Under the hood

This page serves as a quick explanation of what happens under-the-hood when an image is requested from Nixery.

• 1. The image manifest is requested
• 2. Nix fetches and prepares image content
• 3. Layers are grouped, created, hashed, and persisted
• 4. The manifest is assembled and returned to the client
• 5. Image layers are requested

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 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.