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Danila Fedorin b67c1ab615 Add some more to the Nix Blog post

2022-04-09 02:50:04 -07:00

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Prologue

You can skip this section if you'd like.

For the last few days, I've been stuck inside of my room due to some kind of cold or flu, which or {{< sidenote "right" "pcr-note" "may or may not be COVID™." >}} The results of the PCR test are pending at the time of writing. {{< /sidenote >}} In seeming correspondence with the progression of my cold, a thought occurred in the back of my mind: "Your blog deployment is kind of a mess". On the first day, when I felt only a small tingling in my throat, I waved that thought away pretty easily. On the second day, feeling unwell and staying in bed, I couldn't help but start to look up Nix documentation. And finally, on the third day, between coughing fits and overconsumption of oral analgesic, I got to work.

In short, this post is the closest thing I've written to a fever dream.

The Constraints

I run several versions of this site. The first is, of course, the "production" version, hosted at the time of writing on danilafe.com and containing the articles that I would like to share with the world. The second is a version of this site on which drafts are displayed - this way, I can share posts with my friends before they are published, get feedback, and even just re-read what I wrote from any device that has an internet connection. The third is the Russian version of my blog. It's rather empty, because translation is hard work, so it only exists so far as another "draft" website.

My build process (a derivative of what I describe in [rendering mathematics on the back end]({{< relref "./backend_math_rendering.md" >}})) is also fairly unconventional. When I developed this site, the best form of server-side mathematics rendering was handled by KaTeX, and required some additional work to get rolling (specifically, I needed to write code to replace sections of LaTeX on a page with their HTML and MathML versions). There may be a better way now, but I haven't yet performed any kind of migration.

Currently, only my main site is behind HTTPS. However, I would like for it to be possible to adjust this, and possibly even switch my hosts without changing any of the code that actually builds my blog.

Why Flakes

This article is about using Nix Flakes to manage my configuration. But what is it that made me use flakes? Well, two things:

Adding custom packages. The Nix code for my blog provides a package / derivation for each version of my website, and I want to use these packages in my configuration.nix so that I can point various Nginx virtual hosts to each of them. This is typically done using overlays; however, how should my system configuration get my overlay Nix expression? I would like to be able to separate my build-the-blog code from my describe-the-server code, and so I need a clean way to let my system access the former from the latter. flakes solve this issue my letting me specify a blog flake, and pull it in as one of the inputs.
Versioning. My process for deploying new versions of the site prior to flakes boiled down to fethcing the latest commit from the master branch of my blog repository, and updating the default.nix file with that commit. This way, I could reliably fetch the version of my site that I want published. Flakes do the same thing: the flake.lock file contains the hashes of the Git-based dependencies of a flake, and thus prevents builds from accidentally pulling in something else. However, unlike my approach, which relies on custom scripts and extra tools such as jq, the locking mechanism used by flakes is provided with standard Nix tooling. Using Flakes also guarantees that my build process won't break with updates to Hugo or Ruby, since the nixpkgs version is stored in flake.lock, too.

The Final Result

Here's the relevant section of my configuration:

I really like how this turned out for three reasons. First, it's very clear from the configuration what I want from my server: three virtual hosts, one with HTTPS, one with drafts, and one with drafts and in Russian. Second, there's plenty of code reuse. I'm using two builder functions, english and russian, but under the hood, the exact same code is being used to run Hugo and all the necessary post-processing. Finally, all of this can be used pretty much immediately given my blog flake, which reduces the amount of glue code I have to write.

Getting There

A Derivation Builder

As I mentioned earlier, I need to generate multiple versions of my blog. All of these use pretty much the same build process -- run Hugo on the Markdown files, then do some post-processing (in particular, convert the LaTeX in the resulting pages into MathML and nice-looking HTML). I didn't want to write this logic multiple times, so I settled for a function that takes some settings, and returns a derivation:

There are a few things here:

On line 7, the settings src, ssl, and host are inherited into the derivation. The src setting provides a handle on the source code of the blog. I haven't had much time to test and fine-tune the changes enabling multi-language support on the site, so they reside on a separate branch. It's up to the caller to provide which version of the source code should be used for building. The host and ssl settings are interesting because they don't actually matter for the derivation itself -- they just aren't used in the builder. However, attributes given to a derivation are accessible from "outside", and these settings will play a role later.
Lines 10 through 14 deal with setting the base URL of the site. Hugo, my static site generator, does not know how to interpret the --baseURL option when a blog has multiple languages. What this means is that in the end, it is impossible to configure the base URL used in links from the command line, and I need to apply some manual changes to the configuration file. I need to be able to adjust the base URL becasue each version of my website is hosted in a different place: the default (english) website is hosted on danilafe.com, the version with drafts on drafts.danilafe.com, and so on. However, the configuration file only knows one base URL per language, and so it doesn't know when or when not to use the drafts. prefix. The urlSub variable is used in the builder.
On line 15, the publicPath variable is set; while single-language Hugo puts all the generated HTML into the public folder, the multi-language configuration places them into public/[language-code]. Thus, depending on the configuration, the builer needs to look in a different place for final output.

This new website function is general enough to represent all my blog versions, but it's too low-level. Do I really want to specify the publicPath each time I want to describe a version of the site? What about settings.replaceUrl, or the source code? Just like I would in any garden variety language, I defined two helper functions:

Both of these simply make a call to the website function (and thus return derivations), but they make some decisions for the caller, and provide a nicer interface by allowing attributes to be omitted. Specifically, by default, a site version is assumed to be HTTP-only, and to contain non-draft articles. Furthermore, since each function corresponds to a language, there's no need for the caller to provide a blog version, and thus also the output path, or even to specify the "from" part of replaceUrl. The wrapHost function, not included in the snippet, simply adds http or https to the host parameter, which does not otherwise include this information. These functions can now be called to describe different versions of my site:

# Default version, hosted on the main site and using HTTPS
english {
    ssl = true;
    host = "danilafe.com";
}

# English draft version, hosted on draft domain and not using HTTPS.
english {
    drafts = true;
    host = "drafts.danilafe.com";
}

# Russian draft version, hosted on draft (russian) domain, and not using HTTPS.
russian {
    drafts = true;
    host = "drafts.ru.danilafe.com";
}

Configuring Nginx

The above functions are already a pretty big win (in my opinion) when it comes to describing my blog. However, by themselves, they aren't quite enough to clean up my system configuration: for each of these blog versions, I'd need to add an Nginx virtualHosts entry where I'd pass in the corresponding host (like danilafe.com or drafts.danilafe.com), configure SSL, and so on. At one point, too, all paths in /var were by default mounted as read-only by NixOS, which meant that it was necessay to tell systemd that /var/www/challenges should be writeable so that the SSL certificate for the site could be properly renewed. Overall, this was a lot of detail that I didn't want front-and-center in my server configuration.

However, with the additional "ghost" attributes, my derivations already contain most of the information required to configure Nginx. The virtual host, for instance, is the same as replaceUrl.to (since I'd want the Nginx virtual host for a blog version to handle links within that version). The ssl ghost parameter corresponds precisely to whether or not a virtual host will need SSL (and thus ACME, and thus the systemd setting).

To make this really nice, I wanted all of this to be "just another section of my configuration file". That is, I wanted to control my site deployment via regular old attributes in configuration.nix. To this end, I needed a module. Xe recently wrote about NixOS modules in flakes, and what I do here is very similar. In essence, a module has two bits:

The options, which specify what kind of attributes this module understands. The most common option is enable, which tells a module that it should apply its configuration changes.
The configuration, which consists of the various system settings that this module will itself set. These typically depend on the options.

In short, a module describes the sort of options it will accept, and then provides a way to convert these newly-described options into changes to the system configuration. It may help if I showed you the concrete options that my newly-created blog module provides:

There are three options here:

enable, a boolean-valued input that determines whether or not the module should make any changes to the system configuration at all.
sites, which, as written in the code, accepts a list of derivations. These derivations correspond to the various versions of my site that should be served to the outside world.
challengePath, a string to configure where ACME will place files during automatic SSL renewal.

Now, while these are the only three options the user will need to set, the changes to the system configuration are quite involved. For instance, for each site (derivation) in the sites list, the resulting configuration needs to have a virtualHost in the services.nginx namespace. To this end, I defined a function that accepts a site derivation and produces the necessary settings:

Each virtual host always has a root option (where Nginx should look for HTML files), but only those sites for which SSL is enabled need to specify addSSL, enableACME, and acmeRoot. All the virtual hosts are assembled into a single array (below, cfg refers to the options that the user provided to the module, as specified above).

If the enable option is set, we enable Nginx, and provide it with a list of all of the virtual hosts we generated. Below, config (not to be confused with cfg) is the namespace for the module's configuration.

In a similar manner to this, I generate a list of systemd services which are used to configure the challenge path to be writeable. Click the module.nix link above to check out the full file.

Creating a Flake

{{< todo >}} This needs to be done {{< /todo >}}

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