Best Practice: Selectively pulling specific layer types

[tcinbis]

Hey everyone!

With oras-cli version pre 0.13.0 we were (maybe ab-) using the --media-type flag to instruct oras to only pull specific parts of an artifact. For example, we would publish a service artifact containing a container image, as well as, some configuration files and deployment scripts. Since in certain scenarios we would only need access to the scripts or configuration we could save time by just specifying a specific media type.

Because --media-type was removed with version 0.13.0 I am now wondering what the best solution would be to achieve a similar functionality? I saw that the OCI describes Layer Types. Could this help here?

Thanks, everyone!

[qweeah]

Hi Tom, thanks for the question. Can you provide a more specific scenario on: you have a service artifact containing a container image and some configuration files but you only want to pull the configuration layers without the image layers.

For now, oras pull will only download layers that has names stored in OCI title annotation. So if the image layers are not annotated with title, then oras pull will skip them and only download the configuration layers.

From personal perspective, the best practice to pack a service artifact is not to put file layers and image layers all under one OCI artifact manifest, but to put them into different artifacts. We are preparing 0.14.0 and it supports a new manifest type to allow user attaching service files(e.g. service configurations) to a service image. After we released 0.14.0, you can download the configuration file layers via oras pull <oras-artifact-reference>

+---------------+            +---------------------+
| ORAS Artifact +------------> OCI Artifact (image)|
+-----+---------+            +-+---------+---------+
      |                        |         |
+-----v---------+        +-----v--+ +----v------------+
| File Layers...|+       | config | | Image Layers... |+
+---------------+|+      +--------+ +-----------------+|+
 +---------------+|                  +-----------------+|
  +---------------+                    +----------------+

For now, there is no plan to support more fine-grained layer filtering for oras pull

[caozhuozi]

Hi, @qweeah. How can we perform a fine-grained layer filtering now?

[Wwwsylvia]

Hi @caozhuozi , do you use ORAS CLI or SDK? Currently the Go SDK provides such functionality, but CLI doesn't.

[caozhuozi]

Hi, @Wwwsylvia Thanks for looking at my question! 🙏
Could you please specify which part (or specific file) of the Go SDK relates to this option?

Thank you!

[Wwwsylvia]

The CopyGraphOptions.FindSuccessors callback function of oras-go allows you to customize the descriptors of layers to copy. Here is an example of how ORAS CLI uses this function:

oras/cmd/oras/root/attach.go

Lines 191 to 204 in e89bc7b

	graphCopyOptions.FindSuccessors = func(ctx context.Context, fetcher content.Fetcher, node ocispec.Descriptor) ([]ocispec.Descriptor, error) {
	if content.Equal(node, root) {
	// skip duplicated Resolve on subject
	successors, _, config, err := graph.Successors(ctx, fetcher, node)
	if err != nil {
	return nil, err
	}
	if config != nil {
	successors = append(successors, *config)
	}
	return successors, nil
	}
	return content.Successors(ctx, fetcher, node)
	}

Feel free to let us know if you encounter any issues. You can also find us in the ORAS slack channel.