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BundleGen is a command-line tool to convert OCI Images into extended OCI Bundles for use with the Dobby container manager.
In OCI, there are two distinct ways of packing applications for containers - Images and Bundles.
At a high-level an OCI implementation would download an OCI Image then convert that image into an OCI Runtime filesystem bundle. At this point the OCI Runtime Bundle would be run by an OCI Runtime.
An Image is a platform-agnostic distribution format that is designed to store the application and any dependencies it may require such as libraries and configuration files. The specification for images is defined here: https://github.com/opencontainers/image-spec
Iimages contain the following items:
The manifest contains metadata about the contents and dependencies of the image - for example md5 hashes and sizes of the image files.
A higher-level document which points to specific image manifests. This is ideal for supporting multiple platforms, as each platform would have their own manifest.
The final container filesystem is built up of layers where one or more layers are applied on top of each other to create a complete filesystem. Layers can be .tar, .tar.gz or .zstd files.
Each image must have a config file. Note this is different to the runtime spec config.json file.
The image config contains basic image info (such as author, creation date) and information about how the container should be launched, such as the entrypoint, environment variables, networking and volumes.
The config in the image is immutable (since the image ID is based on a hash of this config) and changing it results in a derived image instead of changing the existing image
These are all contained in a single archive that can be distributed and downloaded by anyone. By using the image index, a single image can contain multiple variants of an application each compiled for a different architecture (i.e. a single image could contain versions of an app built for x86 and arm). The image filesystem is built up of multiple layers where each image layer represents changes between itself and the parent layer. To construct the final container filesystem, each layer is unpacked on top of the lower layers. Images are constructed this way since whenever an image builder creates a new image, the differences are saved as a layer. This helps improves performance when building images by only needing to re-build what's changed instead of the whole image again.
Once built, an OCI Image can then be uploaded to a registry server and discovered by name, downloaded, verified by hash, trusted through a signature, and converted into the runtime bundle ready to run by the runtime.
Once the image is unpacked and converted into a bundle, this bundle can be used to launch a container using an OCI compliant runtime. The bundle consists of two parts:
These files must be in a single directory on the local filesystem. In other words, a tar archive of a bundle will have these artefacts at the root of the archive, not nested within a top-level directory. The specification for the runtime configuration and the bundle format are defined here: https://github.com/opencontainers/runtime-spec. Dobby is fully compliant with the latest version 1.0.2 of the specification.
The bundle contains an OCI configuration file (which is named config.json) which can specify host-independent details such as which executable to launch and host-specific settings such as mount locations, hook paths, Linux namespaces and cgroups. Because the configuration includes host-specific settings, application bundle directories copied between two hosts may require configuration adjustments.
It is possible to manually create an OCI bundle from scratch by hand-crafting the runtime configuration and copying files into the rootfs. For more information on how to create OCI bundles from scratch, see the following pages:
As part of the DAC project, Consult Red created a standalone tool to accompany Dobby called BundleGen. This is a command line tool designed to convert OCI images into OCI bundles that can be run by Dobby. BundleGen is designed to run off-box in a cloud environment and uses template files to create the bundles tailed for a given RDK platform. The repo includes sample templates for the Dobby Vagrant VM and the RPi.
For more information and documentation on BundleGen, see the repo here: https://github.com/rdkcentral/BundleGen/tree/master/docs