Image and Layer implementation seems a little coupled

[xiekeyang]

Now Image and Layer seems to exist some coupled problem:

• RootFS includes Layer structure;
• images/image.go includes RootFS function;
• The functionality getting layers is implemented in cmd;
• containerd/image.go has image interface and its structure
• ...

Above is being implemented in both service and driver packages.

We might should set up individual interface and structure for image, layer and rootfs.

Maybe, we should setup layer interface(driver) in new file image/layer.go, and interface(service) in containerd/layer.go.

layer member is according to https://github.com/opencontainers/image-spec/blob/master/layer.md#change-types, to be CreateLayer, DeleteLayer, GetLayers...

For structure implementation, Layer might be inside to Image, that can be easy to be called by APIs.

cc @stevvooe @dmcgowan @estesp

[crosbymichael]

ping @dmcgowan

[dmcgowan]

I think this is a big premature. Once we have a design proposal or working implementation of an OCI image which doesn't use layers, we can figure out how that can be implemented in containerd. While there are many helpers around layers in containerd, none of our interfaces enforce it. Snapshots don't have to be layers, content doesn't have to be layers, and the runtime only cares about mounts. I am going to close this for now, the layerless image discussion should happen elsewhere.