Docker is a platform that allows developers to build, run and manage applications in containers. Its core structure includes Docker Engine, mirroring and containers, client-server architecture, and namespace-based isolation mechanisms. 1. Docker Engine is a core component, including a daemon running in the background (responsible for building images and running containers), a REST API interface, and a command line client (CLI). 2. Mirrors are static templates that contain applications and their dependencies. Containers are running instances of images. Multiple containers can originate from the same image, and the images are built through Dockerfile. 3. Docker uses the client-server architecture, and the CLI communicates with the daemon through the API, and can also connect to remote daemons. 4. The container uses the namespace of the Linux kernel and cgroups to achieve process isolation and resource limitations, ensuring an efficient and secure application operation environment.

Docker is a platform that allows developers to build, run, and manage applications inside containers — lightweight, isolated environments that share the host operating system's kernel but have their own filesystem, processes, and network interfaces. It makes development more consistent across machines and simplifies deployment.

At its core, Docker works by packaging an application and all its dependencies into a container image, which can then be run on any machine that has Docker installed. This eliminates the common problem of “it works on my machine” because everything the app needs is bundled in the image.

Here's how Docker is structured:

1. Docker Engine – The Core Component

The Docker Engine is the heart of Docker. It runs on the host operating system and manages images, containers, networks, and storage. When you install Docker, you're essentially installing the Docker Engine.

• It consists of a server (daemon) that runs in the background and handles tasks like building images and running containers.
• There's also a REST API interface that allows programs to interact with the daemon.
• And a command-line interface (CLI) client ( docker command), which sends commands to the daemon via the API.

So when you type docker run hello-world , the CLI talks to the daemon, which pulls the image and starts the container for you.

2. Images and Containers – What You Actually Work With

A Docker image is a template — a static snapshot containing your application, libraries, and runtime environment. Think of it like a blueprint or recipe.

A container is a running instance of an image. It's dynamic — when you start an image, it becomes a container. Multiple containers can be created from the same image.

For example:

• One image might be nginx:latest
• From that image, you could run two containers, each serving the same content but possibly on different ports or with different configurations

Images are built using a Dockerfile , a text file with instructions like FROM , COPY , and CMD . Each instruction creates a new layer in the image, and these layers are cached to speed up builds.

3. Docker Architecture – Client-Server Model

Docker uses a client-server architecture :

• The Docker client is what you interact with directly — typically via the CLI.
• The Docker daemon ( dockerd ) does the heavy lifting: managing images, containers, networks, and volumes.
• They communicate through a RESTful API, either locally (via Unix socket) or over a network.

This setup means you can even point your local Docker CLI to a remote Docker daemon, allowing you to manage containers running on another machine without having to SSH into it every time.

4. Isolation and Namespaces – How Containers Stay Separate

Containers aren't virtual machines — they don't emulate hardware. Instead, they rely on features of the Linux kernel like namespaces and cgroups to isolate processes.

• Namespaces give containers their own view of the system — separate process trees, network stacks, user IDs, etc.
• Control groups (cgroups) limit resource usage, like CPU or memory, so one container doesn't starve others.

Thanks to these technologies, containers are fast and efficient while still being secure and predictable.

That's the basic structure of Docker. Once you understand how images, containers, and the engine work together, using Docker becomes much less mysterious. It's not magic — just clever use of existing OS features to simplify application delivery.

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