🐋 Docker Crash Course

[JayCesar]

Overview

[JayCesar]

What is Docker?

• Docker is a virtualization software that makes developing and deploying applications much easier.

• It works by packeging an application with all the necessary dependencies, configuration, system tools and runtime.
  - So it is a standardized unit, that has everything the application needs to run.

• It becomes a portable artifact easily shared and distributed.

Why is this a big deal?

Development process before containers

• Each developer needs to install and configure all services directly on their OS on their local machine
• Installation process could be differents (for each OS envirorment)
• Many steps, where something can go wrong.
• If an app uses 10 services, each developer has to install theses 10 services

Development process with containers

So how container solve theses problems?

• With docker I have the services in a own isolated environment
• Ex: Postgres packaged with all dependencies and configs
• I can run different versions of the same app without any conficts

So I can focus more on development!

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• A lot of things can go wrong by this way!
• Also, miscommunications could happen between the teams.

Solution:

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Virtual Machine vs Docker

• Docker is a virtualization tool like VM, so what is the difference?

How an OS is made up?

There are two main layers (SO and Kernel). Kernel talks to Hardware and SO are the applications. Linux Applications talk to the Linux kernel and Windows applications talk to the WIndoes kernel

Docker

• Docker virtualizes the OS Applications Layer
• Docker uses the kernel of the host because it does not have its kernel;

VM

It virtualizes the complete operating system and boots up its host.

Docker images are smaller than VM images

Important note: Docker works only with Linux Distro while VM with all OS's. This is the main difference between Docker and VM (Compatibility).

It means I can not run Linux bases images over Windows kernel, because I can not run Linux application layer in Windows kernel.

But later on, running Linux images in windows became possible but only using docker desktop. (Hypervison layer).

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Install docker

Docker Desktop

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What are images?

An executable artifact in software development and deployment (including Docker) is a file or set of files** that can be directly run or executed to perform a specific task or start an application.

And when I run this image, I mean, this executable application artifact, I have a running instance of this image, which is called Container. Run an image? I create a container.

So, I can run multiple containers from 1 single image:

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docker images = list docker images docker ps = list docker containers

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Docker Registry

How do we get images? Using docker registries:

Docker Hub is where I find images from a lot of services!

So images are versionated!

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DockerHub Registry is the default location when I try to pull an image.

If I do not specify the tag, It will download the latest by default too.

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Run an Image

I can run an image by using these command:

docker run {image}:{tag}

Then a container (with a random name) will be launched, but I will be 'inside' the container. So to exit I just run the command "exit"

I can run the command in background using "d" of "detach":

docker run -d {image}:{tag}

to see the logs:

docker logs {container}

Obs: I can run the images without running the command docker pull image