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Docker

Taking notes from: * https://www.youtube.com/watch?v=RqTEHSBrYFw * https://github.com/sidpalas/devops-directive-docker-course

1 History & Motivation

  • A docker container is a lightweight, stand alone, executable, package of software that includes everything needed to run an application.
  • Bare metal < Virtual machines < Docker containers
  • Docker container is an instance of a docker image.
  • Docker container contains application and binaries and libraries.
  • Reduces dependency hell + is isolated.

2 Technology Overview

  • namespaces, control groups & union filesystem.
  • namespace => isolate context to container
  • cgroup => control how resources shared for a given container. Avoids an application taking up too much resources.
  • Union mount filesystem => Allows you take seperate filesytems to be combined together.
  • Docker desktop does most of the work for us.

3 Installation & Setup

https://docs.docker.com/ * Hello world example for docker: 

docker run rancher/cowsay cowsay "Sup big man"

  • Setup postgres 15.1 on port 5432 on alpine os. (Note we need to proved password env variable) 
    docker run --env POSTGRES_PASSWORD=foobarbaz --publish 5432:5432 postgres:15.1-alpine

4 Using 3rd Party Containers

https://hub.docker.com/ * Any data created or modified in a container is ephemeral. alt text * Solution => volume mount, bind mount * https://github.com/sidpalas/devops-directive-docker-course/blob/main/04-using-3rd-party-containers/README.md

  • To interact with container from the command line use -it flag. 

    docker run --interactive --tty ubuntu:22.04
    same as 
    docker run -it ubuntu:22.04

  • To list all running containers do (-a flag to list all, even exited ones): 

    docker ps -a

  • Docker doesnt remember installing packages after setting up container. Specify install as part of docker image in dockerfile.

  • Docker images don't persist data: 

    ➜  ~ docker run -it --rm ubuntu:22.04
root@6d203c82bcaf:/# mkdir my-data
root@6d203c82bcaf:/# echo "Hello from the container!" > /my-data/hello.txt
root@6d203c82bcaf:/# cat my-data/hello.txt
Hello from the container!
root@6d203c82bcaf:/# exit
exit
➜  ~ docker run -it --rm ubuntu:22.04   
root@ecb159d73754:/# cat my-data/hello.txt
cat: my-data/hello.txt: No such file or directory

Volume mounts

Use volumes and mounts to safely persist the data.

To create a volume: 

docker volume create my-volume

To mount volume my-volume to my-data folder in container: 

docker run -it --rm --mount source=my-volume,destination=/my-data/ ubuntu:22.04

This connects the folder my-data to the volume my-volume. Anything you save into my-data persists in my-volume.

So if you exit out the container and create a new one which also mounts to my-volume, the same files would be accessible.

(Volumes lives on the virtual machine docker creates)

Bind mounts

Binds a folder from host filesystem to a folder in the container

docker run  -it --rm --mount type=bind,source="${PWD}"/my-data,destination=/my-data ubuntu:22.04

Useful for easy visibility of data, but most times volume mounts are preffered.

Diagram: mount diagrame

Use cases of 3rd party containers

  • Databases
  • Interactive test environments
  • CLI utilities

Improving ergonomics:

  • Use a shell function
  • Use alias

5 Demo Web Application

Minimal 3 Tier web app: * React frontend * Two Api implementations: * node (interpreted) * golang (compiled) * PostgreSQL Database

Link: https://github.com/sidpalas/devops-directive-docker-course/tree/main/05-example-web-application

key take away: Trying to run the apis and frontend without containers can be difficult. You have to check what version of node and go you have, what dependencies each api has etc..

This motivates the next section

6 Building Container Images

Link: https://github.com/sidpalas/devops-directive-docker-course/tree/main/06-building-container-images

Dockerfile : A text document that contains all the commands user could call on the command line to assemble an image.

dockerfile + build context + docker ignore = docker build

Docs: https://docs.docker.com/reference/dockerfile/

  • Have Dockerfile where the code lives.
  • docker build . in same dir to build docker image from DockerFile
  • COPY . . in DockerFile to copy entire current working dir to image.
  • To clear docker cache: docker system prune
  • To name a docker image you built: docker build -t api-node:0 .
  • List all docker images docker image list
  • Avoid creating images from scratch (can lead to massive image sizes), use optimised versions from docker hub.