[Perceptes]

I wish this had gone into some more technical detail about what "CNB" does that is actually better. Most of the article was just rehashing some problems with Dockerfiles, but the conclusion is just "CNB fixes it!" The one specific improvement they mention is being able to "rebase" an image without rebuilding the whole thing, which certainly sounds interesting, but is not explained. How does it work? What else is CNB other than a wrapper around `docker build`?

[sclevine]

The presentation to the CNCF TOC covers some of the technical details: https://www.youtube.com/watch?v=uDLa5cc-B0E&feature=youtu.be

Some key points:

- CNBs can manipulate images directly on Docker registries without re-downloading layers from previous builds. The CNB tooling does this by remotely re-writing image manifests and re-uploading only layers that need to change (regardless of their order).

- CNB doesn't require a Docker daemon or `docker build` if it runs on a container platform like k8s or k8s+knative. The local-workstation CLI (pack) just uses Docker because it needs local Linux containers on macos/windows.

[jacques_chester]

> How does it work?

The OCI image format expresses layer order as an array of digests. Essentially, "read the blobs with these SHAs in this order, please".

Cloud Native Buildpacks have predictable layouts and layering. A buildpack can know that layer `sha256:abcdef123` contains (say) a .node_modules directory. It can decide to update only that layer, without invalidating any other layer.

And the operation can be very fast, because you can do it directly against the registry. GET a small JSON file, make an edit, POST it back.

This is a big deal because under the classic Dockerfile model, changes in a lower layer invalidate the higher layers. But this means your image can be invalidated by OS layer changes, dependency changes and so on. It's the right policy for Docker to have -- a conservative policy -- but Buildpacks have the advantage of additional context that lets them rely on other guarantees. Most noticeably ABI guarantees.

[codefinger]

There's some more detail here: https://buildpacks.io/docs/