Why Docker Is Revolutionizing Application Deployment: A Beginner’s Guide

Premise

Docker runs on Linux; while Windows and macOS installers exist, they rely on virtual machines. It assumes the reader has some basic Linux knowledge.

Consensus

Docker follows a client‑server model where the Docker client communicates with the Docker daemon, which handles heavy tasks such as building, running, and publishing containers. The client and daemon can run on the same host or the client can connect to a remote daemon via socket or RESTful API.

Docker and containers are not synonymous; Docker is a suite of virtualization technologies, and containers are just one component.

Docker Use Cases

Faster application delivery : Developers can build containers that include the application and its services locally, then integrate them into CI/CD pipelines.

Simplified development and scaling : Container‑based platforms enable high portability; containers run on developer machines, physical or virtual servers, and cloud platforms.

Higher density and load : Docker provides a lightweight, cost‑effective alternative to VM hypervisors, useful for high‑density environments, private clouds, or PaaS.

Goal

Docker aims to provide a lightweight OS‑level virtualization solution and create portable, lightweight containers for software programs.

Main Components

Docker consists of two main parts:

Docker: the open‑source container virtualization platform. Docker Hub: the SaaS platform for sharing and managing Docker containers.

Underlying Implementation

Docker is built on Linux Container (LXC) technology and further abstracts it so users don’t need to manage containers directly.

Docker uses cgroups for isolation and a union file system to store images and make containers transient.

Cgroups allow resource limits (memory, CPU) and creation of namespaces (PID, UTS, IPC, network, user, mount).

Reference: Docker Cgroups

Union file system layers file systems, as illustrated below:

Relation to Virtual Machines

Operating Docker containers feels like using a fast, lightweight virtual machine, but there are significant differences, shown in the following diagrams:

Virtual machine applications include the OS, binaries, and libraries, whereas Docker shares the host kernel and only contains the application and its dependencies.

Docker Advantages

Docker Features

Interactive Shell: Docker can allocate a virtual terminal attached to any container’s STDIN.

Filesystem Isolation: Each container runs in an independent root filesystem.

Copy‑on‑Write: Root filesystems are created using copy‑on‑write, enabling rapid deployment and saving disk space.

Resource Isolation: cgroups allocate distinct system resources per container.

Network Isolation: Each container has its own network namespace, virtual interface, and IP address.

Logging: Docker captures STDOUT/STDERR/STDIN for each container for real‑time or batch retrieval.

Change Management: Filesystem changes can be committed to new images and reused without templates or manual configuration.

Docker Engine Architecture

The diagram shows Docker’s main components:

1. Server: the daemon that runs continuously in the background.

2. REST API: defines how to interact with the server and issue commands.

3. Client: the command‑line interface (shell) used to send instructions.

Docker Run Process

When running a container, Docker:

Pulls the image (skipping download if already present locally).

Creates a new container.

Assigns a union filesystem with a writable layer; modifications are recorded here and can be committed to a new image or discarded.

Sets up network/bridge interfaces for container‑host communication.

Assigns an IP address from a pool; the container is not reachable via localhost.

Executes the specified program.

Captures and provides application output, including input, output, and error streams.

Source: http://www.uml.org.cn/yunjisuan/201802082.asp