Channels are an important snap concept. They define which release of a snap is installed and tracked for updates.
A channel consists of three parts: risk-levels, tracks and branches.
- Risk-levels represent a progressive potential trade-off between stability and new features.
- Tracks enable snap developers to publish multiple supported releases of their application under the same snap name.
- Branches are optional and hold temporary releases intended to help with bug-fixing.
The complete channel name can be structured as three distinct parts separated by slashes:
There are four risk-levels: stable, candidate, beta and edge. Installing from a less stable risk-level will typically mean more frequent updates.
Snaps are installed using the stable risk-level by default. For example, the following command installs VLC from its stable channel:
$ sudo snap install vlc
--channel option to select a different risk-level. The following command will install the latest beta snap of VLC:
$ sudo snap install --channel=beta vlc
If the beta snap isn’t available, the next most stable snap will be installed.
After installation, the risk-level being tracked can be changed with:
$ sudo snap switch --channel=stable vlc
The switch option, used above, won’t automatically refresh the snap to force the installation of a new snap. To switch channels and update the snap with a single command, add the
--channel option to the refresh command:
$ sudo snap refresh --channel=stable vlc
To check which channel a snap is tracking, look for the tracking field in the output from the
$ snap info vlc [...] snap-id: RT9mcUhVsRYrDLG8qnvGiy26NKvv6Qkd tracking: edge refresh-date: yesterday at 19:54 BST [...]
Risk-levels may not match a project’s internal conventions. Some projects may use alpha instead of edge, for instance. However, a project’s own release nomenclature should be close enough to a snap’s risk-levels to allow you to judge the relative stability of the version you’re installing.
All snaps must have a default track called latest. The previous examples implicitly install from the latest track, but the track can also be specified explicitly:
$ snap install vlc --channel=latest/edge
The releases a track contains is dependent on the snaps’s developer. A track could be used to track minor updates (2.0.1, 2.0.2), major updates (2.1, 2.2), or releases held for long-term support (3.2, 4.1).
Tracks are listed in the channels section of the output from
$ snap info skype [...] channels: stable: 188.8.131.52 (51) 148MB classic candidate: ↑ beta: ↑ edge: ↑ insider/stable: 184.108.40.206 (53) 151MB classic insider/candidate: ↑ insider/beta: ↑ insider/edge: ↑ [...]
In the above output, the Skype snap includes an insider track to publish new builds of the application intended for its internal QA team. You can also see which tracks a snap support by clicking Other versions from its onlone Store entry.
Developers must currently make a request for tracks to be added to their snap via the #store-requests forum category. Releases are verified and checked to ensure that reasonable user expectations are being met. For example, only 3.2.* versions are accepted into a 3.2 track.
Branches are optional. They allow the creation of a short-lived sequences of snaps that can be published on demand by snap developers to help with fixes or temporary experimentation.
Branch names convey their purpose, such as
fix-for-bug123, but the name isn’t exposed in the normal way, such as with
snap info. Instead, they can be tracked by anyone simply knowing the name.
After 30 days with no further updates, a branch will be closed automatically. The replacement snap will then be chosen as it would be with closed channels (see below). For example, beta/fix-for-bug123 will fallback to beta after the fix-for-bug123 branch is closed.
A channel can be closed by a snap publisher when there is no longer a snap that fits a channel’s original purpose or specification.
For example, when a specific risk-level channel is closed, the snap store will select a snap from a more conservative risk-level of the same track. If the original channel is re-opened, snaps will once again be selected from the original channel.
This approach is commonly used for beta testing. If a snap is following a beta channel that’s then closed, the store will offer the snap from the candidate channel. If candidate isn’t available, the snap from stable will be selected. If the beta channel re-opens, the snap will once again be selected from beta.
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