Docker Architecture

The Docker architecture makes it simple for developers to distribute, package, and oversee applications within containers. The parts of it and their interactions are broken down into depth below:

1. Docker Daemon (dockerd)

The main part of Docker that controls containers, images, networks, and volumes is the Docker Daemon. It waits for API requests from the Docker CLI or other apps while operating in the background on the host machine. The Docker daemon (dockerd) is a key component in the Docker architecture responsible for managing Docker containers on the host machine. It is responsible for building, running, and managing Docker images and containers. The main components of the Docker daemon include:

a. Docker Engine

The Docker Engine is the core of Docker, and it includes the following subcomponents:

• Server: A server that manages Docker objects like containers, images, and volumes.
• API: The Docker daemon exposes a REST API that allows Docker clients (such as the
• Docker CLI) and other applications to interact with it.
• Client: The Docker CLI that sends commands to the Docker daemon through the API.

b. Container Runtime

Docker uses container runtimes to manage the lifecycle of containers. The daemon interacts with container runtimes to handle container creation, execution, and removal. Docker's default runtime is <runc>, but it can also use other runtimes like <containerd> or <CRI-O>.

c. Image Manager

The image manager is responsible for pulling, storing, and managing Docker images. It fetches images from Docker registries (like Docker Hub) and manages local images on the host system.

d. Container Manager

The container manager handles the creation, starting, stopping, and removal of containers. It also manages container isolation and the allocation of resources like CPU, memory, and networking.

e. Networking

The Docker daemon manages container networking, allowing containers to communicate with each other and the outside world. It uses network drivers (bridge, overlay, host, etc.) to create and manage virtual networks for containers.

f. Volume Manager

The volume manager is responsible for managing storage and volumes for containers. It handles the creation, mounting, and persistence of volumes to ensure data storage for containers.

g. Logging

Docker captures logs from containers and the daemon itself. These logs can be configured to be sent to different logging drivers (JSON file, syslog, Fluentd, etc.) to ensure appropriate monitoring and auditing.

h. Security

The daemon manages security features, such as namespaces, control groups (cgroups), and capabilities, to ensure container isolation and resource control. It also handles user permissions, secure image downloads, and configurations like AppArmor, SELinux, and Seccomp profiles.

i. Swarm Mode (Optional)

If Docker Swarm mode is enabled, the Docker daemon will also be responsible for managing the orchestration of services, load balancing, and clustering of nodes in a Docker Swarm.

These components work together to enable the Docker daemon to efficiently manage the lifecycle of containers and the resources on the host system.

2. Client for Docker (docker)

The command-line interface (CLI) that users use is called the Docker Client. Using the Docker API, it issues commands to the Docker Daemon. Users are able to complete tasks like:

• Running containers (docker run)
• Building images (docker build)
• Managing containers (docker ps, docker stop, etc.)
• Interacting with Docker registries (docker pull, docker push)

3. Docker Images

Docker images are read-only templates used to create containers. They contain everything needed to run an application, including:

• The application code
• Runtime libraries
• Environment variables
• File system structure

Since images are layered, sharing similar layers among them helps maximize storage and accelerate download times.

4. Docker Containers

Containers are instances of Docker images. They encapsulate the application and its environment, providing an isolated space for it to run. Containers are lightweight, as they share the host OS kernel while maintaining separate filesystems and processes.

Key features of containers include:

• Isolation: Each container runs in its own environment.
• Portability: Containers can run consistently across different environments (development, testing, production).
• Resource Management: Docker can allocate resources like CPU and memory to containers.

5. Docker Registry

A Docker Registry is a storage and distribution system for Docker images. The default registry is Docker Hub, which hosts a vast collection of public images. You can also set up private registries for your organization's images.

Key functions of a registry include:

• Storing images: Images can be pushed to or pulled from the registry.
• Versioning: Registries manage different versions of images, allowing users to specify tags for specific releases.

Docker's architecture makes use of containerization to streamline application development and deployment. You may utilize containerized apps' advantages, such portability, scalability, and economical resource usage, by knowing their components, which include the Docker Daemon, Client, Images, Containers, and Registry.