2022-02-27 19:12:53 +00:00
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---
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title: "Nix Flakes: Packages and How to Use Them"
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date: 2022-02-27
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tags:
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- nix
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- nixos
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- docker
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- systemd
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series: nix-flakes
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vod:
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twitch: https://www.twitch.tv/videos/1409855764
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youtube: https://youtu.be/eUFBD-6yAWQ
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---
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<div class="warning">
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[Nix flakes are still marked as experimental. This documentation has a small
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chance of bitrotting. I will make every attempt to update it if things change,
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however flakes have been fairly consistent for a few years
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now.](conversation://Cadey/coffee)
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2022-04-07 01:43:45 +00:00
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[EDIT(20220327 14:13): A previous version of this article said to use
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`defaultPackage` for the default package. This is deprecated and you should use
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`packages.default` instead.](conversation://Cadey/coffee)
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2022-02-27 19:12:53 +00:00
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</div>
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[What is a package? I've seen this term thrown around with phrases like "Nix is a
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package manager" or "language-specific package manager" or even "download the
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debian package and install it", but it's not really clear to me what a package
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is. What is a package?](conversation://Mara/hmm)
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A package is a bundle of files. These files could be program executables,
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resources such as stylesheets or images, or even a container image. Most of the
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time you don't deal with packages directly and instead you use a _package
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manager_ (a program whose sole goal in life is to deal with packages) to do
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actions for you. This post is going to cover how to define packages in Nix and
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how Nix flakes let you manage multiple packages per project more easily.
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## What is a Package?
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In Nix, you build packages by creating _derivations_ that define the build steps
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and associated inputs (such as the compiler) to end up with the resulting
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outputs (derivation being the product of deriving something). Consider a package
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like this:
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```nix
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# hello-shell.nix
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with import <nixpkgs> { };
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stdenv.mkDerivation {
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name = "hello-HEAD";
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src = ./.;
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installPhase = ''
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echo "Hello" > $out
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'';
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}
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```
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Then we can build this package with `nix-build hello-shell.nix` and a `result`
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symlink will show up in your current working directory. Then you can view what
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it says with `cat`:
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```console
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$ cat ./result
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Hello
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```
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This is all it takes to make a Nix package. You need to name the package, give
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it input source code somehow, and potentially give it build instructions.
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Everything else we'll cover today will build on top of this.
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Let's look back at the Go [example
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package](https://github.com/Xe/gohello/blob/caf54cdff7d8dd9bd9df4b3b783a72fe75c9a11e/flake.nix#L31-L54)
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I walked us through in [the last
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post](https://christine.website/blog/nix-flakes-1-2022-02-21):
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```nix
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# ...
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packages = forAllSystems (system:
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let pkgs = nixpkgsFor.${system};
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in {
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go-hello = pkgs.buildGoModule {
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pname = "go-hello";
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inherit version;
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# In 'nix develop', we don't need a copy of the source tree
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# in the Nix store.
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src = ./.;
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# This hash locks the dependencies of this package. It is
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# necessary because of how Go requires network access to resolve
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# VCS. See https://www.tweag.io/blog/2021-03-04-gomod2nix/ for
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# details. Normally one can build with a fake sha256 and rely on native Go
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# mechanisms to tell you what the hash should be or determine what
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# it should be "out-of-band" with other tooling (eg. gomod2nix).
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# To begin with it is recommended to set this, but one must
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# remeber to bump this hash when your dependencies change.
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vendorSha256 =
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"sha256-pQpattmS9VmO3ZIQUFn66az8GSmB4IvYhTTCFn6SUmo=";
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};
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});
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# ...
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```
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This uses a different builder, one called
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[`pkgs.buildGoModule`](https://nixos.org/manual/nixpkgs/stable/#ssec-language-go).
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This is like the `stdenv.mkDerivation` builder, except it is explicitly made to
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handle Go projects. There are some other flags that you can set in
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`buildGoModule` that can be useful. You can see examples in the NixOS manual
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page [here](https://nixos.org/manual/nixpkgs/stable/#ssec-language-go).
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Another useful builder is [Naersk](https://github.com/nix-community/naersk).
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Naersk will automatically derive build instructions for Rust projects using the
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`Cargo.toml` and `Cargo.lock` files. This means that your build step can look as
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small as this:
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```nix
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naersk-lib.buildPackage ./.
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```
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[You can think of these builders as templates for doing larger builds. This is
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kinda like <a
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href="https://docs.docker.com/engine/reference/builder/#onbuild">the ONBUILD
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Dockerfile instruction</a>, but it isn't limited to
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Docker. The main difference is that Nix builds are more like functions (inputs
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and outputs) and Docker builds focus on the individual commands you run to get
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the result you want. Both eventually compile down to shell commands
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anyways!](conversation://Mara/hacker)
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## A More Useful Package
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This "hello world" program isn't very useful on its own, however we can use it
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as the basis for making something a bit more useful. I have made a template for
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a "Hello world" HTTP server
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[here](https://github.com/Xe/templates/tree/main/go-web-server). Let's make a
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new folder for it and then initialize it:
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[If you want to make your own templates, see how to do that <a
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href="https://peppe.rs/posts/novice_nix:_flake_templates/">here</a>.](conversation://Mara/hacker)
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```shell
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mkdir -p ~/tmp/gohello-http
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cd ~/tmp/gohello-http
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git init
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nix flake init -t github:Xe/templates#go-web-server
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```
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[You may see a message from <a href="https://direnv.net/">direnv</a> about
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needing to approve its content. This will use Nix flake's cached interpreter to
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give you all the advantages of something like <a
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href="https://github.com/nix-community/lorri">Lorri</a> without having to
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install Lorri.](conversation://Mara/hacker)
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Then make an initial commit and run it:
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```shell
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git add .
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git commit -sm "initial commit"
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nix build
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./result/bin/web-server
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```
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[Why are you using `git add .` everywhere? Shouldn't the files be picked up
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implicitly?](conversation://Mara/hmm)
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[Not always. Nix flakes only deals with files that are tracked by git when you
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use it in a git repository. This means that if you want the changes to be
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observed by Nix, you need to add them to git somehow. `git add` is good enough
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for this.](conversation://Cadey/enby)
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Or you can run it directly with `nix run`:
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```shell
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nix run
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```
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## Docker Images
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Most of the time you will build software with Nix, however that doesn't stop you
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from building things like Docker images with Nix. Remember that you can have the
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output of any shell commands be run in a Nix build (the only catch is that they
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can't access the internet directly), so you can build a Docker image out of that
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web server template by defining another package:
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```nix
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# flake.nix
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packages = {
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default = ...;
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2022-02-27 19:12:53 +00:00
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docker = let
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2022-04-07 01:43:45 +00:00
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web = self.packages.${system}.default;
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in pkgs.dockerTools.buildLayeredImage {
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name = web.pname;
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tag = web.version;
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contents = [ web ];
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config = {
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Cmd = [ "/bin/web-server" ];
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WorkingDir = "/";
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};
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};
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};
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```
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This will build a Docker image with the web-server binary in it. To build it,
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run these commands:
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```shell
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git add .
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nix build .#docker
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```
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[What's with that last argument to `nix build`, won't that be read as a shell
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comment?](conversation://Mara/hmm)
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[It's a reference to the package in the flake. Shell only parses comments when
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the `#` is the first character after whitespace, so this is more of a URL
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fragment than a comment. It's telling `nix build` to build the flake package
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named `docker`.](conversation://Cadey/enby)
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It will put the resulting docker image in `./result`. To load it into docker use
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the following command:
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```console
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$ docker load < result
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Loaded image: web-server:20220227
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```
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[Your image tag may differ depending on when you build this
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image. This is deterministic because that date is derived from the date that the
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current git commit was made.](conversation://Mara/happy)
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Then you can run it with `docker run`:
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```shell
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docker run -itp 3031:3031 web-server:20220227
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```
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Then poke it with curl:
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```console
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$ curl http://[::]:3031
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hello from nix!
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```
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You can push this image to the Docker hub like any other image. Another cool
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thing about this is that when you update the program, it'll only actually load
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the images that changed. Let's edit the hello world message:
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```go
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http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
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fmt.Fprintln(w, "hello from nix building a docker image!")
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})
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```
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And then re-build and load it into Docker:
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```shell
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git add .
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nix build .#docker
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docker load < result
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```
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[Woah, when I did that it only updated 2 layers. The first time that I loaded it
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there were something like 7 layers. What's up with
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that?](conversation://Mara/hmm)
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[When you use `buildLayeredImage`, each Nix package that contributes to the
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image gets put in its own Docker layer. This means that only the things that
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have changed actually need to be considered, so when you push an updated image
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to another machine, the only things that will actually be pushed are the
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application binary and the symlink farm pointing to the `contents` of the Docker
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image.](conversation://Cadey/enby)
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## systemd Portable Services
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2022-02-27 22:27:13 +00:00
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<div class="warning">
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[EDIT(20220227 17:24): It seems that these are even more experimental than I
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thought. systemd Portable Services don't seem to work properly with the
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`StateDirectory` and `CacheDirectory` directives unless you are running a git
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HEAD version of systemd. Maybe you should wait a few years before trying to use
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them for anything serious. By then most of the kinks should be worked
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out.](conversation://Cadey/coffee)
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</div>
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2022-02-27 19:12:53 +00:00
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[systemd Portable Services](https://systemd.io/PORTABLE_SERVICES/) function like
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Docker, but they work at the systemd level and allow you to integrate into
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systemd instead of running on the side of it. This gives you access to systemd's
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readiness signaling, logging pipeline and dependency graph so that you can
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integrate like a native service. They are like containers, but without a lot of
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the headaches around networking, stateful storage and logging. They are just
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systemd services at their core.
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[These are kinda like Ubuntu's Snaps or Flatpaks, but they operate purely at the
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system level and are focused at providing things for system services instead of
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user-facing applications. Ubuntu's Snaps do let you create system services, but
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they are basically exclusively used on Ubuntu. systemd Portable Services let you
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target more than just Ubuntu. In the next few years with more releases of
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systemd, Portable Services should be easier to use and will be more integrated
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with the system than Docker is.](conversation://Mara/hacker)
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2022-03-02 16:23:53 +00:00
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There is currently an [open pull request](https://github.com/NixOS/nixpkgs/pull/161278)
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for adding Portable Service building support to nixpkgs, however we can mess
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around with it today thanks to [my portable-svc
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overlay](https://tulpa.dev/cadey/portable-svc) that copies in the contents of
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that pull request.
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[In Nix, an overlay is a set of additional packages or functions that is put on
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top of nixpkgs. This overlay defines the `portableService` function that is
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needed to build portable services.](conversation://Mara/hacker)
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To make this into a portable service, first we need to add my overlay to the
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flake inputs:
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```nix
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# flake.nix
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inputs = {
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nixpkgs.url = "nixpkgs/nixos-unstable";
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utils.url = "github:numtide/flake-utils";
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portable-svc.url = "git+https://tulpa.dev/cadey/portable-svc.git?ref=main";
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};
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```
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Then add it as an argument to the `outputs` function:
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```nix
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outputs = { self, nixpkgs, utils, portable-svc }:
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```
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And then change how we are importing the `pkgs` variable. The `pkgs` variable
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we're currently using is imported like this:
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```nix
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let pkgs = nixpkgs.legacyPackages.${system};
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```
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This works, however there isn't a way to specify an overlay into this. We need
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to change this into a manual import of nixpkgs with the overlay specified, like
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this:
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```nix
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let pkgs = import nixpkgs {
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overlays = [ portable-svc.overlay ];
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inherit system;
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};
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```
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This will let us use the `portableService` function in Nix package definitions.
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Next we need to make a systemd service unit for the web server. The exact path
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to the program binary can and will change with every build, so it would be good
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to have this templated. Make a folder called `systemd`:
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```shell
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mkdir systemd
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```
|
|
|
|
|
|
|
|
And put the following contents in `systemd/web-server.service.in`:
|
|
|
|
|
|
|
|
```systemd
|
|
|
|
[Unit]
|
|
|
|
Description=A web service
|
|
|
|
|
|
|
|
[Service]
|
|
|
|
DynamicUser=yes
|
|
|
|
ExecStart=@web@/bin/web-server
|
|
|
|
|
|
|
|
[Install]
|
|
|
|
WantedBy=multi-user.target
|
|
|
|
```
|
|
|
|
|
|
|
|
Then under the docker package definition, add the package that will template out
|
|
|
|
the systemd unit:
|
|
|
|
|
|
|
|
```nix
|
|
|
|
web-service = pkgs.substituteAll {
|
|
|
|
name = "web-server.service";
|
|
|
|
src = ./systemd/web-server.service.in;
|
2022-04-07 01:43:45 +00:00
|
|
|
web = self.packages.${system}.default;
|
2022-02-27 19:12:53 +00:00
|
|
|
};
|
|
|
|
```
|
|
|
|
|
|
|
|
You can build it with `nix build .#web-service`, the output will look something
|
|
|
|
like this:
|
|
|
|
|
|
|
|
```systemd
|
|
|
|
[Unit]
|
|
|
|
Description=A web service
|
|
|
|
|
|
|
|
[Service]
|
|
|
|
DynamicUser=yes
|
|
|
|
ExecStart=/nix/store/yl863jm907wfr7gq9j0c4bd3d4bdc4vp-web-server-20220227/bin/web-server
|
|
|
|
|
|
|
|
[Install]
|
|
|
|
WantedBy=multi-user.target
|
|
|
|
```
|
|
|
|
|
|
|
|
[The `@web@` in the template was replaced with the nix store path for the web
|
|
|
|
server!](conversation://Mara/happy)
|
|
|
|
|
|
|
|
Then you can add the bit that builds the portable service:
|
|
|
|
|
|
|
|
```nix
|
|
|
|
portable = let
|
2022-04-07 01:43:45 +00:00
|
|
|
web = self.packages.${system}.default;
|
2022-02-27 19:12:53 +00:00
|
|
|
in pkgs.portableService {
|
|
|
|
inherit (web) version;
|
|
|
|
name = web.pname;
|
|
|
|
description = "A web server";
|
|
|
|
units = [ self.packages.${system}.web-service ];
|
|
|
|
};
|
|
|
|
```
|
|
|
|
|
|
|
|
Then you can build it with `nix build`:
|
|
|
|
|
|
|
|
```shell
|
|
|
|
nix build .#portable
|
|
|
|
```
|
|
|
|
|
|
|
|
And then take a look at `./result`:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ file $(readlink ./result)
|
|
|
|
/nix/store/1da6b90i75n03kqlzzfdwxii0j0bzxaf-web-server_20220227.raw:
|
|
|
|
Squashfs filesystem,
|
|
|
|
little endian,
|
|
|
|
version 4.0,
|
|
|
|
xz compressed,
|
|
|
|
9555806 bytes,
|
|
|
|
2010 inodes,
|
|
|
|
blocksize: 1048576 bytes,
|
|
|
|
created: Tue Jan 1 00:00:00 1980
|
|
|
|
```
|
|
|
|
|
|
|
|
<div class="warning" style="padding:1em">
|
|
|
|
|
|
|
|
At the time of writing this article, the most reliable way to test portable
|
|
|
|
services is to use Arch Linux. So you could use something like
|
|
|
|
[waifud](https://github.com/Xe/waifud) to spin up an Arch Linux VM:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ waifuctl create -d arch -h logos -s 20
|
|
|
|
created instance jangmo-o on logos
|
|
|
|
jangmo-o: running
|
|
|
|
jangmo-o: init: IP address: 10.77.129.208
|
|
|
|
```
|
|
|
|
|
|
|
|
Then copy it over with `scp`:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ scp (readlink ./result) xe@10.77.129.208:web-server_20220227.raw
|
|
|
|
```
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
Then you can use `portablectl` to attach it to the system:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ sudo portablectl attach ./web-server_20220227.raw
|
|
|
|
[...]
|
|
|
|
Created symlink /etc/portables/web-server_20220227.raw → /home/xe/web-server_20220227.raw.
|
|
|
|
```
|
|
|
|
|
|
|
|
And then start it like any systemd service:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ sudo systemctl start web-server
|
|
|
|
```
|
|
|
|
|
|
|
|
[If you want the service to start automatically, add `--enable --now` to the
|
|
|
|
`portablectl attach` command. That will enable the service in systemd and then
|
|
|
|
start it, like when you run `systemctl enable --now
|
|
|
|
something.service`.](conversation://Mara/hacker)
|
|
|
|
|
|
|
|
And then inspect the service's status with `systemctl`:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ sudo systemctl status web-server
|
|
|
|
● web-server.service - A web service
|
|
|
|
Loaded: loaded (/etc/systemd/system.attached/web-server.service; disabled; vendor preset: disabled)
|
|
|
|
Drop-In: /etc/systemd/system.attached/web-server.service.d
|
|
|
|
└─10-profile.conf, 20-portable.conf
|
|
|
|
Active: active (running) since Sun 2022-02-27 18:21:01 UTC; 20s ago
|
|
|
|
Main PID: 960 (web-server)
|
|
|
|
Tasks: 5 (limit: 513)
|
|
|
|
Memory: 8.1M
|
|
|
|
CPU: 189ms
|
|
|
|
CGroup: /system.slice/web-server.service
|
|
|
|
└─960 /nix/store/yl863jm907wfr7gq9j0c4bd3d4bdc4vp-web-server-20220227/bin/web-server
|
|
|
|
|
|
|
|
Feb 27 18:21:01 jangmo-o systemd[1]: Started A web service.
|
|
|
|
Feb 27 18:21:01 jangmo-o web-server[960]: 2022/02/27 18:21:01 listening for HTTP on :3031
|
|
|
|
```
|
|
|
|
|
|
|
|
And finally poke it with curl:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ curl http://[::]:3031
|
|
|
|
hello from nix building a docker image!
|
|
|
|
```
|
|
|
|
|
|
|
|
[That ain't Docker, chief!](conversation://Numa/delet)
|
|
|
|
|
|
|
|
[I know, I know, I didn't adjust the message from when I wrote the Docker
|
|
|
|
example.](conversation://Cadey/facepalm)
|
|
|
|
|
|
|
|
And then you can change the handler to something like:
|
|
|
|
|
|
|
|
```go
|
|
|
|
http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
|
|
|
|
fmt.Fprintf(w, "PORTABLE=%s\n", os.Getenv("PORTABLE"))
|
|
|
|
})
|
|
|
|
```
|
|
|
|
|
|
|
|
Rebuild the image with `nix build`:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ git add .
|
|
|
|
$ nix build .#portable
|
|
|
|
```
|
|
|
|
|
|
|
|
Copy it to the arch VM with `scp`:
|
|
|
|
|
|
|
|
```console
|
2022-03-02 16:23:53 +00:00
|
|
|
$ scp $(readlink ./result) xe@10.77.129.208:web-server_20220227.raw
|
2022-02-27 19:12:53 +00:00
|
|
|
```
|
|
|
|
|
|
|
|
And finally run `portablectl reattach` to upgrade it:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ sudo portablectl reattach --now ./web-server_20220227.raw
|
|
|
|
Queued /org/freedesktop/systemd1/job/858 to call RestartUnit on portable service
|
|
|
|
web-server.service.
|
|
|
|
```
|
|
|
|
|
|
|
|
Then you can see that it restarted the unit with `systemctl status`:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ sudo systemctl status web-server
|
|
|
|
● web-server.service - A web service
|
|
|
|
Loaded: loaded (/etc/systemd/system.attached/web-server.service; disabled; vendor preset: disabled)
|
|
|
|
Drop-In: /etc/systemd/system.attached/web-server.service.d
|
|
|
|
└─10-profile.conf, 20-portable.conf
|
|
|
|
Active: active (running) since Sun 2022-02-27 18:30:04 UTC; 37s ago
|
|
|
|
Main PID: 1074 (web-server)
|
|
|
|
Tasks: 6 (limit: 513)
|
|
|
|
Memory: 8.1M
|
|
|
|
CPU: 182ms
|
|
|
|
CGroup: /system.slice/web-server.service
|
|
|
|
└─1074 /nix/store/j1mfz3ydn13qmvcgrql33zi0dwb3x7dk-web-server-20220227/bin/web-server
|
|
|
|
|
|
|
|
Feb 27 18:30:04 jangmo-o systemd[1]: Started A web service.
|
|
|
|
Feb 27 18:30:04 jangmo-o web-server[1074]: 2022/02/27 18:30:04 listening for HTTP on :3031
|
|
|
|
```
|
|
|
|
|
|
|
|
And finally poke it with curl:
|
|
|
|
|
|
|
|
```console
|
|
|
|
$ curl http://[::]:3031
|
|
|
|
PORTABLE=web-server_20220227.raw
|
|
|
|
```
|
|
|
|
|
|
|
|
And there you go! Nix created a portable system service, we spawned it on a
|
|
|
|
newly created Arch Linux VM and then were able to update it so that we could
|
|
|
|
replace the message.
|
|
|
|
|
|
|
|
---
|
|
|
|
|
|
|
|
Nix builds can do more than just turn code into software. They can create Docker
|
|
|
|
images, Portable Services, virtual machine images and more. The only real limit
|
|
|
|
is what you can imagine.
|
|
|
|
|
|
|
|
Flakes make it easier to pull in and munge about packages. Before flakes you'd
|
|
|
|
need to have a few `.nix` files like `docker.nix` for the docker image and
|
|
|
|
`portable.nix` for the portable service. You'd also have to pull in something
|
|
|
|
like [Niv](https://github.com/nmattia/niv) to make sure everything uses the same
|
|
|
|
version of nixpkgs, and even then it's opt-in, not opt-out, so it's easy to mess
|
|
|
|
things up and not use the pinned versions of things. Flakes make that explicit
|
|
|
|
behavior implicit, so you can't bring in dependencies you aren't aware of.
|
|
|
|
|
|
|
|
If you want to see the code repo I developed while writing this post, see
|
|
|
|
[cadey/gohello-http](https://tulpa.dev/cadey/gohello-http) on my git server.
|
|
|
|
|
|
|
|
Thanks for reading!
|