Docker

1. INTRODUCTION

WHAT IS IT ?

Docker is a platform for developers and sysadmins to develop, deploy, and run applications with containers.

HISTORY

ONE APPLICATION ON ONE PHYSICAL SERVER

PROBLEMS

slow deployment times
Huge costs
Wasted resources
Difficult to scale or migrate
Vendor lock in

HYPERVISOR-BASED VIRTUALIZATION

One physical server can contain multiple applications
Each application runs in a virtual machine

BENEFITS

Better resource pooling
- one physical machine divided into multiple VM
Easier to scale
VM's in the cloud
- Pay as you go

LIMITATIONS

Each VM stills requires
- CPU allocation
- storage
- RAM
- An entire guest operation system
More VM's you run, the more resources you need
Guest OS means wasted resources

CONTAINERS

Container based virtualization uses the kernel on the host's operating system to run multiple guest instances

Each guest instance is called a container
Each container has its own
- Root filesystem
- Processes
- Memory
- Network ports

CONTAINERS

CONTAINERS VS VMS

Containers are more lightweight
No need to install guest OS
Less CPU, RAM, storage space required
More containers per machine than VMs
Greater portability

2. CONCEPTS AND TERMS

DOCKER AND THE LINUX KERNEL

Docker Engine (deamon) is the program that enables containers to be built, shipped and run.

INSTALLATION

LINUX:

wget -qO- https://get.docker.com/ | sh

WINDOWS AND MAC OS X

Docker need linux kernel... so you will need a linux Virtual Machine. Docker toolbox contains everything needed.

VERIFY DOCKER INSTALLATION

This command download and run an hello world container

/*lets try*/

docker run hello-world

CHECKING CLIENT AND DAEMON VERSION

docker version

/*lets see*/

IMAGES AND CONTAINER

Images

Read only template used to create containers
Built by you or other Docker users
Stored in the Docker Hub or your local Registry

Containers

Isolated application platform
Contains everything needed to run your application
Based on images

3. IMAGES

WHAT IS IT ?

An image is an executable package that includes everything needed to run an application--the code, a runtime, libraries, environment variables, and configuration files.

SEARCH FOR IMAGES ON DOCKER HUB

lots of Images available for use

https://hub.docker.com/explore/

DISPLAY LOCAL IMAGES

When creating a container, docker will attempt to use a local image first
If no local image is found, the Docker daemon will look in Docker Hub unless another registry is specified.

/*lets take a look*/

docker images

4. GETTING STARTED WITH CONTAINERS

CREATING A CONTAINER

Syntax

docker container run[OPTIONS] IMAGE [COMMAND] [ARG...]

examples

/*Demo time*/

CONTAINER WITH TERMINAL

Use some options:

-i flag tells docker to connect to STDIN on the container
-t flag specifies to get a pseudo-terminal

EXAMPLE

docker container run -i -t ubuntu /bin/bash

/*Demo time. Adding user*/

CONTAINER PROCESSES

A container only runs as long as the process from your command is running
Your command's process is always PID 1 inside the container

CONTAINER ID

Containers can be specified using their ID or name
Long ID and short ID
Short ID and name can be obtained using docker container ls command to list containers
Long ID obtained by inspecting a container
use docker container ls to list running containers
use docker container ls -a to list all containers (includes containers that are stopped)

/*Take a look*/

RUNNING IN DETACHED MODE

Also know as running in the background or as a daemon
Use -d flag
To observe output use docker logs [container id]

Example

/*Take a look*/

docker run -d ubuntu ping 127.0.0.1 -c 50

PORTS MAPPING

Run a web application inside a container
The -P flag to map container ports to host ports

Example

docker container run -p 80:80 nginx

/*Take a look*/

5. DOCKERFILE

INTRO TO DOCKERFILE

A Dockerfile is a configuration file that contains instructions for building a Docker image

Instructions specify what to do when building the image
FROM instruction specifies what the base image should be
RUN instruction specifies a command to execute

FROM ubuntu:14.04
RUN apt-get install -y vim
RUN apt-get install -y curl

IMAGE LAYERS

Images are composed of multiple layers
A layer is also just another image
Every image contains a base layer
Docker uses a copy on write system
Layers are read only

DOCKER BUILD

Build an image from a Dockerfile

Syntax

docker build [OPTIONS] PATH

Example

docker build -t test-image:0.0.1 .

CMD INSTRUCTION

CMD defines a default command to execute when a container is created
CMD performs no action during the image build
Shell format and EXEC format
Can only be specified once in a Dockerfile
Can be overridden at run time

#Shell format
CMD ping 127.0.0.1 -c 30
#Exec format (json)
CMD ["ping", "127.0.0.1", "-c", "30"]

6. MANAGING IMAGES AND CONTAINERS

GETTING TERMINAL ACCESS

Use docker exec command to start another process within a container
Execute /bin/bash to get a bash shell

Exiting from the terminal will not terminate the container

/*lets try */

docker exec -i -t [container ID] /bin/bash

DELETING CONTAINERS

Can only delete containers that have been stopped
Use docker container rm command
Specify the container ID or name

docker container rm cde647f6e1d2

DELETING LOCAL IMAGES

Use docker rmi command
docker image rm [image ID]
if an image is tagged multiple times, you have to delete each tag

docker image rm cde647f6e1d2

7. VOLUMES

VOLUMES

A volumes is a designated directory in a container, wich is designed to persist data, independent of the container's file cycle

Persist when a container is deleted
Can be mapped to a host folder
Can be shared between containers

MOUNT A VOLUME

Volumes are mounted when creating or executing a container
Can be mapped to a host directory
Volume paths specified must be absolute

#Execute a new container and mount the folder into its file system
docker run -it --name test-volume -v myvol2:/app ubuntu:14.04#Execute a new container and map the /data/src folder from the host into the /test/src folder in the container
docker run -it --name test-volume-2 -v $PWD:/app ubuntu:14.04

USES OF VOLUMES

De-couple the data that is stored from the container which created the data (exemple logs)
Good for sharing data between containers
- Can setup a data containers which has a volume you mount in other containers
Mounting folders from the host is good for testing purpose but generally not recommended for production use

8. DOCKER COMPOSE

Docker Compose is a tool for creating and managing multi container applications

Containers are all defined in a single file called "docker-compose.yml"
Each container runs a particular component/service of your application.
For example:
- Web front end
- User authentification
- Payments
- Database
Compose will spin up all your containers in a single command

CONFIGURING THE COMPOSE YML FILE

Defines the services that make up your application
Each service contains instructions for building and running a container

version: '3'

services:
  db:
    image: postgres
  web:
    build: .
    command: python3 manage.py runserver 0.0.0.0:8000
    volumes:
      - .:/code
    ports:
      - "8000:8000"
    depends_on:
      - db

BUILD AND IMAGE INSTRUCTION

Build defines the path to the Dockerfile that will be used to build the image
Container will be run using the image built
Image defines the image that will be used to run the container
All service must have either a build or image instruction

RUNNING YOUR APPLICATION

Use docker-compose build
- Build the image for each service
Use docker-compose up
- Create and start the containers

SOURCES AND REFERENCE:

https://docs.docker.com/get-started/

https://training.docker.com/

https://labs.play-with-docker.com/