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Filesystem Bundle

Container Format

This section defines a format for encoding a container as a filesystem bundle - a set of files organized in a certain way, and containing all the necessary data and metadata for any compliant runtime to perform all standard operations against it. See also OS X application bundles for a similar use of the term bundle.

The definition of a bundle is only concerned with how a container, and its configuration data, are stored on a local file system so that it can be consumed by a compliant runtime.

A Standard Container bundle contains all the information needed to load and run a container. This includes the following three artifacts which MUST all reside in the same directory on the local filesystem:

  1. config.json : immutable, host independent configuration. This REQUIRED file, which MUST be named config.json, contains settings that are host independent and application specific such as security permissions, environment variables and arguments. See config.json for more details.

  2. runtime.json : mutable, host dependent configuration. This REQUIRED file, which MUST be named runtime.json, contains settings that are host specific such as memory limits, local device access and mount sources. The goal is that the bundle can be moved as a unit to another runtime and run the same application if runtime.json is reconfigured. See runtime.json for more details.

  3. A directory representing the root filesystem of the container. While the name of this REQUIRED directory may be arbitrary, users should consider using a conventional name, such as rootfs. This directory MUST be referenced from within the config.json file.

While these three artifacts MUST all be present in a single directory on the local filesytem, that directory itself is not part of the bundle. In other words, a tar archive of a bundle will have these artifacts at the root of the archive, not nested within a top-level directory.