What are the common container image formats and standards and how do they differ?

1 OCI Image Format and Specification

One of the most widely adopted container image formats and standards is the Open Container Initiative (OCI) Image Format and Specification, which defines how to create, store, and distribute container images using a common set of rules and conventions. The OCI Image Format and Specification is based on the Docker Image Format, which was the de facto standard for container images before the OCI was formed in 2015. The OCI Image Format and Specification consists of two main components: the image manifest, which describes the image metadata and layers, and the image index, which references one or more image manifests for different platforms and architectures. The OCI Image Format and Specification aims to provide interoperability and portability across different container runtimes and registries, as well as support for image signing and encryption.

2 Docker Image Format and Registry API

The Docker Image Format and Registry API are the original container image format and standard that were developed by Docker, the company behind the popular container platform. The Docker Image Format and Registry API are still widely used and supported by many container tools and services, as they provide a simple and efficient way to create, store, and distribute container images. The Docker Image Format and Registry API use a similar structure to the OCI Image Format and Specification, with some minor differences in the image manifest and index fields. The Docker Image Format and Registry API also define a RESTful interface for interacting with container registries, which are the servers that store and serve container images. The Docker Image Format and Registry API support various features, such as image tagging, layering, compression, and content-addressable identifiers.

3 AppC Image Format and Discovery

Another container image format and standard that emerged in the early days of the container ecosystem is the App Container (AppC) Image Format and Discovery, which was developed by CoreOS, the company behind the rkt container runtime. The AppC Image Format and Discovery are designed to be simple, flexible, and secure, and to enable decentralized and federated distribution of container images. The AppC Image Format and Discovery use a different structure than the OCI and Docker formats, with an image manifest that contains the image name, version, labels, annotations, dependencies, and execution parameters. The AppC Image Format and Discovery also define a discovery mechanism that allows users to find and download container images from various sources, such as HTTP, HTTPS, DNS, or local file systems. The AppC Image Format and Discovery support features such as image signing, verification, encryption, and compression.

4 Other Container Image Formats and Standards

In addition to OCI, Docker, and AppC formats and standards, various organizations and communities have developed or proposed other container image formats and standards. Examples include the Cloud Native Application Bundle (CNAB) format, used for packaging and managing multi-service applications; the Singularity Image Format (SIF), designed for high-performance computing (HPC) and scientific workloads; the Nix Image Format, which uses the Nix package manager and functional programming language; and the Flatpak Image Format, created for desktop applications. Each of these formats has its own benefits and drawbacks, making them more suitable for certain use cases, scenarios, or environments. However, they may also struggle with compatibility, adoption, and support from other container tools and services.