Getting started with docker
Contents
Introduction
Using docker you can wrap your application with all its dependencies into a container by having your host machine clean.
Installation
Follow below links to setup docker on your machine.
Basics of docker
Before getting started with docker, you need to understand below terminologies.
docker-machine
Docker follows client-server architecture. docker-machine is a command to create Virtual machine with a minimal linux os running on it. All the images that you build will be hosted on this server.
docker
docker is a command to interact with your docker-machine. Think of it as a client which talks to a server(docker-machine) to get the work done.
Image
It is a pre-built image like the ones you download for OSes which can be ran on the container.
Container
Light weight virtual machine which runs your image.
Dockerfile
It is a manifest file which is used for building your own images. This manifest file contains set of instructions which guides docker to run list of commands to setup your application. Docker rebuilds the image only if it finds any new instruction after the first build.
Below is the sample Dockerfile
# Filename: Dockerfile
# Pull image from the docker hub
FROM ruby
RUN which ruby
On mac, you may need to start docker Quickstart Terminal before trying out docker commands.
Now, run docker build -t myruby . within the same directory. Your output will look like the below.
Use -t option to name your image.
Sending build context to Docker daemon 197.7 MB
Step 1 : FROM ruby
---> c6ebc3270d1c
Step 2 : RUN which ruby
---> Running in 425293ec8f98
/usr/local/bin/ruby
---> bc3c6f463349
Removing intermediate container 425293ec8f98
Successfully built bc3c6f463349
If you try to rerun the same command, you will find that docker says that it is Using cache.
Sending build context to Docker daemon 197.7 MB
Step 1 : FROM ruby
---> c6ebc3270d1c
Step 2 : RUN which ruby
---> Using cache
---> bc3c6f463349
Successfully built bc3c6f463349
Run docker images to find your images.
To start a new container, run docker run -t myruby which will start the irb console.
To enter into the container with new TTY, run docker run -it myruby /bin/bash.
Docker hub
It is a public repository which hosts all the docker images.
Dockerize Applications
Step by step guide to move things to docker.
Rails on docker
Requirements:
- ruby
- build-essential
- nokogiri dependencies
- nodejs
Dockerfile:
# Pull image from the docker hub
FROM ruby
RUN apt-get update -qq && apt-get install -y build-essential
# Nokogiri dependencies
RUN apt-get install -y libxml2-dev libxslt1-dev
# JS runtime dependencies
RUN apt-get install -y nodejs
RUN gem install rails
Run docker build -t rails . to create a new rails image.
Rails application
Lets take a sample rails application which uses MySQL database with redis and a sidekiq for running background jobs.
Docker compose
It's a tool provided by docker(You may need to install this separately from here) which takes a YAML file with all the components, links and the ports to be exposed. And it will take care of linking, exposing ports, mounting volumes etc. This command will look for docker-compose.yml file or you can also pass the custom YAML file to it.
Sample docker-compose.yml may look like this:
redis:
image: redis
ports:
- '6379'
Run docker-compose build inside the directory which contains docker-compose file to build all the images and then do docker-compose up to start the services. When you run build, it will fetch latest redis image from the docker hub since we are using image option. When do docker-compose up, redis container will be started in its default port which we are exposing it to our host machine by setting it in the ports options.
Creating Dockerfile
Lets create a Dockerfile with ruby and install bundler.
# Pulls latest ruby version from the docker hub
FROM ruby
RUN apt-get update -qq && apt-get install -y build-essential
# Nokogiri dependencies
RUN apt-get install -y libxml2-dev libxslt1-dev
# JS runtime dependencies
RUN apt-get install -y nodejs
RUN apt-get install -y mysql-client
RUN gem install bundler
ENV BUNDLE_PATH /bundle
Creating docker-compose file
Create a docker-compose file within the same directory where you have created Dockerfile. Typically you will have these files inside the root of your rails application.
MySQL:
For MySQL server you would need a persistent storage meaning you wouldn't want to have data inside the same container. For these reasons, docker provides you with data volume containers or you could use data volume. Lets create a data volume container for MySQL. Data volume container is a container which will save your data but use a light weight and existing image for running these containers.
Now, lets create a docker-compose file with mysql data volume.
Filename: docker-compose.yml
mysql_data_volume:
image: redis
volumes:
- /var/lib/mysql
command: echo 'Data container for mysql'
We have named the image as mysql_data_volume. Here we are using redis as a light weight image and /var/lib/mysql is a path that is created in this container. Whenever we start the services, docker runs a startup command. For data containers, you don't want your containers to be running all the time.
So, we just run a command which will create a container and exit. But, it will not be removed.
Lets add MySQL server to it.
mysql_data_volume:
image: redis
volumes:
- /var/lib/mysql
command: echo 'Data container for mysql'
mysql:
image: mysql:5.5
volumes_from:
- mysql_data_volume
environment:
- MYSQL_ROOT_PASSWORD=$DOCKER_MYSQL_ROOT_PASSWORD
Here, we are setting mysql version as 5.5 which will be downloaded from the docker hub. mysql will be the image name. Set the env. variable DOCKER_MYSQL_ROOT_PASSWORD in your host machine. When you start the services, docker will pick up the env. variables.
Redis:
redis:
image: redis
I'm not creating data container for redis. But, you can try creating a data container for redis like we have done for mysql.
Sidekiq:
sidekiq:
build: .
working_dir: /opt/myapp/current
environment:
- BUNDLE_GEMFILE=/opt/myapp/current/Gemfile
command: bundle exec sidekiq --environment staging -C /opt/myapp/shared/config/sidekiq.yml
volumes:
- '$PWD:/opt/myapp/current'
links:
- mysql
- redis
sidekiq will be our image name. Setting build to . will tell docker-compose to look for Dockerfile within this directory. You need to set working directory for running rails commands. Use environment to set environment variables for the sidekiq container. Running sidekiq as startup command. You can mount host volume on the docker container using volumes option. Syntax goes like this [HOST_FILE_PATH]:[DOCKER_CONTAINER_FILE_PATH]. Since, sidekiq needs mysql and redis to run, we are linking both our images to it using links option.
Application
myapp:
build: .
working_dir: /opt/myapp/current
environment:
- BUNDLE_GEMFILE=/opt/myapp/current/Gemfile
command: bundle exec thin start
ports:
- '3000:3000'
volumes:
- '$PWD:/opt/myapp/current'
links:
- mysql
- redis
We are using same Dockerfile for both sidekiq and the application. In ports options, we are exposing default port of the thin server on the host machine.
Running your application on docker
First, run docker-compose build . to build all the images. Once it completes, we need to run bundle install. You do not want to run it all the time. Lets cache the gems by creating a data volume container for bundler.
bundle_data:
image: redis
volumes:
- /bundle
command: echo 'Data container for bundler'
Now, you need to use mount these volume inside sidekiq and myapp images.
Your final docker-compose will look the following,
bundle_data:
image: redis
volumes:
- /bundle
command: echo 'Data container for bundler'
mysql_data_volume:
image: redis
volumes:
- /var/lib/mysql
command: echo 'Data container for mysql'
mysql:
image: mysql:5.5
volumes_from:
- mysql_data_volume
environment:
- MYSQL_ROOT_PASSWORD=$DOCKER_MYSQL_ROOT_PASSWORD
redis:
image: redis
sidekiq:
build: .
working_dir: /opt/myapp/current
environment:
- BUNDLE_GEMFILE=/opt/myapp/current/Gemfile
command: bundle exec sidekiq --environment staging -C /opt/myapp/shared/config/sidekiq.yml
volumes:
- '$PWD:/opt/myapp/current'
volumes_from:
- bundle_data
links:
- mysql
- redis
myapp:
build: .
working_dir: /opt/myapp/current
environment:
- BUNDLE_GEMFILE=/opt/myapp/current/Gemfile
command: bundle exec thin start
ports:
- '3000:3000'
volumes:
- '$PWD:/opt/myapp/current'
volumes_from:
- bundle_data
links:
- mysql
- redis
To run bundle install using docker-compose, do docker-compose run myapp bundle install.
Finally, run docker-compose up to start all the services.
Setup nginx to host your rails application
Using nginx, we are going to expose port 80 to outside world which serves your rails application. We're going to link rails app with nginx container. So far we have seen how to link containers within a directory. Lets see how to link external containers within the current directory which contains docker-compose.yml for nginx.
Lets create a nginx conf file.
nginx.conf
user www-data;
worker_processes 4;
events {
worker_connections 768;
}
http {
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 65;
types_hash_max_size 2048;
default_type application/octet-stream;
access_log /var/log/nginx/access.log;
error_log /var/log/nginx/error.log;
gzip on;
gzip_disable 'msie6';
include /etc/nginx/conf.d/*.conf;
include /etc/nginx/sites-enabled/myapp;
}
sites-enabled/myapp
server {
listen 80;
server_name localhost;
root /opt/myapp/current/public;
location / {
proxy_pass http://rails_app_myapp_1:3000;
}
}
docker-compose.yml
web_server:
image: nginx
volumes:
- './config:/etc/nginx'
- '/opt/myapp:/opt/myapp'
- '/var/log/nginx:/var/log/nginx'
ports:
- '80:80'
external_links:
- rails_app_myapp_1
Mounting volumes for nginx configuration, your application directory and nginx log path. Also exposing port 80 on your host machine. rails_app is directory name of your rails app which contains docker files and myapp_1 is nothing but your image name of your app. When you start any container docker will name the container with the image name and suffixes it with _[0-9.*] and thats why we are using rails_app_myapp_1. To verify it, just run docker ps command if you rails app container is running. You will find this container name.
Now just run docker-compose up to start the nginx container.