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Docker Internals

Docker is a tool designed to make it easier to create, deploy, and run applications using containers. The internal working of Docker involves several components, each playing a specific role in the containerization process. Below, I will explain the key components and how they interact within Docker using diagrams in Mermaid.

Docker Architecture Overview​

The core components of Docker are:

  • Docker Client: The interface used by users to interact with Docker (CLI or API).
  • Docker Daemon: The background service that manages Docker containers, images, networks, etc.
  • Docker Images: Pre-configured templates to create containers.
  • Docker Containers: Running instances of Docker images.
  • Docker Registry: A storage for Docker images, often Docker Hub.

Here’s a high-level architecture diagram:

Docker Client and Daemon Interaction​

The Docker Client sends commands to the Docker Daemon (server-side), which then executes those commands. For example, when you run docker run, the Docker Client sends this command to the Docker Daemon, which decides what to do based on the request (e.g., pull an image, create a container, etc.).

Docker Images and Containers​

A Docker Image is a lightweight, portable, and executable package that includes everything needed to run a piece of software, including the code, runtime, libraries, and environment variables. A Container is a running instance of a Docker Image.

The process of creating a container from an image looks like this:

Docker Container Internals​

Internally, each Docker container is isolated using Linux kernel features such as:

  • Namespaces: Provide isolation for resources (PID, Mount, Network, etc.).
  • Cgroups: Control resource allocation (CPU, Memory, IO).
  • Union File System (UFS): Combines multiple layers of Docker images into a single filesystem.

Here’s a diagram showing the isolation:

Docker Registry​

Docker images are stored in a Docker Registry (like Docker Hub). The registry holds different versions of images, which can be pulled to create containers. This allows developers to manage and share images easily.

Docker Network​

Docker uses its own network driver to manage container communication. Containers can be connected to different networks (bridge, host, overlay). Here's a simple diagram showing containers in a Docker network: