without making any changes.
snap validate --refresh --enforce xSfWKGdLoQBoQx88vIM1MpbFNMq53t1f/myset1
After enforcement is enabled, snapd ensures the consistency of the enforced validation sets, and the snaps they reference, during install, refresh and remove operations.
During auto-refreshes or manual refreshes, enforced validation set assertions on the system may be updated to a newer revision.
If a validation set assertion is not pinned to a specific sequence, the system will move to the latest sequence available in the store, provided that the installed snaps, including any newer revisions available in store, satisfy its constraints.
However, if the assertion is pinned to a specific sequence, or if a newer revision of the same sequence is available, the system will move to that revision without re-evaluating whether its constraints are still satisfied. This can cause the validation set to become invalid without warning.
To ensure safe rollbacks and consistent behavior, always increment the sequence number when creating a new revision of a validation set used in production.
In this way, the snapcraft edit-validation-sets command can be used to control the updates of multiple snaps at the same time.
For brief periods during multi-snap updates, different snap revisions, from previous and incoming validation set sequence points, can co-exist. Validation set enforcement is not intended to deal with any breaking hard version dependencies during transitions.
As with monitor mode, enforcing can be disabled for select validation sets with the āsnap validate --forgetā command.
When using snap install and snap refresh, the --ignore-validation flag can be added to bypass validation set enforcement for the snaps affected. Doing so will ignore the validation of the given snap, and for subsequent refresh operations. This may result in the validation set becoming invalid in snap validate output.