Managing user environments with Nix

For some reason, I'm having quite some discussions about Nix's user environment mechanism lately. For example, in my last two talks I have spent a considerable amount of time answering questions that were related to it.

The user environment mechanism is an important key feature of Nix, which primary purpose is to allow someone to conveniently launch programs installed by the Nix package manager, but it has a few additional use cases as well, such as determining which packages are in use (and which are not) in order to safely garbage collect obsolete packages.

I have only briefly described them in some of my blog posts, but I have never covered in detail how they work on my blog. Because of the amount of questions I have received lately, I think it would be a good idea to write about it.

The Nix store

As described in my exhaustive explanation blog post on Nix (and the original explanation recipe), every Nix package is stored in an isolated directory in a so called "Nix store". In the Nix store every package has a special filename that may look like this:

/nix/store/spy13ngvs1fyj82jw2w3nwczmdgcp3ck-firefox-23.0.1

The first part of the basename (spy13ngvs1fyj82jw2w3nwczmdgcp3ck) is a cryptographic hash derived from all build inputs used to build Firefox. For example, if we would build Firefox with a different compiler or for a different architecture, the generated hash code will be different, which makes it possible to safely store multiple variants of packages next to each other, because none of them will have the same name.

Although this storing convention looks a bit odd, it provides a number of interesting benefits. Since Nix executes builds inside environments in which as many side effects are removed, we can use the hash codes to uniquely identify package variants regardless on what machine they have been built.

They also provide better reproducibility, since these paths to Nix packages including their hash codes must always be explicitly specified, in order to allow a package build to find its dependencies. The advantage of this is that unspecified dependencies cannot accidentally allow a build to succeed, limiting reproducibility guarantees.

More details on the benefits can be found in my two explanation recipes. However, storing packages this way also causes a major inconvenience for end-users. To launch a particular program, the user must always provide the full path to the executable that resides inside the Nix store, which is quite impractical due to the fact that every Nix package has a hash code in it.

Nix profiles

As a means for users to conveniently launch software, the Nix package manager provides the concept of Nix profiles, which are user environments exposing a generation of installed packages to a user. Software packages can be automatically installed in Nix profiles by using the nix-env command-line utility.

To install software, such as Info ZIP's zip and unzip utilities, we can run:

$ nix-env -i zip unzip
installing `zip-3.0'
installing `unzip-6.0'
building path(s) `/nix/store/1zhgva9cmfz0wam9m7ibgaih2m3cic6l-user-environment'
created 33 symlinks in user environment

Allowing us to run any of the installed tools without specifying any paths with hash codes, e.g.:

$ zip --version
Copyright (c) 1990-2008 Info-ZIP - Type 'zip "-L"' for software license.
This is Zip 3.0 (July 5th 2008), by Info-ZIP.
Currently maintained by E. Gordon.  Please send bug reports to
the authors using the web page at www.info-zip.org; see README for details.

One of the things the above nix-env command-line invocation does is automatically building the requested packages (or downloading its substitutes) and composing a user environment, which is a symlink tree that synthesizes the contents of the currently installed packages (zip and unzip) into one single component. A user environment is also a package residing in the Nix store, as shown in the following figure:

Besides creating a user environment that exposes all installed packages from a single location, a few additional symlink indirections are used. Each time nix-env performs a deployment activity, a generation symlink is generated (called profile-1 in the figure above). There is also a default symlink (profile) pointing to the generation symlink that is currently active. That symlink is also referenced from the user's home directory through ~/.nix-profile. By adding the latter symlink to the user's PATH, e.g.:

export PATH=~/.nix-profile/bin:$PATH

Packages installed through nix-env can be found automatically through a few layers of symlink indirections.

When we perform another deployment activity with nix-env, such as removing zip:

$ nix-env -e zip
uninstalling `zip-3.0'

Then a new symlink tree is composed containing the then currently installed packages (in this case only unzip), a new generation symlink is generated (profile-2), and the default symlink is updated to point to the new generation. Flipping symlinks is actually an atomic operation on UNIX. As a consequence, we can perform upgrades atomically, because there is never a situation in which we refer to packages belonging to the old and new user environment at the same time.

As may be observed in the picture above, the old generation user environment is still present allowing us to do rollbacks as well. For example, if we regret uninstalling zip, we can switch back to the previous generation by running:

$ nix-env --rollback

Then the default symlink's reference is updated to point to the previous generation symlink that provides the zip utility. Again, this is an atomic operation.

Apart from exposing packages installed by users, the directory containing profile generations also serves a second goal; it is used to determine which packages are in use and which are not.

The generations of the Nix profiles of all users of the system are treated as so called garbage collector roots -- any package that is installed in a profile generation and any of its dependencies are considered packages that are live (a.k.a. in use) and should not be garbage collected. Furthermore, Nix also treats open files and running processes as garbage collector roots. Therefore, it is safe to run the Nix garbage collector at any time.

We can also remove older Nix profile generations if we don't need them anymore:

$ nix-env --delete-generations old

The above command only removes the older generation symlinks, but not the packages that have become obsolete. To actually remove packages from the system that are no longer in use, we must explicitly run the garbage collector:

$ nix-collect-garbage

So far, I have shown examples that work on a per-user profile. In multi-user Nix installations, such as in NixOS, every user has a personal Nix profile (residing in /nix/var/nix/profiles/per-user) and there is also a system-wide default Nix profile (residing in /nix/var/nix/profiles/default) containing packages that are exposed to all users of a system.

NixOS system profiles

Apart from individual packages, NixOS -- the Linux distribution built around the Nix package manager, also uses Nix profiles to manage generations of system configurations.

As described in my earlier blog post about NixOS, a complete system configuration is deployed from a single declarative specification, including all system services and their configurations, the desktop, and user packages. Each time we deploy a system configuration, e.g. by running:

$ nixos-rebuild switch

a Nix profile is built which consists of (mostly) a symlink tree containing all packages and configuration files that constitutes all static parts of a system, including the Linux kernel and the kernel modules. This profile is called a system profile and offers the same benefits as ordinary package profiles, such as the fact that we can store multiple generations next to each other and perform atomic upgrades and rollbacks.

There is a subtle difference with NixOS deployment compared to ordinary package deployment. Merely flipping symlinks is not enough to get a new NixOS configuration deployed. We also have to perform a collection of imperative deployment steps, such as setting up dynamic directories, such as /var, starting and stopping system services, and updating the GRUB bootloader configuration. This is done by an activation script (/run/current-system/bin/switch-to-configuration) part of the NixOS system profile, that is executed each time a new version of a NixOS configuration is deployed.

As can be seen in the screenshot above, once a new NixOS system configuration has been deployed, the GRUB bootloader can be used to boot into any previous NixOS configuration that has not been garbage collected yet.

Generating user environments from Nix expressions

In addition to using nix-env, user environments can also be generated from Nix expression by using the buildEnv {} function. The following Nix expression produces the same user environment (shown earlier) containing zip and unzip:

with import <nixpkgs> {};

buildEnv {
  name = "zipprofile";
  paths = [ zip unzip ];
}

This expression can be built by running:

$ nix-build zipprofile.nix

Conclusion

In this blog post, I have explained how the Nix user environment mechanism works. Essentially user environments are symlink trees that blend all currently installed packages into a single component so that they can be referenced from a single location. Another symlink layer manages generations of user environments. We can atomically switch between these generations by flipping symlinks. By adding the latter symlink to a user's PATH, packages can be automatically found without providing any absolute paths with hash codes in them that are difficult to remember.

Besides exposing packages in a user's PATH, Nix profiles are also used in NixOS to automatically find other resources, such as manual pages (through MANPATH) and KDE desktop menu items (through XDG_DATA_DIRS).

More information on Nix profiles can be found in Eelco Dolstra's PhD thesis which can be obtained from his publications page. Additionally, my PhD thesis also covers Nix profiles in its background chapter. Furthermore, there is a chapter that extends their concepts to distributed systems. This chapter is loosely based on our HotSWUp 2008 paper titled: "Atomic Upgrading of Distributed Systems" which can be obtained from my publications page.