Docker vs LXD vs rkt: What are the differences?

Introduction

In the world of containerization, Docker, LXD, and rkt are popular technologies that allow developers to package, distribute, and run applications in isolated environments. Each of these technologies has its own unique features and advantages.

1. Containerization Technology: Docker is primarily a containerization technology that focuses on creating lightweight, portable, self-sufficient containers that can run on any machine. It provides a complete ecosystem for building, shipping, and running containers. In contrast, LXD is more of a system container manager that allows users to run full-fledged Linux distributions within containers. It offers a more traditional virtualization-like experience with greater isolation from the host system. On the other hand, rkt is a security-focused container runtime that promotes Pod-native infrastructure and emphasizes cluster orchestration and management.

2. Image Distribution: Docker utilizes its Docker Hub for storing and sharing container images, making it easy for users to access a vast repository of pre-built images. LXD relies on its image stores, where users can upload, download, and share images specifically designed for use with LXD containers. In comparison, rkt does not have a centralized image distribution platform like Docker Hub. Users typically rely on container registries like quay.io or self-hosted registries for sharing and managing rkt images.

3. Security Model: Docker has a strong focus on security, with features like namespaces, control groups, and capabilities to isolate containers from each other and the host system. It also offers content trust, which allows users to verify the integrity and provenance of container images. LXD provides a secure-by-default approach, isolating containers with Linux security mechanisms while limiting their access to host resources. On the other hand, rkt emphasizes security through its use of App Container (appc) specifications, which define every aspect of a container's runtime environment to enhance predictability and security.

4. Orchestration and Clustering: Docker comes with built-in orchestration tools like Docker Swarm and Docker Compose, which enable users to deploy and manage containerized applications across multiple hosts. LXD, being more focused on system containers, does not offer built-in support for orchestration but can be used in conjunction with tools like Juju or Kubernetes for cluster management. rkt, on the other hand, is designed to be a flexible component that can be integrated with various orchestrators and cluster managers, such as Kubernetes, Mesos, or Nomad.

5. Runtime Architecture: Docker uses a client-server architecture, where the Docker client interacts with the Docker daemon to build, run, and manage containers. LXD operates as a daemon-less service, enabling users to interact with the LXD API directly to manage containers and virtual machines. rkt follows a pod-centric model, where a pod is a group of containers that share resources and networking, allowing users to define complex, multi-container applications more easily than with Docker or LXD.

In Summary, Docker, LXD, and rkt each offer unique approaches to containerization, catering to different use cases and preferences, ranging from container management, security, image distribution, orchestration capabilities, and runtime architecture.