Export to OCI layout format on disk

Exporting to OCI layout format is an experimental feature available on pack since version 0.30.0

1. Enable experimental feature

Verify your pack version is equal or greater than 0.30.0

pack version

Enable the experimental features on pack

pack config experimental true

You can confirm everything is fine, checking the config.toml file in your PACK_HOME installation folder. For example:

cat ~/.pack/config.toml
experimental = true
layout-repo-dir = "<$HOME>/.pack/layout-repo"

The configuration shows the experimental mode was enabled and a local directory to save images on disk was configured to path <$HOME>/.pack/layout-repo. layout-repo-dir is being used as a local repository to save images requires by pack build command in OCI layout format.

2. Build the app

Please first follow the steps to build an app, once you have successfully built an application you can export the sample application to disk in OCI layout format.

The OCI layout feature must be enabled using the convention oci:<path/to/save/image> in the <image-name> parameter when invoking pack build.

For example:

pack build oci:sample-app --path samples/apps/java-maven --builder cnbs/sample-builder:bionic

It will save the image in a folder ./sample-app created in your current directory.

3. Check your image

Congratulations!

You can verify your application image was saved on disk in a folder called ./sample-app in your current directory in OCI layout format. For example:

tree sample-app

sample-app
├── blobs
│ └── sha256
│     ├── 141bfb0cd434d425bc70edb9e56ea11d07aed76450eb0e73e6110645f251a8d3
│     ├── 2fa192256ce255c6ea6c1296eadfe2feba8094f40e6aa85e699645caca2e85d8
│     ├── 5a44e4f7b58d74fe6f92dd7028075c91191128d1e2e7f39846fe061a9a98836e
│     ├── 72d9f18d70f395ff9bfae4d193077ccea3ca583e3da3dd66f5c84520c0100727
│     ├── 827746ec7ba80f4e4811b6c9195b6f810fbc2d58a6c9cc337bf0305791f24e97
│     ├── ad13830c92258c952f25d561d8bf7d9eb58b8a3003960db1502cbda8239130b5
│     ├── b97b58b190d5f731c879b0f7446a2bd554863b51851e03757199c74dd922ce61
│     ├── c44222730efa142cd5bedc0babf82a9a07d325494be7f5c3cfde56f43166b65f
│     ├── e1048fb89c3194a1f0542c0847aa086a7034dd7867c48fe8c93675cf36f90610
│     ├── f0a30c5bc44742065b1b4ffa95271a39994f05ba7a03dd7e7143d1d3e45fa0b1
│     └── f9d6350d0c44c0e7165a522155f53181ce8c163a6b8ead1f6baea22d1a8d8a78
├── index.json
└── oci-layout  

3 directories, 13 files

If you want to keep playing with the image in OCI layout format, one tool you can take a look at is umoci. It can help you to create a runtime bundler that can be executed with another tool like runc

Extra configuration

Skip saving your run-image layers on disk

Before using this option we suggest to remove your local layout directory (the one configured in your pack config.toml with the key layout-repo-dir) and your application image folder (if you are planning to use the same folder). The reason for this is pack doesn’t remove the blobs saved in the layout-repo-dir if you use the --sparse flag

If you don’t need your run-image layers on disk, you can skip them using --sparse flag in your pack build command invocation.

For example:

pack build oci:sample-app --sparse --path samples/apps/java-maven --builder cnbs/sample-builder:bionic

Verify your application image

sample-app
├── blobs
│ └── sha256
│     ├── 2ebed3ab57806441e2bf814eaf0648ed77289e058340d2b76d32b422fbaac5d8
│     ├── 2fa192256ce255c6ea6c1296eadfe2feba8094f40e6aa85e699645caca2e85d8
│     ├── 5a44e4f7b58d74fe6f92dd7028075c91191128d1e2e7f39846fe061a9a98836e
│     ├── 741e558b7b807fea350b26b8152170a2463277cb3d1268b60de76ec12608518a
│     ├── 907c84671180d979a38affb62d9a6ea8e9a510e27639e0b60a34a42f1a846ddc
│     ├── ad13830c92258c952f25d561d8bf7d9eb58b8a3003960db1502cbda8239130b5
│     ├── c44222730efa142cd5bedc0babf82a9a07d325494be7f5c3cfde56f43166b65f
│     └── f9d6350d0c44c0e7165a522155f53181ce8c163a6b8ead1f6baea22d1a8d8a78
├── index.json
└── oci-layout

3 directories, 10 files

As you can see, there are 3 missing files at sample-app/blobs/sha256 folder. The missing 3 blobs are the blobs from the run-image that were not downloaded but if you check your config file you’ll notice you have the same number of layers as when you export the full image.

Implementation notes

Media Types

According to the OCI specification, the compatibles media types for the index.json files must be:

• application/vnd.oci.image.index.v1+json or
• application/vnd.docker.distribution.manifest.list.v2+json

If you are trying to use the lifecycle directly without using pack to export your image, take on consideration that tools like:

skopeo

skopeo copy -a docker://<your-image> <dest>

It will give you application/vnd.oci.image.index.v1+json media type, which is currently working

But crane

crane pull <your-image> <dest> --format=oci

It will give you application/vnd.docker.distribution.manifest.list.v2+json, which will fail because of the state of our current implementation, we will improve this behavior in future versions.