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diff --git a/tools/nixery/docs/src/under-the-hood.md b/tools/nixery/docs/src/under-the-hood.md new file mode 100644 index 000000000000..3791707b1cd2 --- /dev/null +++ b/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 |