Channel branches

Nixpkgs is the git repository containing all packages and NixOS modules/expressions. Installing packages directly from the master branch of the Nixpkgs repo is possible, but risky, since git commits are merged into master before being heavily tested. That's where channels are useful.

A "channel" is a name for the latest "verified" git commits in Nixpkgs. Each channel has a different definition of what "verified" means. Each time a new git commit is verified, the channel declaring this verification gets updated. Contrary to a user of the git master branch, a channel user will benefit from both verified commits and binary packages from the binary cache.

Channels are reified as git branches in the nixpkgs repository and as disk images in the channels webpage. There is also the handy nix channel status webpage that tracks the age of channels.

There are several channels, each with its own use case and verification phase.

The official channels

Channels can be broadly categorized into stable and unstable channels, and large and small channels:

• Stable/unstable:
  • Stable channels (nixos-21.05) provide conservative updates for fixing bugs and security vulnerabilities, but do not receive major updates after initial release. New stable channels are released every six months.
  • Unstable channels (nixos-unstable, nixpkgs-unstable) correspond to the main development branch (master) of Nixpkgs, delivering the latest tested updates on a rolling basis.

• Large/small:
  • Large channels (nixos-21.05, nixos-unstable) provide binary builds for the full breadth of Nixpkgs.
  • Small channels (nixos-21.05-small, nixos-unstable-small) are identical to large channels, but contain fewer binaries. This means they update faster, but require more to be built from source.

Most users will want the stable/large channel, currently nixos-21.05.

Like packages installed via nix-env, channels are managed at user-level. NixOS uses the channels set for the root user to update the system-wide configuration; channels set for other users control only the user environment for that user. If you wish to change the channel used by the system-level configuration (/etc/nixos/configuration.nix), ensure you run the correct nix-channel command as root:

Common nix-channel commands

Listing current channels	nix-channel --list
Adding a primary channel	nix-channel --add https://nixos.org/channels/channel-name nixos
Adding other channels	nix-channel --add https://some.channel/url my-alias
Remove a channel	nix-channel --remove channel-alias
Updating a channel	nix-channel --update channel-alias
Updating all channels	nix-channel --update

Note that updating channels won't cause a rebuild in itself; if you want to update channels and rebuild, you can use nixos-rebuild with the --upgrade flag to do both in one step.

Channel update process

The channel update process begins when anyone with commit access pushes changes to either master or one of the release-XX.XX branches.

Hydra Build

Then, for each unstable channel, a particular job at hydra.nixos.org is started which must succeed:

• For NixOS: the trunk-combined/tested job, which includes some automated NixOS tests.
• For nixos-small: the unstable-small/tested job.
• For nixpkgs: the trunk/unstable job, which contains some critical release packages.

Success Conditions

For a channel update to succeed, two conditions need to be satisfied:

• Particular jobset evaluation needs to be completely built ie. no more queued jobs, even if some jobs may fail
• Particular jobset evaluation's tested/unstable job needs to be built succesfully

The nixos.org server has a cronjob for which nixos-channel-scripts are executed and poll for the newest jobset that satisfies the above two conditions and trigger a channel update.

Channel Update

Once the job succeeds at a particular nixpkgs commit, cache.nixos.org will download binaries from hydra.nixos.org. When the download completes, the channel updates.

For the NixOS channel command-not-found index is generated, which can take some time since it has to fetch all packages. nixpkgs is quickly updated since none of the above needs to happen once a channel update is triggered.

To find out when a channel was last updated, check this page.

When unstable lags behind master

As https://status.nixos.org shows, a downside of nixos-unstable is that when the channel is blocked due to hydra failures, other (security) fixes will also not get in. While of course we try to keep hydra green, it is expected that this happens every once in a while. When you want to upgrade or downgrade a single package while leaving the rest of your system on nixos-unstable, you could use this approach.

Channel history

You can find the channel history at https://channels.nix.gsc.io/

Check build status

hydra-check

$ hydra-check --channel unstable bash
Build Status for nixpkgs.bash.x86_64-linux on unstable
✔ bash-4.4-p23 from 2021-05-23 - https://hydra.nixos.org/build/143785213

also useful for finding build logs