[Nix-dev] NixOS channel

[Eelco Dolstra]

Hi all,

Since a few weeks there is a NixOS channel, which is now the default mechanism
for keeping NixOS up to date.  (A channel is a Nix mechanism for distributing a
consistent set of Nix expressions and binaries.)  A quick summary on how to use it:

$ nix-channel --add http://nixos.org/releases/nixos/channels/nixos-unstable
$ nix-channel --update
$ nixos-rebuild switch

The NixOS channel contains a copy of Nixpkgs (but only the binaries for
i686-linux and x86_64-linux), so after "nix-channel --update" you can install
packages via nix-env:

$ nix-env -i thunderbird

or, if you prefer the "-A" option:

$ nix-env -iA nixos.pkgs.thunderbird

The advantages of using a channel over the previous way (doing "svn up
/etc/nixos/*" a.k.a. nixos-checkout) are:

- Correctness: The channel is only updated to a new revision of NixOS/Nixpkgs if
it passes certain tests - specifically, if all jobs in the "nixos:tested" view
succeed (http://hydra.nixos.org/view/nixos/tested).  So you have a reasonable
certainty that the new version will actually work.  By contrast, if you update
to the latest revision using nixos-checkout, it's quite possible that somebody
just committed something that broke the entire system.

- Efficiency: The channel is only updated after all Hydra builds in that
revision have finished, *and* all binaries have been mirrored.  (Binary patches
are generated asynchronously, however, since they can take much longer to
produce.)  So you don't have to worry about somebody just having committed
something that causes a huge recompilation.

This makes keeping NixOS up to date a rather more pleasant activity, IMHO.

Some things to note:

- The downloaded channel sources are stored in
/nix/var/nix/profiles/per-user/root/channels/nixos (which is actually in the Nix
store).  The $NIX_PATH environment variable contains this directory, so <nixos>
and <nixpkgs> in Nix expressions resolve there unless overriden through
$NIX_PATH or the "-I" flag.

- In principle, you can do rollbacks on the channels profile ("nix-env -p
/nix/var/nix/profiles/per-user/root/channels --rollback") if you don't like the
result of a "nix-channel --update".  However, this doesn't roll back the
corresponding binaries manifest in /nix/var/nix/manifest so its utility is
limited [1].

- The NixOS installation CD no longer contains or installs
/etc/nixos/{nixos,nixpkgs} but instead uses the NixOS channel.  You can of
course run "nixos-checkout" if you want a working tree for development.  (In
other news, "nixos-checkout" now accepts a argument to specify the desired
location of the working trees, e.g. "nixos-checkout /home/eelco/Dev".  The
default location /etc/nixos may eventually go away since it's a weird location
for keeping large source trees.)

- To use a development tree in nixos-rebuild, just use the -I flag, e.g.
"nixos-rebuild switch -I /home/eelco/Dev".

- The "nixpkgs_sys" link in ~/.nix-defexpr is no longer created.  Instead you
can use "nixos.pkgs" in "nix-env -iA" calls, as shown above.  "nixos.pkgs" is
the Nixpkgs in the NixOS channel, so it's guaranteed to have binaries available.

- The command "nixos-version" prints out the NixOS version you're currently
running, e.g. "0.1pre34067-34075".  The version string include the NixOS and
Nixpkgs revisions, respectively.  If you install from SVN it just prints
"0.1pre-svn".

- Old versions can be installed from http://nixos.org/releases/nixos/.  These
are periodically deleted because they take up a lot of space.  (Note that the
nixos-unstable channel is really just a link to the latest NixOS release.)

[1] In the future we may get rid of manifests, in which case the Right Thing
would happen magically.

tl;dr

-- 
Eelco Dolstra | http://www.st.ewi.tudelft.nl/~dolstra/