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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 0000000000..4b79830010 --- /dev/null +++ b/tools/nixery/docs/src/under-the-hood.md @@ -0,0 +1,129 @@ +# 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 fetches and prepares image content](#2-nix-fetches-and-prepares-image-content) +- [3. Layers are grouped, created, hashed, and persisted](#3-layers-are-grouped-created-hashed-and-persisted) +- [4. The manifest is assembled and returned to the client](#4-the-manifest-is-assembled-and-returned-to-the-client) +- [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 fetches and prepares image content + +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 uses the `symlinkJoin` function to +create a special layer with the "symlink farm" required to let the +image function like a normal disk image. + +Nix then returns information about the image contents as well as the +location of the special layer to Nixery. + +## 3. Layers are grouped, created, hashed, and persisted + +With the information received from Nix, Nixery determines the contents +of each layer while optimising for the best possible cache efficiency +(see the [layering design doc][] for details). + +With the grouped layers, Nixery then begins to create compressed +tarballs with all required contents for each layer. As these tarballs +are being created, they are simultaneously being hashed (as the image +manifest must contain the content-hashes of all layers) and persisted +to storage. + +Storage can be either a remote [Google Cloud Storage][gcs] bucket, or +a local filesystem path. + +During this step, Nixery checks its build cache (see [Caching][]) to +determine whether a layer needs to be built or is already cached from +a previous build. + +*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. + +## 4. The manifest is assembled and returned to the client + +Once armed with the hashes of all required layers, Nixery assembles +the OCI Container Image manifest which describes the structure of the +built image and names all of its layers by their content hash. + +This manifest is returned to the client. + +## 5. Image layers are requested + +The client now inspects the manifest and determines which of the +layers it is currently missing based on their content hashes. Note +that different container runtimes will handle this differently, and in +the case of certain engine and storage driver combinations (e.g. +Docker with OverlayFS) layers might be downloaded again even if they +are already present. + +For each of the missing layers, the client now issues a request to +Nixery that looks like this: + +`GET /v2/${imageName}/blob/sha256:${layerHash}` + +Nixery receives these requests and handles them based on the +configured storage backend. + +If the storage backend is GCS, it *redirects* 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. + +If the storage backend is the local filesystem, Nixery will attempt to +serve the layer back to the client from disk. + +--------- + +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 +[layering design doc]: https://storage.googleapis.com/nixdoc/nixery-layers.html |