docs: Add an "under-the-hood" page explaining the build process

This page describes the various steps that Nixery goes through when
"procuring" an image.

The intention is to give users some more visibility into what is going
on and to make it clear that this is not just an image storage
service.

tools/nixery/docs/src/SUMMARY.md

@@ -1,4 +1,5 @@
 # Summary
 
 - [Nixery](./nixery.md)
+- [Under the hood](./under-the-hood.md)
 - [Nix, the language](./nix-1p.md)

tools/nixery/docs/src/nixery.md

@@ -52,10 +52,6 @@ The instance at `nixery.dev` tracks a recent NixOS channel, currently NixOS
 Private registries might be configured to track a different channel (such as
 `nixos-unstable`) or even track a git repository with custom packages.
 
-### Is this an official Google project?
-
-**No.** Nixery is not officially supported by Google.
-
 ### Should I depend on `nixery.dev` in production?
 
 While we appreciate the enthusiasm, if you would like to use Nixery in your
@@ -63,6 +59,10 @@ production project we recommend setting up a private instance. The public Nixery
 at `nixery.dev` is run on a best-effort basis and we make no guarantees about
 availability.
 
+### Is this an official Google project?
+
+**No.** Nixery is not officially supported by Google.
+
 ### Who made this?
 
 Nixery was written mostly by [tazjin][].

tools/nixery/docs/src/under-the-hood.md

@@ -0,0 +1,105 @@
+# Under the hood
+
+This page serves as a quick explanation of what happens under-the-hood when an
+image is requested from Nixery.
+
+<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-refresh-toc -->
+
+- [1. The image manifest is requested](#1-the-image-manifest-is-requested)
+- [2. Nix builds the image](#2-nix-builds-the-image)
+- [3. Layers are uploaded to Nixery's storage](#3-layers-are-uploaded-to-nixerys-storage)
+- [4. The image manifest is sent back](#4-the-image-manifest-is-sent-back)
+- [5. Image layers are requested](#5-image-layers-are-requested)
+
+<!-- markdown-toc end -->
+
+--------
+
+## 1. The image manifest is requested
+
+When container registry clients such as Docker pull an image, the first thing
+they do is ask for the image manifest. This is a JSON document describing which
+layers are contained in an image, as well as some additional auxiliary
+information.
+
+This request is of the form `GET /v2/$imageName/manifests/$imageTag`.
+
+Nixery receives this request and begins by splitting the image name into its
+path components and substituting meta-packages (such as `shell`) for their
+contents.
+
+For example, requesting `shell/htop/git` results in Nixery expanding the image
+name to `["bashInteractive", "coreutils", "htop", "git"]`.
+
+If Nixery is configured with a private Nix repository, it also looks at the
+image tag and substitutes `latest` with `master`.
+
+It then invokes Nix with three parameters:
+
+1. image contents (as above)
+2. image tag
+3. configured package set source
+
+## 2. Nix builds the image
+
+Using the parameters above, Nix imports the package set and begins by mapping
+the image names to attributes in the package set.
+
+A special case during this process is packages with uppercase characters in
+their name, for example anything under `haskellPackages`. The registry protocol
+does not allow uppercase characters, so the Nix code will translate something
+like `haskellpackages` (lowercased) to the correct attribute name.
+
+After identifying all contents, Nix determines the contents of each layer while
+optimising for the best possible cache efficiency.
+
+Finally it builds each layer, assembles the image manifest as JSON structure,
+and yields this manifest back to the web server.
+
+*Note:* While this step is running (which can take some time in the case of
+large first-time image builds), the registry client is left hanging waiting for
+an HTTP response. Unfortunately the registry protocol does not allow for any
+feedback back to the user at this point, so from the user's perspective things
+just ... hang, for a moment.
+
+## 3. Layers are uploaded to Nixery's storage
+
+Nixery inspects the returned manifest and uploads each layer to the configured
+[Google Cloud Storage][gcs] bucket. To avoid unnecessary uploading, it will
+first check whether layers are already present in the bucket and - just to be
+safe - compare their MD5-hashes against what was built.
+
+## 4. The image manifest is sent back
+
+If everything went well at this point, Nixery responds to the registry client
+with the image manifest.
+
+The client now inspects the manifest and basically sees a list of SHA256-hashes,
+each corresponding to one layer of the image. Most clients will now consult
+their local layer storage and determine which layers they are missing.
+
+Each of the missing layers is then requested from Nixery.
+
+## 5. Image layers are requested
+
+For each image layer that it needs to retrieve, the registry client assembles a
+request that looks like this:
+
+`GET /v2/${imageName}/blob/sha256:${layerHash}`
+
+Nixery receives these requests and *rewrites* them to Google Cloud Storage URLs,
+responding with an `HTTP 303 See Other` status code and the actual download URL
+of the layer.
+
+Nixery supports using private buckets which are not generally world-readable, in
+which case [signed URLs][] are constructed using a private key. These allow the
+registry client to download each layer without needing to care about how the
+underlying authentication works.
+
+---------
+
+That's it. After these five steps the registry client has retrieved all it needs
+to run the image produced by Nixery.
+
+[gcs]: https://cloud.google.com/storage/
+[signed URLs]: https://cloud.google.com/storage/docs/access-control/signed-urls