The advent of Docker in 2013 revolutionized software development by enabling the packaging of applications and their dependencies into standardized units called containers. Docker ensured consistency across development, testing, and production environments, significantly streamlining development and deployment processes.

In 2014, Kubernetes emerged as a powerful cluster orchestration tool, managing containerized applications across multiple nodes. Initially a single-node runtime, Kubernetes quickly evolved into a robust system for automating deployment, scaling, and operations of containerized applications. Its open-source nature and strong community support helped it become the de facto standard for container orchestration, facilitating high availability and scalability.

By 2015, the need for standardized specifications within the container ecosystem led to the formation of the Open Container Initiative (OCI). This organization aimed to create open, vendor-neutral standards for container formats and runtimes, fostering broader adoption and ensuring interoperability across different platforms and vendors.

CoreOS, introduced in 2013, was a pioneering container-optimized operating system built on general-purpose Linux. CoreOS offered automated updates, a minimal footprint, and enhanced security, focusing on reliability and scalability for containerized workloads. Its innovations significantly influenced the development of other container-focused operating systems and tools.

Understanding Container-Optimized OS

Container-optimized OS are designed to streamline and secure containerized environments. Key characteristics include:

  • Minimal Distribution: Includes only essential components for running containers, reducing overhead and improving efficiency.
  • Secure Immutable File System: Ensures core system files remain untampered, enhancing security and stability.
  • Declarative Provisioning: Allows system configuration to be defined declaratively, simplifying management and replication.
  • Automated Updates: Keeps the system up-to-date with the latest security patches and features without manual intervention.

Notable Container-Optimized OS

  • CoreOS Container Linux (October 2013 – May 2020): Based on Chromium OS and Gentoo, CoreOS pioneered many concepts now standard in container environments.
  • Fedora CoreOS / Red Hat CoreOS (since June 2018): Based on Fedora, continuing the legacy of CoreOS with integration into the Red Hat ecosystem.
  • Flatcar Container Linux (since November 2019): A "friendly fork" of CoreOS, maintaining compatibility while evolving independently.
  • Google COS, AWS Bottlerocket, and Azure Linux are other examples of container-optimized OSs.

Flatcar Container Linux Overview

Flatcar Container Linux is built on Gentoo Linux, leveraging its customizable nature and performance optimization capabilities. Gentoo’s Portage package management system allows fine-grained control over software installations, leading to a lean and efficient operating system.

Installing Flatcar

Flatcar Container Linux can be deployed on various platforms, including cloud providers, virtualization platforms, and bare metal servers. Detailed installation guides are available for each provider:

Flatcar also supports installation on various virtualization platforms and bare metal servers, ensuring versatility for different deployment scenarios.

Source: YouTube – Reinventing Container Linux for the Wasm Era (and More) with System Extensions - Andrew Randall

Image-based Operating Systems

Image-based operating systems like Flatcar and Fedora CoreOS enhance reliability, security, and manageability through:

  1. Stateless Installation: Entire OS is replaced or upgraded without impacting user applications or data. The root filesystem is reloaded from a clean, known-good image on each boot.
  2. Read-Only OS Partition: Core OS partition is immutable during normal operation, preventing modifications. Writable data is stored separately, preserving OS integrity.
  3. Verifiable and Attestable: Immutability allows continuous verification using cryptographic hashes, enhancing security.


These operating systems employ advanced update mechanisms:

  • Atomic Updates: Updates are downloaded and deployed in the background, activated upon reboot.
  • Automated Rollbacks: Rollback mechanisms ensure system stability by reverting to previous versions if issues are detected.
  • Controlled Rollout: Updates are coordinated across clusters to minimize disruption and ensure consistent deployment.

Flatcar and Fedora CoreOS Implementation:

  • Flatcar: Uses an A/B partition scheme, allowing one partition to be updated while the other remains active.
  • Fedora CoreOS: Utilizes ostree for de-duplication, optimizing storage and reducing bandwidth for updates.


Ignition is a crucial utility used by Fedora CoreOS, RHEL CoreOS, and Flatcar Linux for disk management tasks during the initramfs stage of the boot process. It handles disk partitioning, formatting, writing files, and user configuration. On the first boot, Ignition reads its configuration from a designated source and applies the specified settings.

Key Features of Ignition

  • JSON Format: Simple and straightforward to parse.
  • Declarative Configuration: Specifies files, systemd services, network configurations, users, filesystems, and partitions.
  • External References: Allows referencing external sources for configuration data.
  • Initramfs Application: Ensures correct system setup from the first boot.

Simplifying Ignition Configuration

  • Butane: Converts human-readable YAML files into the JSON format required for Ignition.

Examples of Kubernetes Deployments Using Ignition

  • Kubernetes Cluster API: Automates provisioning and configuration of Kubernetes clusters.
  • Typhoon: Uses Butane and Terraform for consistent and repeatable Kubernetes deployments.
  • OKD / OpenShift: Automates initial cluster configuration and setup.

Extending the Immutable with systemd-sysext

systemd-sysext overlays extensions on the read-only /usr partition. This enables adding custom software on top of the base system without altering it. Flatcar has deprecated Torcx in favor of systemd-sysext for deploying custom Docker/containerd versions.

Source: YouTube – System and Configuration Extensions for Image-based Linux Distros and Beyond

Key Features of systemd-sysext

  • Overlay Extensions: Adds custom software on top of the read-only partition.
  • Flexible Integration: Integrates user-provided software deeply into the OS.
  • Deprecation of Torcx: Offers a more generic method for extending the immutable base system.

Further Documentation and Resources:

By using systemd-sysext, their users can overcome the limitations of a read-only root filesystem, allowing for the addition of necessary host-level software while maintaining the security and integrity of the base system.

CoreOS Layering with OSTree

Fedora CoreOS uses OSTree for managing and layering the OS. OSTree manages the filesystem as immutable snapshots, allowing atomic updates and rollbacks.

Key Features of OSTree

  • Git-Like Management: Version control for system binaries and configurations.
  • Atomic Updates: Applied as single operations, ensuring stability.
  • Layered Filesystem: Simplifies rollback by managing updates as new layers.
  • Platform-Agnostic: Integrated with various package managers.
  • Efficiency and Performance: Reduces overhead associated with traditional package management.

Further Documentation and Resources

By leveraging these advanced features and technologies, container-optimized operating systems provide robust, efficient, and secure environments for containerized applications, ensuring high availability, scalability, and ease of management for cloud computing experts.

Final thoughts

The landscape of container-optimized operating systems continues to evolve, driven by innovations from Docker, Kubernetes, CoreOS, and many others. These systems provide the foundation for modern, scalable, and secure containerized environments, catering to the needs of cloud computing experts and Linux users. Through features like minimal distributions, secure immutable file systems, and automated updates, container-optimized OSs enhance the efficiency and reliability of managing containerized applications. As the technology progresses, tools like Ignition, systemd-sysext, and OSTree further simplify and secure the deployment and management processes, ensuring a robust infrastructure for the future of cloud computing.

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