#+TITLE: NixOS
#+DESCRIPTION: NixOS
#+TAGS: GNU/Linux
#+TAGS: nixos
#+TAGS: nix
#+DATE: 2019-07-19
#+SLUG: nixos
#+LINK: ansible https://www.ansible.com/
#+LINK: configuration-management-wiki https://en.wikipedia.org/wiki/Software_configuration_management
#+LINK: fhs https://refspecs.linuxfoundation.org/fhs.shtml
#+LINK: glibc https://www.gnu.org/software/libc/
#+LINK: gnu https://www.gnu.org
#+LINK: guile-scheme https://www.gnu.org/software/guile/
#+LINK: guix https://guix.gnu.org/
#+LINK: linux https://www.kernel.org/
#+LINK: man-symlink-2 http://man7.org/linux/man-pages/man2/symlink.2.html
#+LINK: mozilla-firefox https://www.mozilla.org/en-US/firefox/
#+LINK: nftables https://wiki.nftables.org/wiki-nftables/index.php/Main_Page
#+LINK: nix https://nixos.org/nix/
#+LINK: nix-expression https://nixos.wiki/wiki/Nix_Expression_Language
#+LINK: nix-expressions https://nixos.wiki/wiki/Nix_Expression_Language
#+LINK: nix-paper https://www.usenix.org/legacy/events/lisa04/tech/full_papers/dolstra/dolstra.pdf
#+LINK: nixos https://nixos.org/
#+LINK: nixos-manual https://nixos.org/nixos/manual/index.html
#+LINK: nixos-paper https://nixos.org/~eelco/pubs/nixos-icfp2008-final.pdf
#+LINK: puppet https://puppet.com/
#+LINK: python https://python.org
#+LINK: salt-stack https://www.saltstack.com/
#+LINK: wiki-dsl https://en.wikipedia.org/wiki/Domain-specific_language
#+LINK: wiki-dsl https://en.wikipedia.org/wiki/Domain-specific_language
#+LINK: wiki-luks https://en.wikipedia.org/wiki/Linux_Unified_Key_Setup
#+LINK: wiki-raid https://en.wikipedia.org/wiki/RAID
#+LINK: wiki-unix-epoch https://en.wikipedia.org/wiki/Unix_time


#+BEGIN_PREVIEW
[[nixos][NixOS]] is a new kind of [[gnu][GNU]]/[[linux][Linux]] distribution,
borrowing the ideas of functional programming languages to bring about a
revolution of how we think about operating systems and software development.
#+END_PREVIEW

** NixOS

[[nixos][NixOS]] is a "functional" [[gnu][GNU]]/[[linux][Linux]] distribution
extending the ideas of functional programming languages to operating systems.

To illustrate, most operating systems are more analogous to imperative
languages where the programmer instructs, via the various incantations of the
language, the computer to perform some operation.  Many of these instructions
modify state-- variables, files, network packets.  Functional programming
languages can be instructive, but many of them feel more declarative in nature.
That is, instead of telling what the computer to do, the programmer defines the
result of the computation.  Side-effects, as they are known, are either
disallowed completely or require a certain amount of "ceremony" to be
performed.

Following this imperfect analogy, [[gnu][GNU]]/[[linux][Linux]] distributions
typically follow the imperative paradigm: the operator issues instructions that
modify the state of the system.  Installing and configuring some software
package, for example, requires the operator to run a series of commands that
first install the package, another series of commands to configure the software
to the desired state, and finally, the system and the resulting software is
ready for use.

In contrast, [[nixos][NixOS]] follows the functional paradigm.  The entire
system, from the boot loader to the available software and its configuration
can all be traced to a single file: ~/etc/nixos/configuration.nix~.

In the system configuration file for [[nixos][NixOS]], we declare the various
end states of the system:

- system packages

- system services

- users and groups

- kernel modules loaded during boot

- boot loader configuration

- mount points

The vast majority of the system can be codified into this configuration file.
Currently missing is disk partitioning and allocation and user data.

To understand better the concepts behind [[nixos][NixOS]], we will take a brief
detour through [[nix][Nix]], the package manager.

*** Nix (the package manager)

Beyond the comparisons of language paradigms, there are some other _really_
neat ideas that come from the [[nix][Nix]] family.  Specifically, I want to
discuss the dependency management of [[nix][Nix]] and, therefore,
[[nixos][NixOS]].

Recall the description of the "imperative" operating systems above: many
software packages have dependencies on various other components and packages.
Typically the distribution managers resolve the dependencies into a coherent
tree such that all packages resolve to the correct (read available) version of
[[glibc][~glibc~]].  However, maintainers are human and maintainership is
non-trivial.  Packages slip through, and core breakages happen.  Worse, the
possible matrix of configurations and various packages is mind-numbingly
large.

Concretely, what this may look like can be demonstrated by a simple example.
Let's say an operator installs package ~A~ which depends on package ~C~.
Later, our example operator installs another package ~B~, which also depends on
package ~C~.  For now, we will say that package ~C~ is the same version.  So
far, all is good.  Nothing is broken, and the system is stable.  However, some
time later, package ~B~ needs to be updated and causes a resulting update to
package ~C~.  Now package ~A~ may not work.  Its version of package ~C~ is now
replaced by the version pulled in by package ~B~.  This may be fine, but by a
similar token, it may also be plainly broken.  Worse, it may be only _subtly_
broken, the breakage is not noticed by cursory testing.

Using [[nix][Nix]], this situation is impossible.  Package ~A~ has its own
complete dependency graph, including package ~C~'s dependency graph.  The same
holds for package ~B~, [[nix][Nix]] stores the entire dependency graph of each
package separate from each other package.

Therefore, given our above example of package management mishaps, when package
~B~ updates and pulls in a new version of package ~C~.  The version of ~C~ used
by ~A~ is left untouched and package ~A~ works just the same as before.

*** Nix Store

After examining the above example, how does [[nix][Nix]] accomplish this?

If we think about the [[fhs][Filesystem Hierarchy Standard]], an executable
package is installed into some ~bin~ directory, its dynamically linked objects
are in a ~lib~ directory, its configuration may be in a ~etc~ directory.

With a [[nix][Nix]] package, the package has all same directories, however,
they are isolated in the "store".  The [[nix][Nix]] store is typically a
directory ~/nix/store/~ that contains each package in its ~hash-name-version~
folder.  For example, let's look at [[mozilla-firefox][Firefox]]:

#+begin_example
% ls -al $(which firefox)
lrwxrwxrwx 1 root root 68 Dec 31  1969 /run/current-system/sw/bin/firefox -> /nix/store/78gl44fjjira5jsgyj8vdwsnw8wdwngs-firefox-68.0/bin/firefox
% ls -l /nix/store/78gl44fjjira5jsgyj8vdwsnw8wdwngs-firefox-68.0/
total 0
dr-xr-xr-x 2 root root 21 Dec 31  1969 bin
dr-xr-xr-x 3 root root 21 Dec 31  1969 lib
dr-xr-xr-x 2 root root 42 Dec 31  1969 nix-support
dr-xr-xr-x 4 root root 39 Dec 31  1969 share
#+end_example

There's a few things to notice here.  One, the directories are all read-only;
two, the modified time is set to [[wiki-unix-epoch][UNIX Epoch]]; third, the
entire folder structure necessary for [[mozilla-firefox][Firefox]] to run is in
this store.

Let's examine a package that requires dynamic linking to correctly execute, the
[[python][Python]] interpreter:

#+begin_example
% ls -l $(which python)
lrwxrwxrwx 1 root root 68 Dec 31  1969 /run/current-system/sw/bin/python -> /nix/store/dmh36s38dcpc91grfsh6wqrm65rz5hfh-pythonOverlay/bin/python
% ls -l /nix/store/dmh36s38dcpc91grfsh6wqrm65rz5hfh-pythonOverlay
total 4
dr-xr-xr-x 2 root root 4096 Dec 31  1969 bin
lrwxrwxrwx 1 root root   65 Dec 31  1969 include -> /nix/store/10rqw9cx8x2knwdaxhlyb4drla8v8zzk-python3-3.7.4/include
dr-xr-xr-x 3 root root  113 Dec 31  1969 lib
dr-xr-xr-x 3 root root   17 Dec 31  1969 share
#+end_example

Now, let's examine the linked objects:

#+begin_example
% ldd $(which python)
linux-vdso.so.1 (0x00007ffd3c3d9000)
libpython3.7m.so.1.0 => /nix/store/10rqw9cx8x2knwdaxhlyb4drla8v8zzk-python3-3.7.4/lib/libpython3.7m.so.1.0 (0x00007f6ccf964000)
libpthread.so.0 => /nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib/libpthread.so.0 (0x00007f6ccf943000)
libdl.so.2 => /nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib/libdl.so.2 (0x00007f6ccf93e000)
libcrypt.so.1 => /nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib/libcrypt.so.1 (0x00007f6ccf904000)
libncursesw.so.6 => /nix/store/adc71v5apk4dzcxg7cjqgszjg1a6pd0z-ncurses-6.1-20190112/lib/libncursesw.so.6 (0x00007f6ccf892000)
libutil.so.1 => /nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib/libutil.so.1 (0x00007f6ccf88b000)
libm.so.6 => /nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib/libm.so.6 (0x00007f6ccf6f5000)
libgcc_s.so.1 => /nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib/libgcc_s.so.1 (0x00007f6ccf4df000)
libc.so.6 => /nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib/libc.so.6 (0x00007f6ccf329000)
/nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib/ld-linux-x86-64.so.2 => /nix/store/681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27/lib64/ld-linux-x86-64.so.2 (0x00007f6ccfccf000)
#+end_example

Each of the linked objects, sans the kernel object, are found in the
[[nix][Nix]] store.

Furthermore, if we examine the [[python][Python]] ~lib~ folder:

#+begin_example
% ls -l /nix/store/dmh36s38dcpc91grfsh6wqrm65rz5hfh-pythonOverlay/lib
total 12
lrwxrwxrwx 1 root root   78 Dec 31  1969 libpython3.7m.so -> /nix/store/10rqw9cx8x2knwdaxhlyb4drla8v8zzk-python3-3.7.4/lib/libpython3.7m.so
lrwxrwxrwx 1 root root   82 Dec 31  1969 libpython3.7m.so.1.0 -> /nix/store/10rqw9cx8x2knwdaxhlyb4drla8v8zzk-python3-3.7.4/lib/libpython3.7m.so.1.0
lrwxrwxrwx 1 root root   75 Dec 31  1969 libpython3.so -> /nix/store/10rqw9cx8x2knwdaxhlyb4drla8v8zzk-python3-3.7.4/lib/libpython3.so
lrwxrwxrwx 1 root root   71 Dec 31  1969 pkgconfig -> /nix/store/10rqw9cx8x2knwdaxhlyb4drla8v8zzk-python3-3.7.4/lib/pkgconfig
dr-xr-xr-x 3 root root 8192 Dec 31  1969 python3.7
#+end_example

The shared objects and folders are also symbolic links to other packages and
folders in the [[nix][Nix]] store.

This leads us to the following observation: packages in the [[nix][Nix]] store
are comprised of the outputs of the package and associated symbolic links to
the package's inputs.

*** Nix Profiles

Packages in [[nix][Nix]] are directory trees found in the [[nix][Nix]] store,
what are profiles?  Perhaps, more appropriately, what are user environments?

When setting up [[nix][Nix]] as a package manager either in [[nixos][NixOS]] or
a different [[gnu][GNU]]/[[linux][Linux]] distribution, there's typically a
symbolic link in the user's home directory:

#+begin_example
% ls -l ~/.nix-profile
lrwxrwxrwx 1 kb users 41 May 15  2017 /home/kb/.nix-profile -> /nix/var/nix/profiles/per-user/kb/profile
#+end_example

However, this link is to another link.  Let's follow the rabbit:

#+begin_example
% ls -l /nix/var/nix/profiles/per-user/kb/profile
lrwxrwxrwx 1 kb users 14 Jul 30 15:15 /nix/var/nix/profiles/per-user/kb/profile -> profile-3-link
#+end_example

This indirect symbolic link points to a symbolic link in the same directory.
Let's keep following:

#+begin_example
% ls -l /nix/var/nix/profiles/per-user/kb/profile-3-link
lrwxrwxrwx 1 kb users 60 Jul 30 15:15 /nix/var/nix/profiles/per-user/kb/profile-3-link -> /nix/store/d7d6hcv8v2crb98nhh00nrr2bkh034kc-user-environment
% ls -l /nix/store/d7d6hcv8v2crb98nhh00nrr2bkh034kc-user-environment
dr-xr-xr-x 2 root root 156 Dec 31  1969 bin
lrwxrwxrwx 1 root root  63 Dec 31  1969 etc -> /nix/store/k0hyks88khah4hvb19i0d6swsawyzz5a-awscli-1.16.170/etc
dr-xr-xr-x 2 root root  35 Dec 31  1969 lib
lrwxrwxrwx 1 root root  60 Dec 31  1969 manifest.nix -> /nix/store/lqc7v4cb77fzgnrqn0miz1fpkzb3dxc2-env-manifest.nix
lrwxrwxrwx 1 root root  65 Dec 31  1969 share -> /nix/store/k0hyks88khah4hvb19i0d6swsawyzz5a-awscli-1.16.170/share
#+end_example

After following three links, we are referred to a directory tree in the
[[nix][Nix]] store.

Let's examine the ~bin~ directory quickly:

#+begin_example
% ls -l /nix/store/d7d6hcv8v2crb98nhh00nrr2bkh034kc-user-environment/bin
total 0
lrwxrwxrwx 1 root root 67 Dec 31  1969 aws -> /nix/store/k0hyks88khah4hvb19i0d6swsawyzz5a-awscli-1.16.170/bin/aws
lrwxrwxrwx 1 root root 82 Dec 31  1969 aws_bash_completer -> /nix/store/k0hyks88khah4hvb19i0d6swsawyzz5a-awscli-1.16.170/bin/aws_bash_completer
lrwxrwxrwx 1 root root 77 Dec 31  1969 aws_completer -> /nix/store/k0hyks88khah4hvb19i0d6swsawyzz5a-awscli-1.16.170/bin/aws_completer
lrwxrwxrwx 1 root root 89 Dec 31  1969 cargo-generate-nixfile -> /nix/store/s11zqp9r8h4r65iqv272b477igb9a9mw-rust_carnix-0.10.0/bin/cargo-generate-nixfile
lrwxrwxrwx 1 root root 91 Dec 31  1969 cargo_generate_nixfile.d -> /nix/store/s11zqp9r8h4r65iqv272b477igb9a9mw-rust_carnix-0.10.0/bin/cargo_generate_nixfile.d
lrwxrwxrwx 1 root root 73 Dec 31  1969 carnix -> /nix/store/s11zqp9r8h4r65iqv272b477igb9a9mw-rust_carnix-0.10.0/bin/carnix
lrwxrwxrwx 1 root root 75 Dec 31  1969 carnix.d -> /nix/store/s11zqp9r8h4r65iqv272b477igb9a9mw-rust_carnix-0.10.0/bin/carnix.d
#+end_example

Currently, this example profile only has two packages installed:

#+begin_example
% nix-env -q
awscli-1.16.170
rust_carnix-0.10.0
#+end_example

But from this example so far, we see that user environments are comprised of
symbolic link "forests" of the packages that make up the current profile.

Let's follow this example again, however, we going to modify the user
environment by adding a package:

#+begin_example
% nix-env -i autogen
#+end_example

Starting with second link:

#+begin_example
% ls -l /nix/var/nix/profiles/per-user/kb/profile
lrwxrwxrwx 1 kb users 14 Aug 13 05:52 /nix/var/nix/profiles/per-user/kb/profile -> profile-4-link
#+end_example

The profile link now points to a different link.  Let's keep going:

#+begin_example
% ls -l /nix/var/nix/profiles/per-user/kb/profile-4-link
lrwxrwxrwx 1 kb users 60 Aug 13 05:52 /nix/var/nix/profiles/per-user/kb/profile-4-link -> /nix/store/xslnn9gs5gkgdvzgb0w3b0iggbsszag5-user-environment
#+end_example

The user-profile now points to a completely different symlink forest in the
[[nix][Nix]] store.

The old profile still exists.  Let's switch (rollback) to it:

#+begin_example
% nix-env --rollback                                                (1)
switching from generation 4 to 3
% ls -l /nix/var/nix/profiles/per-user/kb/profile
lrwxrwxrwx 1 kb users 14 Aug 13 05:56 /nix/var/nix/profiles/per-user/kb/profile -> profile-3-link
% ls -l /nix/var/nix/profiles/per-user/kb/profile-3-link
lrwxrwxrwx 1 kb users 60 Jul 30 15:15 /nix/var/nix/profiles/per-user/kb/profile-3-link -> /nix/store/d7d6hcv8v2crb98nhh00nrr2bkh034kc-user-environment
#+end_example

Rolling back to a previous profile was effortless and we went back to _exactly_
the same store path that we had previously.

Since the [[man-symlink-2][~symlink(2)~]] operation is atomic, changing profile
generations is atomic.  Adding a package to the profile is atomic: that is,
once the package is downloaded, built, added to the store, and the set of links
are compiled into a new profile, the switch to this new profile is entirely
atomic.  If the any of the previous steps fail, the user profile is not
adversely affected.

*** System Profiles

After user profiles, we are left with system profiles.  What exactly is
[[nixos][NixOS]]?  If any of the above provides any foreshadowing, the answer
may seem obvious: a system profile is a forest of symbolic links to the
packages, services, and other system configuration that comprise a
[[gnu][GNU]]/[[linux][Linux]] system.

However, that may not be obvious.

Let's start by first reiterating that [[nixos][NixOS]] is different than
traditional [[gnu][GNU]]/[[linux][Linux]] distributions, very different.  One
of the most notable differences that is important to this discussion is the
lack of adherence to the [[fhs][Filesystem Hierarchy Standard]].  Chiefly, in
the root of the filesystem of a [[nixos][NixOS]] system, there is almost no
need for ~/bin~, ~/usr~, and ~/lib~.

#+begin_example
% ls -l /
total 57
drwxr-xr-x   2 root root  4096 Aug  2 09:04 bin
drwxr-xr-x   5 root root  1024 Jun  5 17:01 boot
drwxr-xr-x  21 root root  4140 Aug 13 05:37 dev
drwxr-xr-x  26 root root  4096 Aug  2 09:04 etc
drwxr-xr-x   3 root root    19 May 16 10:54 gnu
drwxr-xr-x   3 root root    29 May  8 07:24 home
drwx------   2 root root 16384 Jun  5 16:02 lost+found
drwxr-xr-x   4 root root    30 May 16 09:49 nix
drwxr-xr-x   5 root root  4096 Jun  5 20:01 opt
dr-xr-xr-x 257 root root     0 Aug  2 09:03 proc
drwx------   6 root root  4096 Aug 12 23:04 root
drwxr-xr-x  20 root root   640 Aug 14 20:56 run
dr-xr-xr-x  13 root root     0 Aug  2 09:03 sys
drwxrwxrwt  55 root root 16384 Aug 14 21:00 tmp
drwxr-xr-x   3 root root  4096 Jun  5 17:01 usr
drwxr-xr-x   9 root root  4096 Aug  2 09:04 var
#+end_example

In the above output, there /is/ both ~/bin~ and ~/usr~, but no ~/lib~.  What is
in ~/bin~ and ~/usr~?  Two things: one, ~/bin/sh~ and two, ~/usr/bin/env~.
These are kept around as ways to resolve issues with porting packages into the
[[nix][Nix]] environment.

#+begin_example
% ls -l /bin/
total 4
lrwxrwxrwx 1 root root 75 Aug  2 09:04 sh -> /nix/store/93h01q6yg13xdrabvqbddzbk11w6a928-bash-interactive-4.4-p23/bin/sh
% ls -lR /usr
/usr:
total 4
drwxr-xr-x 2 root root 4096 Aug  2 09:04 bin
/usr/bin:
total 4
lrwxrwxrwx 1 root root 66 Aug  2 09:04 env -> /nix/store/d9s1kq1bnwqgxwcvv4zrc36ysnxg8gv7-coreutils-8.30/bin/env
#+end_example

Notice, however, that these files are in fact symbolic links into the
[[nix][Nix]] store.

If there is nothing in ~/bin~ and nothing in ~/usr~, where does the system find
all of the install programs?

The answer: ~/run/current-system~:

#+begin_example
% ls -l /run/current-system
lrwxrwxrwx 1 root root 88 Aug  2 09:04 /run/current-system -> /nix/store/9c3k3ky5lg3x937984902v1d7148m7c5-nixos-system-phenex-19.03.173147.77295b0bd26
% ls -l /nix/store/9c3k3ky5lg3x937984902v1d7148m7c5-nixos-system-phenex-19.03.173147.77295b0bd26
total 48
-r-xr-xr-x 1 root root 16455 Dec 31  1969 activate
lrwxrwxrwx 1 root root    91 Dec 31  1969 append-initrd-secrets -> /nix/store/vynm9pvxlzd8rracmmkhpj2a3g79whbw-append-initrd-secrets/bin/append-initrd-secrets
dr-xr-xr-x 2 root root    37 Dec 31  1969 bin
-r--r--r-- 1 root root     0 Dec 31  1969 configuration-name
lrwxrwxrwx 1 root root    51 Dec 31  1969 etc -> /nix/store/a04f5cdfinc8p4n6x0hw9a0jn5l2mi9i-etc/etc
-r--r--r-- 1 root root    57 Dec 31  1969 extra-dependencies
dr-xr-xr-x 2 root root     6 Dec 31  1969 fine-tune
lrwxrwxrwx 1 root root    65 Dec 31  1969 firmware -> /nix/store/ak22608y0db7m4bzwmps23gi4f0s13dc-firmware/lib/firmware
-r-xr-xr-x 1 root root  5568 Dec 31  1969 init
-r--r--r-- 1 root root     9 Dec 31  1969 init-interface-version
lrwxrwxrwx 1 root root    57 Dec 31  1969 initrd -> /nix/store/n7x32hhg41mflx9xvmmw61piwjdr81m1-initrd/initrd
lrwxrwxrwx 1 root root    65 Dec 31  1969 kernel -> /nix/store/sgkk7pqh7jqvy6rvgnkk367amrpknw91-linux-4.19.59/bzImage
lrwxrwxrwx 1 root root    58 Dec 31  1969 kernel-modules -> /nix/store/nrmlrwxyqmp4dbcldrlvibv0h61356bf-kernel-modules
-r--r--r-- 1 root root    10 Dec 31  1969 kernel-params
-r--r--r-- 1 root root    24 Dec 31  1969 nixos-version
lrwxrwxrwx 1 root root    55 Dec 31  1969 sw -> /nix/store/11pfbzzamqvnbfxis4pbnzhrvarn3pj1-system-path
-r--r--r-- 1 root root    12 Dec 31  1969 system
lrwxrwxrwx 1 root root    64 Dec 31  1969 systemd -> /nix/store/9zkhhvix7rlqlj8pf8s2kbw8b88rky75-systemd-239.20190219
#+end_example

The various files and directories in this derivation are what is necessary for
the current generation of the system.  Similar to user environments, upgrades
and rollbacks are atomic.  Installing a package into the system packages, for
example, will happen in isolation.  Only after the build process is complete
and successful does the forest of links get changed.

However, because there is some necessary artifacts of state when running a
system, this isolation is certainly not perfect.  Particularly so with the
interaction of services and their underlying configuration.

** Alternatives

Aside from [[nixos][NixOS]], there is also [[guix][Guix]], a [[gnu][GNU]]
alternative to [[nix][Nix]] and [[nixos][NixOS]].  Many of the ideas of
[[guix][Guix]] are derived from [[nix][Nix]].  In fact, early on in the life of
the [[guix][Guix]] project, [[guix][Guix]] interacted with the [[nix][Nix]]
daemon directly.  This is no longer the case as [[guix][Guix]] has its own
daemon now, however, the design is very similar.

The configuration language of [[guix][Guix]] is, in proper [[gnu][GNU]] style,
[[guile-scheme][Guile Scheme]] instead of a [[wiki-dsl][DSL]]--
[[nix-expressions][nix]].

[[guix][Guix]], as a [[gnu][GNU]] project, also takes a hard-line stance on
software freedom and therefore does not and will not include any non-free code
in the package repositories.  Furthermore, this also means that [[guix][Guix]]
the package manager will not be supported on other operating systems such as
Apple MacOS and Microsoft Windows.

Another alternative is using any number of
[[configuraiton-management-wiki][Configuration Management]] solutions on a
typical [[gnu][GNU]]/[[linux][Linux]] distribution.  However, solutions such as
[[ansible][Ansible]], [[puppet][Puppet]], and [[salt-stack][Salt]] fail in a
very similar way that tradition distrubtions fail: they are dependent on
ordering and distrubtion managers and software maintainers to create
appropriate software dependency graphs.  Failure in dependency management
yields failures in the system.  Furthermore, once a package is no longer
available in the official repositories, it is no longer trivially available to
be installed via the software configuraiton tools.

** Impressions and Thoughts

[[nix][Nix]] and by extension [[nixos][NixOS]] (hopefully) solve some really
annoying problems I tend to keep running into when developing software and
immutable, reproducible infrastructure.  I'm really excited about the
possibilities [[nix][Nix]] can bring.  However, until I can really sit down and
_use_ [[nix][Nix]] for development, I can't say anything with certainty.

In time, I will provide more detail on my impressions and review [[nix][Nix]]
and [[nixos][NixOS]] in more depth.  However, if nothing else, being able to
codify systems and environments in a reproducible manner is already a huge win.

I highly recommend reading [[nixos-paper][NixOS: A Purely Functional Linux
Distribution]] and [[nix-paper][Nix: A Safe and Policy-Free System for Software
Development]] by Dolstra /et al./.  Both papers are very good explanations and
breakdowns of the motivation and ideas behind [[nix][Nix]] and
[[nixos][NixOS]].