Docker on CentOS for beginners

[comment]: # Docker on CentOS for beginners

Introduction

The article will introduce Docker on CentOS.

Key concepts

Docker

Docker is the world's leading software containerization platform.

Docker is using union file systems which is a layered file system.

When docker run a container, every image consists of a serials of read-only layers,

and the container provides a read-write layer, and Docker combines these layers into one image,

and forms a single coherent file system on the host OS.

Q: What is the relationship between the docker host OS and the container base image OS?

A: The container's kernel is going to be the one from ubuntu, but nothing more.

Docker for Windows and Docker for Mac

Docker only support applications on Linux.

Docker cannot run on Windows and Mac directly. Docker will create a VM via Virtual Box, and run command in the VM.

Docker engine

Docker engine is a light weight Docker, is used to create Docker images and run Docker containers.

Docker daemon and Docker client

Docker uses a client-server architecture. The Docker client talks to the Docker daemon, which does the heavy lifting of building, running, and distributing your Docker containers.

Docker registries

Docker has a good ecological environment, there are some registries that store images and other materials on the Internet.

Docker Hub is a public registry and stores images.

images

An image is a file and is a read-only template, Docker can use it to create containers.

Docker images can be built from 2 ways.

• Build it from Dockfile
• Build it from a container.

containers

Docker containers are similar to a directory. A Docker container holds everything that is needed for an application to run. Each container is created from a Docker image. Docker containers can be run, started, stopped, moved, and deleted. Each container is an isolated and secure application platform. Docker containers are the run component of Docker.

Dockerfile

Dockerfile files can be used to create images by Docker.

Dockerfiles are files whose names are 'Dockerfile'. A Dockerfile is a scripts to create an image.

In general, a Docker file is based on another image. For example: a image inheritance chain is:

spark : hadoop : centos : scratch.

Base images like 'centos' are images that are from the scratch image which is an explicitly empty image from the official Docker.

We can consider 'scratch' is a reversed word by Docker. (You cannot pull, push an image named as 'scratch'.)

In fact, base images are not always OS images. like hello-world image, its Dockerfile is

FROM scratch
COPY hello /
CMD ["/hello"]

Why Docker is better than VMs

• Performance
  
  It is said that the performance of running applications in Docker is same as running them on native machine.
  
  Meanwhile, VMs performance is slower twice times than native machines.

Installation & configuration the Docker repository file

Method 1: Run the Docker installation scripts

curl -fsSL https://get.docker.com/ | sh

Method 2: From scripts

• Install Docker

$ sudo yum install docker

or light weight docker

$ sudo yum install docker-engine

• Create Docker repository file
  
  The configuration can help Docker to pull images from a registry.

sudo tee /etc/yum.repos.d/docker.repo <<-'EOF'
[dockerrepo]
name=Docker Repository
baseurl=https://yum.dockerproject.org/repo/main/centos/7/
enabled=1
gpgcheck=1
gpgkey=https://yum.dockerproject.org/gpg
EOF

Configuration Docker service

# configure load the Docker service at boot
sudo chkconfig docker on
# start Docker
sudo service docker start
# create a Docker group
sudo groupadd docker
# alow non-root user to run Docker
sudo usermod -aG docker $(whoami)

Verify Docker is installed

docker run hello-world
docker version
docker info

Docker key commands

Docker help

docker --help
docker [command] --help

Search an image

You may go to Docker Hub, search an image you want to use.

or

# search images related to 'spark'
docker search spark

Output;

INDEX NAME DESCRIPTION STARS OFFICIAL AUTOMATED

docker.io docker.io/sequenceiq/spark An easy way to try Spark 287 [OK]

docker.io docker.io/gettyimages/spark A debian:jessie based Spark container 27 [OK]

docker.io docker.io/singularities/spark An Apache Spark development Docker image 12 [OK]

docker.io docker.io/shopkeep/spark Docker container with Spark, Scala, SBT, a... 7 [OK]

Pull an image

# pull the image named 'sequenceiq/spark'
docker pull sequenceiq/spark

List local images

docker images

Output:

REPOSITORY TAG IMAGE ID CREATED SIZE

docker.io/hello-world latest c54a2cc56cbb 8 weeks ago 1.848 kB

docker.io/shopkeep/spark latest 4be7b9be182e 12 months ago 1.693 GB

Create a container

# create a container from image name
docker create -t -i shopkeep/spark

# Or create a container from image id
docker create -t -i c54

Output:

5bb2d0aaa227154920733fb9f13e1571fe40254dd1b3373b617550cdde71e57e

List local containers

# Show all containers
docker ps -a

Output:

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

5bb2d0aaa227 shopkeep/spark "/bin/bash" 15 minutes ago Created insane_kowalevski

# Show running containers
sudo docker ps

Run a container

docker start -a -i 5bb

Output:

root@5bb2d0aaa227:/opt/spark#

Note: Docker is smart enough to match partial container id 5bb with the container id 5bb2d0aaa227...

Enter a running container with the existing TTY

# in most cases, the running container was started with a command 'docker start 5bb'
docker attach 5bb

Enter a running container with new TTY

docker exec -ti 5bb bash

Exit from a running container

root@5bb2d0aaa227:/opt/spark# exit

NOTE: the exit command from the last TTY will stop the container.

Stop a container

docker stop 5bb

Save a container as an image

docker commit 5bb my/image1

Output:

sha256:5cf350490e9e8b7495acb753f4041e34788e4881300b28ffc958db06d45fb4b3

Save an image to a tar archive

docker save my/image1 > ~/my_image1.tar

Load an image from a tar archive

docker load -i ~/my_image1.tar

Docker images/containers locations

• /var/lib/docker
• C:\Users[user name].docker

References

• Docker website
• Docker Overview
• Docker installation
• How is Docker different from a normal virtual machine?
• What is the relationship between the docker host OS and the container base image OS?