rkt

rkt, pronounced "rocket," is an open-source container runtime for Linux that was developed by CoreOS and released in 2014. It was designed as a secure, standards-based alternative to Docker, emphasizing a modular architecture, reproducible builds, and strong image provenance. rkt implements the App Container (appc) specifications and originally used App Container Images (ACI) as its primary image format.

Workloads in rkt are run as pods containing one or more applications. Images are referenced as ACIs and are verified with cryptographic signatures before execution. The runtime uses a pluggable stage1 isolation architecture and can operate with various execution environments, including systemd-nspawn. rkt provides a command-line interface for fetching images, verifying their signatures, and running or exporting pods. It also offered optional integration with Kubernetes via a separate runtime interface called rktlet, allowing Kubernetes clusters to use rkt as a container runtime.

For image sources, rkt originally focused on appc/ACI and provided tools to convert Docker images to the ACI format through docker2aci; support for OCI or Docker-native images was limited and not its primary path. The design emphasized security, including the management of trusted keys and image signatures locally or remotely.

Over time, rkt's development slowed as other runtimes such as Docker, containerd, and CRI-O gained prominence. After Red Hat acquired CoreOS, the project shifted away from active development, and in 2020 the rkt project was archived, with the ecosystem broadly recommending migration to OCI-compliant runtimes. The project remains of historical interest as an early attempt to create a secure, standards-driven container runtime separate from Docker.

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