Docker Containers

What is Virtualization:

• It’s a process where software is used to create an abstraction layer over computer hardware.
• It allows H/W components of a single machine to be divided into multiple VMs which share these computing resources
• The abstraction software is called hypervisor, which separates VMs from one another and allocates processors, memory and storage among them

What are Containers:

• More agile way of handling virtualization, it abstracts the OS layer rather than H/W
• Rather than spinning up entire VM, containerization packages together everything needed to run an application/microservice (along with run time libraries they require to run)
• Container includes the code, its dependencies and even the OS itself. This allows application to almost anywhere.
• Container uses a form of virtualization where the leverage the feature of host OS to isolate processes' access to CPU, memory and disk
• Containers share the same OS kernel

Installation - Ubuntu:

Update the Ubuntu System Packages

--updates package info so that the system knows which one to upgrade

sudo apt-get update

--performs the upgrades of the packages installed on the system to the newest versions

sudo apt-get upgrade

Install the required dependencies for connecting to a Repo over https

sudo apt-get install apt-transport-https ca-certificates curl gnupg-agent software-properties-common

• This is part of a dependency package which is required for Docker to run
• apt-transport-https
• Ubuntu uses this to allow apt-get to download packages via HTTPS
• ca-certificate
• This package contains a list of common Certificate Authorities
• CURL
• Client URL Request Library
• A tool to get and send files using a URL syntax
• Used for downloading repo files for a package
• gnupg-agent
• This is used to manage secrets(private keys).
• You need to add Docker's official GPG key on your system
• software-properties-common
• Tool used to manage package sources
• Without this you would have to manually add/remove package repos on your system

Add the official GPG key from Docker

• Most Linux packages are verified using its GPG key - to verify authenticity & integrity of packages
• Use below command to download and add official GPG key from Docker into the Ubuntu systems
• The downloaded key is added to the list of trusted keys in the system

curl - fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

• In order to verify the authenticity of the downloaded key, fingerprints are used.
• Docker has provided the fingerprint in their docs so that it can be verified with the one we downloaded
• Use below command to verify the fingerprint

sudo apt-key finger 0EGHSSA - replace this with fingerprint

Add the Docker stable repo to your system

sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

• The info will be added in /etc/apt/sources.list

sudo cat /etc/apt/sources.list

• Run apt-get update again to update new packages that were installed earlier

sudo apt-get update

Install Docker CE

sudo apt-get install docker-ce

• The other package dependencies that will be installed alongside docker:

○ Docker-ce-cli

○ Containerd.io

• This will also start the docker daemon (service)

• Verify the docker

sudo docker version

○ This provides info about client and engine both

The Docker Ecosystem:

• Docker CLI

• Docker Daemon

• Docker Commands - https://docs.docker.com/engine/reference/commandline/docker

Docker Images, Containers and Hub:

• Image is static, read-only template

• Non-running representation of all the components required to run a corresponding container application

• Container, however, is a run time instance of the image.

• Hub is a container registry or repo. It is public repo and cloud based

○ Used for creating, sorting, testing and distributing Docker images

○ It has got below services & features:

§ Repo - storage where images are saved for either public or private use

§ Official Images - set of repos providing base OS images, download-and-use image of programming languages and platforms

§ Publisher images - images from software vendors like Microsoft, Google, IBM etc.

§ Builds - allows automatically build and upload images from Git repos

§ Webhooks - integrate with other services through automation

□ Eg. Creating an automation server when you successfully upload a Docker image to Docker hub

§ https://hub.docker.com

§ Docker search microsoft | grep sql

Docker image naming convention:

• REGISTRY[:PORT]/REPO/IMAGE[:TAG]

• Registry hostname is a DNS comply name, default value is docker hub: https://index.docker.io

○ Run docker info command to get the details or registry

○ Available public container registeries

§ Microsoft Container Registry (MCR) : mcr.microsoft.com

§ Amazon Elastic Container Registry (ECR) : dkr.<region>.amazonaws.com

§ Google Container Registry (GCR) : gcr.io

• Repository name

○ Repo structure depends on how the images are stored on the container registry

○ For eg. Docker.io/microsoft is MS official repo on Docker Hub

• Image name

○ Identifies the contents of the Docker image

○ With numerous versions, editions, and OS combo, relying solely on image name isn't enough to get the right image

○ That is where tags come in

• Tags

○ To further identify the contents of the Docker image, providing additional details

○ If tag name is not provided, Docker will pull the latest tag that the corresponding image has in the repo

Docker run hello-world is equivalent to docker run hello-world:nanoserver-sac2016 or docker run hello-world:latest

○ List available tags for SQL server on Linux image - https://hub.docker.com/_/microsoft-mssql-server

Running a SQL Server on Linux Container:

• Pull the image

○ Docker pull mcr.microsoft.com/mssql/server:2017-CU14-ubuntu

• Run the container

○ Docker run -e "ACCEPT_EULA=Y" -e "SA_PASSWORD=D0cker@098!QAZ" -p 1433:1433 --name sqldevlinuxcon01 -d -h linuxsqldev01 mcr.microsoft.com/mssql/server:2017-CU14-ubuntu

○ -e : sets required env. Variables for SQL

○ -p : publishes container's TCP port(range) to the host - ip:hostPort:containerPort - mapping port 1433 on the host to port 1433 on the container

○ --name : custom name to help ifentify the container instead of a system-generated one

○ -d : runs the container in detached mode (background process) and prints the container ID. It ensures container still runs in background post docker run command.

○ -h : server hostname that you want to assign to the container

• Check for errors/status of containers

○ Docker images

○ Docker ps -a

Lifecycle of a container:

• Docker ps -a

• Created: when you create a container using docker create command but not run it

• Running: This is when container is up and running and doing its job

• Exited: This is when container has gone through running and completed its job. When primary process is not running, the container exits

• Paused: This is when you choose to pause the container using docker pause command, suspending all the processes

• Restarting: A container where a restart policy has been configured

• Dead: This is when a docker daemon attempted to stop the container but failed

• Docker run = docker create + docker start

○ Docker create hello-world

○ Docker start -a containerID

Some important docker commands:

Docker version -

Docker info

Docker run

Docker search

Docker pull

Docker images

Docker ps -a