content.md

Containers with Docker

The problems for developers

• Consistent environment
  Ability to create predictable environments that are isolated from other applications.
• Run anywhere
  Ability to run virtually anywhere: on Linux, Windows, and Mac operating systems; on virtual machines or bare metal; on a developer’s machine or in data centers on-premises; in the public cloud.
• Isolation Ability to virtualize CPU, memory, storage, and network resources at the OS-level, providing developers with a sandboxed view of the OS logically isolated from other applications.

What are containers?

• Standardized unit of software that allows developers to isolate their application from its environment.
• Packages code and all its dependencies, so the application runs quickly and reliably from one computing environment to another.
• Container platforms:
  • Docker
  • LXC (Linux Containers)
  • rkt (CoreOS Rocket)

Docker container

• Docker since 2013
• Became an industry standard
• Easy to use
• An excellent building block (for software delivery according to DevOps perspectives)

Docker

• Portable anywhere (Linux, Windows, Data center, Cloud, Serverless, etc.)
  Docker created the industry standard for containers.
• Lightweight
  Containers share the machine’s OS system kernel and therefore do not require an OS per application, driving higher server efficiencies and reducing server and licensing costs.
• Secure
  Applications are safer in containers and Docker provides the strongest default isolation capabilities in the industry.

Container vs Virtual Machines vs Bare metal

Docker architecture

• client (CLI via REST API)
• server (Docker host) - deamon, containers, images, volumes
• registry - Docker Hub

Docker components

Docker engine:

• Server (daemon process)
• REST API
• Client (CLI)

Docker objects

• Images
• Containers
• Networks
• Volumes
• ... some more objects

Docker objects are available to be observed and controlled using the command docker <object-name>.

Example of a Docker workflow

1. Developers write code locally and share their work with their colleagues using Docker containers.
2. They use Docker to push their applications into a test environment and execute automated and manual tests.
3. When developers find bugs, they can fix them in the development environment and redeploy them to the test environment for testing and validation.
4. When testing is complete, getting the fix to the customer is as simple as pushing the updated image to the production environment.

CLI commands

• docker help - explore commands
• docker ps - list runnig containers
• docker run <CONTAINER_NAME> - list runnig containers
• docker container - manage containers
• docker image - manage images
• docker volume - manage volumes
• docker network - manage networks
• docker exec - run a command in a running container

Example: docker run -i -t ubuntu /bin/bash

Building Docker images

Dockerfile - configuration file for building images.

Python:

FROM ubuntu:15.04
COPY . /app
RUN make /app
CMD python /app/app.py

Node.js:

FROM node:12
WORKDIR /usr/src/app
COPY package.json .
RUN npm install
COPY . .
CMD [ "npm", "start" ]

To build an image:

docker build .

Storage on Docker

By default all the files created inside a container are stored on writable container layer. This means:

• the data is not persistent
• You can’t easily move the data somewhere else

Storage types:

• volumes - prefered
• bind mounts
• tmpfs mount

Example:

docker run -v path/on/host:/app nginx:latest

Running multi-container applications with Docker Compose

Docker Compose

• is a tool for defining and running multi-container Docker applications.
• Uses a compose (configuration) file
• A way to document and configure all of the application’s service dependencies (databases, queues, caches, web service APIs, etc)
• Only one single command to create and start containers - docker-compose up

Use cases

1. Development environments
   When you’re developing software, the ability to run an application in an isolated environment and interact with it is crucial. The Compose command line tool can be used to create the environment and interact with it.

2. Automated testing environments
   An important part of any Continuous Deployment or Continuous Integration process is the automated test suite. Automated end-to-end testing requires an environment in which to run tests. Compose provides a convenient way to create and destroy isolated testing environments for your test suite.

3. Single host deployments
   To deploy to a remote Docker Engine.

Using Docker Compose

A three-step process:

1. Define your app’s environment with a Dockerfile so it can be reproduced anywhere.
2. Define the services that make up your app in docker-compose.yml so they can be run together in an isolated environment.
3. Run docker-compose up and Compose starts and runs your entire app.

Docker Compose example

Installing Wordpress website.

version: '3.3'

services:
   db:
     image: mysql:5.7
     volumes:
       - db_data:/var/lib/mysql
     restart: always
     environment:
       MYSQL_ROOT_PASSWORD: somewordpress
       MYSQL_DATABASE: wordpress
       MYSQL_USER: wordpress
       MYSQL_PASSWORD: wordpress

   wordpress:
     depends_on:
       - db
     image: wordpress:latest
     ports:
       - "8000:80"
     restart: always
     environment:
       WORDPRESS_DB_HOST: db:3306
       WORDPRESS_DB_USER: wordpress
       WORDPRESS_DB_PASSWORD: wordpress
       WORDPRESS_DB_NAME: wordpress
volumes:
    db_data: {}

Docker Compose commands

Base commands:

• docker-compose up - Create and start containers
• docker-compose down - Stop and remove containers, networks, images, and volumes
• docker-compose exec - Execute a command in a running container
• docker-compose scale - Set number of containers for a service