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Open Container Initiative: A technical overview of its specifications and benefits

Understand the importance of open standards for container technology with this technical blog post on the Open Container Initiative (OCI).

David Thor Mar 14

The Open Container Initiative (OCI) is a project aimed at creating open standards for container technology. Containers, such as those provided by Docker, allow developers to package their applications into a single, portable unit, making it easy to deploy and run the application on any system that supports container technology.

The OCI was formed in 2015 by a group of industry leaders, including Docker, Google, and Red Hat, with the goal of creating a set of standards for container technology that would be open and vendor-neutral. The initiative provides a framework for building and running portable containers, including specifications for container images, runtime, and configuration.

The OCI has three main specifications: the Image Specification, the Distribution Specification, and the Runtime Specification. The Image spec defines the physical structure of a container, the Distribution spec defines the API protocol by which containers are sent and received, and the Runtime spec defines how a container can be run by a supported container platform. Together they provide a comprehensive framework for building, sharing, and running containers, and ensure the portability and interoperability of containerized applications across different platforms.

The Image Specification

The OCI Image Specification is arguably the most important and definitely my favorite of the OCIs contributions. It defines the anatomy of an image itself and how the container’s filesystem is to be constructed. 

It may seem like a mundane thing to spend time documenting so meticulously, but remember that containers have to encapsulate everything an application needs to run. If the application needs Node.js or Python, those have to be included in the image. If it has a dozen different libraries or packages it depends on, those have to be included as well. In order to ensure that containers don’t take up enormous amounts of disk space, the Image spec includes a clever way of slicing filesystem snapshots into smaller, reusable portions called layers

The concept of a layer is pretty easy for developers to understand: it’s a diff! A set of changes to the filesystem that need to be made when compared to the layer before it. By stacking these layers in a specific order, dictated by an image manifest, the desired file system can be reconstructed from the smaller parts. For a single image the value may not be apparent, but this layering pays tons of dividends when dealing with multiple images that share common elements. Two images that both need Node.js for example would be able to share the layer(s) that install Node.js on the filesystem, thus saving a huge chunk of duplicate content from being stored. This is what makes it possible for developers to work with so many different images on a single machine at the same time.

The Distribution Specification

The distribution spec is actually the most recent of the OCIs contributions to the container ecosystem and has been increasingly important as more and more businesses adopt containers for their cloud applications. This specification dictates the API protocol by which containers are sent and received, but where developers are most likely to be exposed to it is via container registries.

Almost all public cloud providers, like AWS, GCP, Azure, and more, offer managed container registries in which developers can store their container artifacts and layers. All it takes from a developer’s perspective is to run `docker push` or `docker pull` to store and retrieve images from each of these registries, but it’s the OCIs distribution spec that ensures that these commands work across all the different vendors. Without it, developers would have to learn and adopt entirely new tools and commands to switch between vendors. 

The Runtime Specification

To round off the OCIs contributions, we have the Runtime spec. The Image spec helps developers store and reproduce a snapshot of their filesystem, the Distribution spec helps developers ship containers to registries and their favorite cloud platforms, but it’s the Runtime spec that delivers the instructions to the platforms on what to do with the container and its filesystem once it’s arrived. This includes details like what command to run on the image filesystem, how permissioning works between the container and the host OS, and even how the host platform should report back runtime status updates.

By standardizing the way containers advertise runtime instructions, container images are able to run virtually anywhere without developers having to learn anything about the target platform. This standardization is what enables containers to run on Kubernetes, AWS ECS, Heroku, and even Docker Desktop, without changing anything about the image itself. This portability benefits both developers and the platform ecosystem by allowing cloud platforms to support more customers with a more stable experience.

In summary, the Open Container Initiative (OCI) is a project that aims to establish open standards for container technology. These standards ensure that containers built to the OCI specifications will run consistently across different environments and different vendors, thus ensuring the portability and interoperability of containerized applications across different platforms. Developers should be aware of the OCI specifications and their advantages, but the point of these standards is also to ensure that developers don’t have to think or worry about the stability of their applications or VMs. 

That being said, the OCI is a community-driven initiative and the specifications require continuous investment and improvement to meet the latest technological advancements. Open standards that achieve such wide industry support are hard to come by, and developers should stay updated with the latest updates and consider participating in the OCI community if we hope to ensure that the container ecosystem remains open and prosperous.

Learn more about containers

If you’d like to learn more about containers, check out these other posts from Architect!

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