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diff --git a/pkg/v1/partial/README.md b/pkg/v1/partial/README.md new file mode 100644 index 0000000..53ebbc6 --- /dev/null +++ b/pkg/v1/partial/README.md @@ -0,0 +1,82 @@ +# `partial` + +[](https://godoc.org/github.com/google/go-containerregistry/pkg/v1/partial) + +## Partial Implementations + +There are roughly two kinds of image representations: compressed and uncompressed. + +The implementations for these kinds of images are almost identical, with the only +major difference being how blobs (config and layers) are fetched. This common +code lives in this package, where you provide a _partial_ implementation of a +compressed or uncompressed image, and you get back a full `v1.Image` implementation. + +### Examples + +In a registry, blobs are compressed, so it's easiest to implement a `v1.Image` in terms +of compressed layers. `remote.remoteImage` does this by implementing `CompressedImageCore`: + +```go +type CompressedImageCore interface { + RawConfigFile() ([]byte, error) + MediaType() (types.MediaType, error) + RawManifest() ([]byte, error) + LayerByDigest(v1.Hash) (CompressedLayer, error) +} +``` + +In a tarball, blobs are (often) uncompressed, so it's easiest to implement a `v1.Image` in terms +of uncompressed layers. `tarball.uncompressedImage` does this by implementing `UncompressedImageCore`: + +```go +type UncompressedImageCore interface { + RawConfigFile() ([]byte, error) + MediaType() (types.MediaType, error) + LayerByDiffID(v1.Hash) (UncompressedLayer, error) +} +``` + +## Optional Methods + +Where possible, we access some information via optional methods as an optimization. + +### [`partial.Descriptor`](https://godoc.org/github.com/google/go-containerregistry/pkg/v1/partial#Descriptor) + +There are some properties of a [`Descriptor`](https://github.com/opencontainers/image-spec/blob/master/descriptor.md#properties) that aren't derivable from just image data: + +* `MediaType` +* `Platform` +* `URLs` +* `Annotations` + +For example, in a `tarball.Image`, there is a `LayerSources` field that contains +an entire layer descriptor with `URLs` information for foreign layers. This +information can be passed through to callers by implementing this optional +`Descriptor` method. + +See [`#654`](https://github.com/google/go-containerregistry/pull/654). + +### [`partial.UncompressedSize`](https://godoc.org/github.com/google/go-containerregistry/pkg/v1/partial#UncompressedSize) + +Usually, you don't need to know the uncompressed size of a layer, since that +information isn't stored in a config file (just he sha256 is needed); however, +there are cases where it is very helpful to know the layer size, e.g. when +writing the uncompressed layer into a tarball. + +See [`#655`](https://github.com/google/go-containerregistry/pull/655). + +### [`partial.Exists`](https://godoc.org/github.com/google/go-containerregistry/pkg/v1/partial#Exists) + +We generally don't care about the existence of something as granular as a +layer, and would rather ensure all the invariants of an image are upheld via +the `validate` package. However, there are situations where we want to do a +quick smoke test to ensure that the underlying storage engine hasn't been +corrupted by something e.g. deleting files or blobs. Thus, we've exposed an +optional `Exists` method that does an existence check without actually reading +any bytes. + +The `remote` package implements this via `HEAD` requests. + +The `layout` package implements this via `os.Stat`. + +See [`#838`](https://github.com/google/go-containerregistry/pull/838). |