Yichun Zhao JAVA Notes: September 2022

Docker Concepts

Docker

In an English dictionary, a docker is a person employed in a port to load and unload ships. Docker refers to a product and a vendor that provides software to run containerized applications.

Container

A container is enclosed, self-contained, and independent. It represents an independent os or a partial os that may run an application.

Containers are isolated from each other, and it is also isolated from the underlying OS. It is a self-contained system and can be run on any OS.

Image

Image may be considered as a blueprint, a piece of instructions to build a container. Docker uses the image as a blueprint to create containers and running them on top of the host OS.

Docker Volume

Docker containers are supposed to be ephemeral, created and destroyed by automation tools at any time so that they cannot persist data. To persist data from a container, we have to use a docker volume to plug a host file folder into a container. If a docker is not mounted a volume explicitly, a container will automatically create a volume inside the host. Otherwise, it allows mounting an explicit volume on a container.

docker volume ls: to show all volumes

there are three types of volumes, a default volume, a mounted specific host folder, or a named volume. In production, a named volume is often applied.

Docker File

A DockerFile is a text document that contains all the commands that a user could call on the command line to assemble an image. Using the docker build, users can create an automated build that executes several command-line instructions in succession.

commands in DockerFile

FROM: define the base image, on which we will be building

ADD: add files to the container being built. Add<source><destination in container>

RUN: add layers to the base image, by installing component

CMD: is used to run commands at the start of the container. these commands run only when there is no argument specified while running the container.

ENTRYPOINT: os is used strictly to run commands the moment the container initializes. The difference between CMD and ENTRYPOINT will run irrespective of whether the argument is specified.

ENV: define environment variables in the container runtime.

run apt-get update

run apt-get -y install apache2

Docker Commands

docker pull <image>: Pull an image or a repository from a registry

docker ps: show all running containers

docker ps -a: show all containers, including running and exited ones

docker run -it -d <image>: run a docker container from a image, returning a <container-id>

-it: give me an interactive terminal

-d: run the container as a daemon

docker run -it <container > : leaving user an active console that showing app logs and input

docker run <container > : console input is deactivated; user cannot interact with the console

docker exec <container-id>: get the terminal of the running container.

docker exec -it container-id bash

-it: give me an interactive terminal

bash: is a Linux terminal to interact with

step out of a container, typing 'exit';

docker start <container>: Start one or more stopped containers

docker stop <container>: Stop one or more running containers

docker container rm <containername|containerId>: Removing one or more exited containers

docker container rename <containername> <newname>: renaming a container

docker kill <container-id>: stoping the container in a forced way.

docker rm <container-id>: removing the container from the docker supervision.

docker rmi <image-id>: removing a specific image.

docker rmi $(docker images) : removing all docker images

docker volume ls: list all volumes

docker volume crate postgres_vol-1: explicitly create a volume and give it a name.

flag -v and specify a name for the volume when you run the Container and map it to the directory, like this

docker run -v postgres_vol_2:/var/lib/postgresql/data --name volume-postgres -p 5433:5432 -d craig/postgres:version1

-p: binding a laptop port to a container port

-v: binding a host file directory to a virtual folder of the container; this case binding a named-volume to a specific virtual folder of the container.

docker volume --help: finding out other docker volume options

docker logs <containerName|containerId>: showing specific container logs.

show docker container log in a real time

docker logs --follow docker_container_id

'--follow' a full option name; or for a short, just '-f'. this option will tail the latest logs on the screen in a real time.

Docker Network

Docker network is another docker concept. When deploying more than one container on the isolated docker network, containers can talk with others directly by using container names, without localhost, port number, etc.

docker network ls : list all docker networks.

docker network create new-network-name: create a new docker network

Docker Compose

it is a tool to define and instruct how running several docker containers together.

version: '3'

services:

web:

image: nginx:latest

ports:

- "8080:80"

depends_on:

- db

db:

image: mysql:5.7

environment:

MYSQL_ROOT_PASSWORD: examplepassword

MYSQL_DATABASE: testdb

MYSQL_USER: user

MYSQL_PASSWORD: userpassword

volumes:

- db-data:/var/lib/mysql

volumes:

db-data:

Run Apache2 within Ubuntu container

now we run the Ubuntu container and mapping Apache2 80 to the Local system

dcoker run -it -p 80:80 -d ubuntu

-p: port

Apache2 normally working on port 80;

Apache2 will be running inside a container on the top of the docker, which resides on the local OS.

So we need to map the port of this container onto my local system.

now go inside the container

docker exec -it <container-id>

apt-get update

checking if Apache2 is running

service apache2 status

start the Apache2

service apache2 start

Run Postgres from Docker Container

Click the link above.

Initialization scripts

If you would like to do additional initialization in an image derived from this one, add one or more *.sql, *.sql.gz, or *.sh scripts under /docker-entrypoint-initdb.d (creating the directory if necessary). After the entrypoint calls initdb to create the default postgres user and database, it will run any *.sql files, run any executable *.sh scripts, and source any non-executable *.sh scripts found in that directory to do further initialization before starting the service.

Difference between Docker and VM

To fully understand Docker, we also need to talk about the difference between Docker and a Virtual Machine (VM). The latter often runs in cloud environments like AWS and Azure. Whenever you create a VM you are sharing the hardware with others and other VMs. What these cloud environments are doing is 'virtualise' the hardware. Docker doesn't do that and does it differently. In a VM you can have multiple Operating Systems running on the same hardware, whilst with Docker, you virtualise the Operating System. Therefore, the big difference between VMs and Docker containers is that the former can have multiple (Guest) Operating Systems on the same hardware, through for example VMWare (which is called a Hypervisor). when you install Docker, you are going to use the Docker Engine to create isolated entities on the OS. These entities are called containers. Docker, therefore, allows you to automate the deployment of applications in these containers.

run mySql from container

docker run --name=testMySql -e MYSQL_ROOT_PASSWORD=test -p 3306:3306 -d mysql:latest

sudo docker exec -it testMySql bash

bash-4.4# mysql -u root -p

mysql> show databases;

docker run --name=testMySql -e MYSQL_ROOT_PASSWORD=test -p 3306:3306 -d mysql:5.7.39

Yichun Zhao JAVA Notes

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