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70 lines
4.0 KiB
Markdown
70 lines
4.0 KiB
Markdown
# Open Container Initiative
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## Image Format Specification
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This specification defines an OCI Image, consisting of a [manifest](manifest.md), an [image index](image-index.md) (optional), a set of [filesystem layers](layer.md), and a [configuration](config.md).
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The goal of this specification is to enable the creation of interoperable tools for building, transporting, and preparing a container image to run.
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### Table of Contents
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- [Introduction](spec.md)
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- [Notational Conventions](#notational-conventions)
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- [Overview](#overview)
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- [Understanding the Specification](#understanding-the-specification)
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- [Media Types](media-types.md)
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- [Content Descriptors](descriptor.md)
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- [Image Layout](image-layout.md)
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- [Image Manifest](manifest.md)
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- [Image Index](image-index.md)
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- [Filesystem Layers](layer.md)
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- [Image Configuration](config.md)
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- [Annotations](annotations.md)
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- [Conversion](conversion.md)
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- [Considerations](considerations.md)
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- [Extensibility](considerations.md#extensibility)
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- [Canonicalization](considerations.md#canonicalization)
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## Notational Conventions
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC 2119](http://tools.ietf.org/html/rfc2119) (Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997).
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The key words "unspecified", "undefined", and "implementation-defined" are to be interpreted as described in the [rationale for the C99 standard][c99-unspecified].
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An implementation is not compliant if it fails to satisfy one or more of the MUST, MUST NOT, REQUIRED, SHALL, or SHALL NOT requirements for the protocols it implements.
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An implementation is compliant if it satisfies all the MUST, MUST NOT, REQUIRED, SHALL, and SHALL NOT requirements for the protocols it implements.
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## Overview
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At a high level the image manifest contains metadata about the contents and dependencies of the image including the content-addressable identity of one or more [filesystem layer changeset](layer.md) archives that will be unpacked to make up the final runnable filesystem.
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The image configuration includes information such as application arguments, environments, etc.
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The image index is a higher-level manifest which points to a list of manifests and descriptors.
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Typically, these manifests may provide different implementations of the image, possibly varying by platform or other attributes.
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Once built the OCI Image can then be discovered by name, downloaded, verified by hash, trusted through a signature, and unpacked into an [OCI Runtime Bundle](https://github.com/opencontainers/runtime-spec/blob/master/bundle.md).
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### Understanding the Specification
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The [OCI Image Media Types](media-types.md) document is a starting point to understanding the overall structure of the specification.
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The high-level components of the spec include:
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* [Image Manifest](manifest.md) - a document describing the components that make up a container image
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* [Image Index](image-index.md) - an annotated index of image manifests
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* [Image Layout](image-layout.md) - a filesystem layout representing the contents of an image
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* [Filesystem Layer](layer.md) - a changeset that describes a container's filesystem
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* [Image Configuration](config.md) - a document determining layer ordering and configuration of the image suitable for translation into a [runtime bundle][runtime-spec]
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* [Conversion](conversion.md) - a document describing how this translation should occur
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* [Descriptor](descriptor.md) - a reference that describes the type, metadata and content address of referenced content
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Future versions of this specification may include the following OPTIONAL features:
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* Signatures that are based on signing image content address
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* Naming that is federated based on DNS and can be delegated
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[c99-unspecified]: http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf#page=18
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[runtime-spec]: https://github.com/opencontainers/runtime-spec
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