60 lines
3.6 KiB
Markdown
60 lines
3.6 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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- [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", "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, REQUIRED, or SHALL requirements for the protocols it implements.
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An implementation is compliant if it satisfies all the MUST, REQUIRED, and SHALL 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 one or more 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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* An archival format for container images, consisting of an [image manifest](manifest.md), an [image index](image-index.md) (optional), an [image layout](image-layout.md), a set of [filesystem layers](layer.md), and [image configuration](config.md) (base OCI layer)
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* A [process of referencing container images by a cryptographic hash of their content](descriptor.md) (base OCI layer)
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* A format for [storing CAS blobs and references to them](image-layout.md) (optional OCI layer)
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* Signatures that are based on signing image content address (optional OCI layer)
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* Naming that is federated based on DNS and can be delegated (optional OCI layer)
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[c99-unspecified]: http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf#page=18
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