Posted On: Sep 8, 2022

Seekable OCI (SOCI) is a technology open sourced by AWS that enables containers to launch faster by lazily loading the container image. SOCI works by creating an index (SOCI Index) of the files within an existing container image. This index is a key enabler to launching containers faster, providing the capability to extract an individual file from a container image before downloading the entire archive.

Most methods for launching containers download the entire container image from a remote container registry before starting the container. Waiting for all of the data is wasteful in cases when only a small amount of data is needed for startup. Prior research has shown that the container image downloads account for 76% of container startup time, but on average only 6.4% of the data is needed for the container to start doing useful work.

There are various solutions to this problem, including reducing the size of a container image and pre-fetching container images to local storage. Lazy loading is an approach where data is downloaded from the registry in parallel with the application startup. Container images are stored as an ordered list of layers, and layers are most often stored as gzipped tar files. It’s usually not possible to fetch individual files from gzipped tar files. Some projects enable lazy loading through format conversion. One such project is stargz-snapshotter, which takes an existing OCI image and builds a new OCI image with an embedded table of contents. With SOCI, we borrowed some of the design principles from stargz-snapshotter, but took a different approach. A SOCI index is generated separately from the container image, and is stored in the registry as an OCI Artifact and linked back to the container image by OCI Reference Types. This means that the container images do not need to be converted, image digests do not change, and image signatures remain valid.

An open-source build tool is used to create SOCI indices for existing OCI container images and a remote snapshotter, called soci-snapshotter, provides containerd the ability to lazy load images that have been indexed by SOCI. SOCI and the soci-snapshotter are open sourced under Apache 2.0, and you can learn more about the project on GitHub. We look forward to working and engaging with the community on improving SOCI and making container launches faster.