Docker has revolutionized software development and deployment by providing lightweight, portable containers that package applications and their dependencies together. But how exactly does Docker work under the hood to achieve this isolation and efficiency? This article takes a comprehensive look at Docker’s internal architecture, explaining the key components and how they interact to create and run containers.

Table of Contents

Introduction

Docker containers provide isolated environments that behave like virtual machines but share the host OS kernel, making them highly efficient. This is achieved through a combination of Linux kernel features, container runtimes, and Docker’s own architecture. Understanding these layers helps in optimizing container usage and troubleshooting complex issues.

Docker Client-Server Architecture

Docker operates on a client-server model:

  • The Docker CLI is the command-line interface where users issue commands like docker run or docker build.
  • The Docker Daemon (dockerd) runs in the background on the host machine, listening for API requests from the CLI.
  • The CLI communicates with the daemon over a Unix socket or network interface, enabling local or remote management.
  • The daemon manages Docker objects: images, containers, networks, and volumes.
  • When necessary, the daemon interacts with container runtimes to start containers and perform low-level system calls.

This separation allows Docker to cleanly manage container lifecycle and resources while providing a simple user interface[1][2][3][5].

Key Docker Components

Docker CLI

The CLI is the user-facing tool to interact with Docker. It translates user commands into REST API calls directed at the Docker Daemon. For example, when you run:

docker run nginx

the CLI sends a REST API request to the daemon to start an Nginx container[1][3].

Docker Daemon (dockerd)

The daemon is the core service that orchestrates Docker operations. It:

  • Listens for API requests.
  • Manages images, containers, networks, and volumes.
  • Coordinates with container runtimes to instantiate containers.
  • Communicates with registries to pull/push images.
  • Maintains the overall Docker environment on the host[1][2][3][5].

Container Runtime (containerd and runc)

Docker uses container runtimes to actually create and run containers:

  • containerd is the container lifecycle manager. It manages image transfer, storage, and container execution.
  • runc is the low-level runtime that leverages Linux kernel features to create containers. It uses system calls such as clone(), setns(), and mount() to set up namespaces, cgroups, and filesystems for container isolation[1][4][5].

The process flow is typically:

Docker CLI → Docker Daemon → containerd → containerd-shim → runc → Linux Kernel → Container Process

containerd-shim acts as a supervisor to keep containers running independently of the daemon process[1].

Linux Kernel Features

Docker relies heavily on Linux kernel features to provide isolation and resource control:

  • Namespaces: Provide isolated views of system resources (process IDs, network, mount points, IPC, hostname). Each container gets its own namespace instance, giving it a virtualized environment independent of the host or other containers[1][4].

    Important namespaces include:

    • PID (process IDs)
    • NET (network stack)
    • MNT (filesystem mounts)
    • IPC (inter-process communication)
    • UTS (hostname/domain name)
    • USER (user and group IDs)

  • Control Groups (cgroups): Limit and monitor resource usage such as CPU, memory, and I/O for containers. This prevents any single container from exhausting host resources[1][4].

  • Capabilities: Fine-grained permissions to restrict root privileges inside containers for enhanced security[1].

  • OverlayFS: A union filesystem that layers container images efficiently, allowing containers to share common base layers while maintaining separate writable layers[1][4].

How Docker Creates and Runs a Container

When you run a container, the following sequence happens internally:

  1. CLI sends API request: The Docker CLI sends a request to the Docker Daemon to create a container.

  2. Daemon processes request: The daemon verifies the image exists locally or pulls it from a registry.

  3. containerd manages lifecycle: containerd sets up the container environment including filesystem and network.

  4. containerd-shim supervises: It maintains container processes even if the daemon stops.

  5. runc creates container: Using kernel syscalls like clone(), namespaces are created, filesystems mounted using OverlayFS, and resource limits applied via cgroups.

  6. Kernel enforces isolation: The Linux kernel isolates the container’s processes, network, and filesystem.

  7. Container process runs: The container runs its default command (e.g., /bin/sh, nginx) inside the isolated environment[1][2][3][5].

Filesystem Management with OverlayFS

Docker images consist of layered filesystems stacked on top of each other. OverlayFS is a union filesystem that merges these layers into a single coherent view for containers:

  • Read-only layers: Base image layers are immutable and shared across containers.
  • Writable layer: Each container gets its own top writable layer where changes are stored.
  • This layering enables efficient image storage and fast container startup[1][4].

Networking and Isolation

Docker provides each container with its own isolated network stack using the NET namespace:

  • Containers get unique IP addresses and network interfaces.
  • Docker manages virtual bridges and routing internally.
  • Networking drivers (bridge, host, overlay, macvlan) allow flexible configurations for container communication and connectivity to the host or external networks[1][2][4].

Resource Management via cgroups

Control groups limit container resource usage:

  • CPU shares and quotas.
  • Memory limits and swap.
  • Disk I/O throttling.
  • Network bandwidth restrictions.

This ensures containers run within defined resource boundaries, preventing noisy neighbor effects[1][4].

Conclusion

Docker’s powerful yet elegant architecture combines a client-server model with Linux kernel features to provide process isolation, resource control, and efficient filesystem layering. The Docker Daemon orchestrates container lifecycle, delegating actual container creation to runtimes like containerd and runc, which leverage namespaces, cgroups, and OverlayFS at the kernel level. This design allows Docker to deliver highly portable, lightweight, and secure containers that have become foundational to modern software development and cloud-native applications.

Understanding these internals equips developers and operators with the knowledge to optimize, troubleshoot, and extend Docker for complex real-world scenarios.

If you want to explore further, consider diving into:

  • Linux namespaces and cgroups documentation.
  • Container runtime specifications (OCI).
  • Docker networking and storage drivers.

This foundational understanding will deepen your mastery of container technology and its ecosystem.