Docker - How it functios in the Operating System (OS) | Overview.

Docker & the Operating System (OS) - Overview.

Scope:

• Intro,
• Docker Key Components,
• Namespaces & Types,
• Control Groups (cgroups),
• Union File System (UnionFS),
• How Docker Runs on Different Operating Systems (Linux, windows, MacOS),
• Docker Engine Components (runs as a client-server architecture),
• How a Container Is Created,
• Visual Architecture (Simplified).

Intro:

• Docker works by leveraging features of the host operating system (OS)... particularly the Linux kernel, to run applications in isolated environments called containers.
• Unlike virtual machines (VMs), which require a full OS for each instance, containers share the host OS kernel but isolate the application processes.
• Here's twtech overview of how Docker works on an OS:

 1. Docker and the OS: Key Components

Docker relies on several OS-level features to work:

a. Namespaces

• Provide isolation for containers (e.g., process IDs, network interfaces, mount points, user IDs).
• Each container sees its own isolated set of resources.

Types of namespaces used:

• pid: process isolation
• net: network isolation
• mnt: filesystem mount points
• uts: hostname/domain isolation
• ipc: inter-process communication

b. Control Groups (cgroups)

• Limit and prioritize resources (CPU, memory, disk I/O(input/output)) that a container can use.
• Prevent a single container from consuming all system resources.

c. Union File System (UnionFS)

• Provides layered file systems (used in Docker images).
• Enables image reuse and efficient storage.

 2. How Docker Runs on Different Operating Systems

On Linux

• Docker runs natively using the Linux kernel features (namespaces, cgroups).
• Containers are fast and efficient because they directly use the host kernel.

 On Windows & macOS

• These Operating Systems (Windows & macOS) don’t support Linux kernel features natively.
• Docker also runs a Linux virtual machine (e.g., using WSL2 on Windows or a lightweight VM on macOS).
• Containers still behave similarly, but with added overhead due to the VM layer.

 3. Docker Engine Components (Docker runs as a client-server architecture):

Component	Role
Docker Client.	CLI (docker commands) that communicates with the daemon
Docker Daemon (dockerd).	Background service managing images, containers, etc.
Docker Images.	Read-only templates used to create containers
Docker Containers.	Running instances of images, isolated via kernel features
Docker Registry.	Stores and distributes images (e.g., Docker Hub)

 4. How a Container Is Created

1. User runs: docker run nginx
2. Docker client sends the command to the Docker daemon.
3. Docker daemon:
• Pulls the image (if not cached).
• Creates a container using the image.
• Sets up namespaces and cgroups.
• Attaches filesystem layers.
• Starts the process in the container.

 Visual Architecture (Simplified)

 twtech-Summary (Docker works on an OS by):

• Using kernel features like namespaces and cgroups (control groups) for isolation and resource control respectively.
• Running directly on Linux (but through a VM) on macOS and Windows.
• Managing containers via a client-server model (Docker CLI → Docker daemon).