Channel branches

Nixpkgs is the Git repository containing all package recipes and NixOS module declarations. Installing packages directly from the master branch of the Nixpkgs repository is possible but risky, since Git commits are merged into master before being heavily tested. That is where channel branches are useful.

A "channel branch" is Git branch with the "verified" Git commits in Nixpkgs that is exposed at channels.nixos.org. There is also a channel status webpage that tracks the age of channel branches.

Each channel branch follows a corresponding development branch to which new commits are first added. These new commits are then "verified" using the Hydra continuous integration system, where each channel branch corresponds to building any new or updated packages for that branch and perform the associated tests. A channel branch is updated once its builds succeeds for a new commit. Contrary to users of the development branches, channel branch users will benefit from both "verified" commits and pre-built packages from the public binary cache.

There are several types of channel branches, 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-24.05) only provide conservative updates for fixing bugs and security vulnerabilities, but do not receive major updates after the initial release. New stable channels are released every six months.
- Unstable channels (nixos-unstable, nixpkgs-unstable) follow the master branch of Nixpkgs, delivering the latest tested updates on a rolling basis.

Large/small:
- Large channels (nixos-24.05, nixos-unstable) are updated only after Hydra has finished building the full breadth of Nixpkgs.
- Small channels (nixos-24.05-small, nixos-unstable-small) are identical to large channels, but are updated as soon as Hydra has finished building a defined set of commonly-used packages. Thus, users following these channels will get faster updates but may need to build any packages they use from outside the defined set themselves. These channels are intended to be used for server setups, for example.

Most users will want the stable/large channel, currently nixos-24.05.

NixOS uses Nix profiles for the root user to refer different versions of the system-wide configuration. Profiles set for other users only control the user environment for that user.

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 (see above), 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.

Updates for the -unstable channels typically take a few days after commits land in the master branch.

To find out when a channel was last updated, check https://status.nixos.org/. The progress of a particular pull request can be tracked via the (third-party) Nixpkgs Pull Request Tracker.

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.

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