Docker

What is virtualization？

chroot is an operation in Unix and Linux systems that changes the apparent root directory for a running process and its children. A process running in such an environment cannot access files outside the designated root directory.

Virtualization technologies: VMware, VirtualBox, KVM

Virtualization adds a hypervisor layer on top of the operating system, which virtualizes resources such as CPU, memory, disk, and network. Operating systems can then be installed on top of these virtualized resources, creating a virtual execution environment—i.e., Virtual Machines (VMs).

Advantages of VMs

  • Improve IT efficiency and reduce maintenance costs
  • Faster workload deployment
  • Higher server availability

Disadvantages of VMs

  • Higher resource overhead, weaker performance
  • Poor scalability and migration capability

Why Docker

Scenarios

  • Developers code locally and share their work via Docker containers.
  • Push applications to a test environment with Docker and run automated/manual tests.
  • Fix issues in dev, redeploy to test, and verify quickly.
  • After testing, promoting to production is as simple as pushing an updated image.

Needs

Deliver applications quickly and consistently, package environments into images to avoid configuration drift, simplify the development lifecycle, and support fast, agile iteration.

Container vs Virtual Machine

Feature Container Virtual Machine
Startup speed Seconds Minutes
Performance Near-native Weaker
Memory overhead Very small Large
Disk usage Usually MB Usually GB
Runtime density Thousands per host Dozens per host
Isolation Process-level isolation Full OS isolation
Portability Excellent Average

Core Concepts

Docker Engine (Daemon)
Docker uses a client/server architecture. The Docker client communicates with the Docker daemon (engine) via a Unix socket or REST API. The engine performs the heavy lifting of building, running, and distributing containers.

Docker Image & Dockerfile
A Docker image is a read-only template used to create containers, built layer-by-layer from instructions (e.g., copy files, run commands).

Docker Registry (Hub)
Registries can be public or private. The largest public registry is Docker Hub, which hosts a vast number of images.

Basic Operations

Images

# Pull an image
docker pull mysql:5.7.30

# List images
docker images

# Tag an image
docker tag mysql:5.7.30 mysql5

# Inspect details
docker inspect mysql:5.7.30

# Search images
docker search mysql

# Remove an image
docker rmi mysql:5.7.30

# Push an image
docker push mysql[:TAG]

Containers

# Create a container
docker create -it nginx

# Start a container
docker start 9cfcce23593a

# Show running containers
docker ps

# Show all containers
docker ps -a

# Run and remove after exit
docker run -it --rm --network host tomcat:8.5.56-jdk8-openjdk

Practical Example

  1. Create a volume 
    docker volume create test_volume
  2. Mount the same volume in two containers 
    docker run -it --rm -v test_volume:/test nginx:latest /bin/bash
docker run -it --rm -v test_volume:/test nginx:latest /bin/bash

cd /test
touch a.txt
ls /test   # The file is visible from both containers

Notes

  • Docker does not support relative paths for mount points inside containers.
  • Multiple containers can use the same named volume concurrently (e.g., one writes while another reads).
  • If a named volume already exists, Docker will reuse it automatically; otherwise it creates it on first use.
  • You can also bind-mount a host directory into a container; if the host path doesn’t exist, Docker creates it at container start.