831f68fd71
After receiving feedback during containerd summit walk through of the pull POC, we found that the resolution flow for names was out of place. We could see this present in awkward places where we were trying to re-resolve whether something was a digest or a tag and extra retries to various endpoints. By centering this problem around, "what do we write in the metadata store?", the following interface comes about: ``` Resolve(ctx context.Context, ref string) (name string, desc ocispec.Descriptor, fetcher Fetcher, err error) ``` The above takes an "opaque" reference (we'll get to this later) and returns the canonical name for the object, a content description of the object and a `Fetcher` that can be used to retrieve the object and its child resources. We can write `name` into the metadata store, pointing at the descriptor. Descisions about discovery, trust, provenance, distribution are completely abstracted away from the pulling code. A first response to such a monstrosity is "that is a lot of return arguments". When we look at the actual, we can see that in practice, the usage pattern works well, albeit we don't quite demonstrate the utility of `name`, which will be more apparent later. Designs that allowed separate resolution of the `Fetcher` and the return of a collected object were considered. Let's give this a chance before we go refactoring this further. With this change, we introduce a reference package with helps for remotes to decompose "docker-esque" references into consituent components, without arbitrarily enforcing those opinions on the backend. Utlimately, the name and the reference used to qualify that name are completely opaque to containerd. Obviously, implementors will need to show some candor in following some conventions, but the possibilities are fairly wide. Structurally, we still maintain the concept of the locator and object but the interpretation is up to the resolver. For the most part, the `dist` tool operates exactly the same, except objects can be fetched with a reference: ``` dist fetch docker.io/library/redis:latest ``` The above should work well with a running containerd instance. I recommend giving this a try with `fetch-object`, as well. With `fetch-object`, it is easy for one to better understand the intricacies of the OCI/Docker image formats. Ultimately, this serves the main purpose of the elusive "metadata store". Signed-off-by: Stephen J Day <stephen.day@docker.com> |
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reference.go | ||
reference_test.go |