Dockerfiles and Automation

19 Jan, 2019

Iterative Development with Containers

I've always loved the idea of containers, and have written papers in grad school about their various forms, from logical storage buckets to standardized units of capitalism. But how technology adopts the concept of standardization works best: wrap code and the dependencies it needs to run in its own environment, and connect it to its siblings over an internal network. Build the containers when code changes, deploy and destroy them often. With a little work at persistence, its a great way to develop things faster and put them in real-world environments. Doing so from a laptop is even nicer.

Using Docker images and a node server, dynamically generate a suuuuper dumb dummy application running in containers. The app won't have any data, but this system works to connect a rails or nodejs server, a postgres database, and an nginx load balancer -- a real-world setup that could easily scale to millions of users.

After testing locally, deploy the same app to a Kubernetes cluster and see how it could scale to billions.

Setup

  • Docker
  • Docker Compose
Docker

The simplest way to install docker is through their website, which provides a full taskbar application for managing containers locally. Containers start as normal code that we use everywhere, and have to be built into a container image before we can use it. Below is a simple node server that responds to requests with an environment variable, and a Dockerfile to package it.

The node server responds to requests with the MESSAGE environmental variable. We can use this to test multiple deployments.

Node
// ./node-server-container/index.js
var http = require('http');
var fs = require('fs');

http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/html'});
  res.end(`<h1>${process.env.MESSAGE}</h1>`);
}).listen(8000);

The Dockerfile specs how the container image will behave. In this case, it uses a node image, makes a directory, copies the js file, opens the port, and starts the script. Once this image is built, we can deploy 100s of copies of the container, over and over.

# ./node-server-container/Dockerfile
FROM node
RUN mkdir -p /usr/src/app
COPY index.js /usr/src/app
EXPOSE 8000
CMD [ "node", "/usr/src/app/index" ]
NGINX

Define an nginx.conf with upstream servers outlined. NGINX will proxy requests to the upstream block, which contains the node app. We can access the node containers by name in docker:

upstream our-cool-app {
  server server1:8000;
  server server2:8000;
}

server {
    listen 8080;
    server_name our-cool-app;
    location / {
        proxy_pass http://our-cool-app;
        proxy_set_header Host $host;
    }
}
FROM nginx
RUN rm /etc/nginx/conf.d/default.conf
COPY nginx.conf /etc/nginx/conf.d/default.conf
Docker Compose
version: '3.2'

services:
  # Build node server Containers
  server1:
    build: ./node-server-container
    tty: true
    environment:
      - 'MESSAGE=no. 1'
    volumes:
      - './simple-server/data:/docker-vol'
  server2:
    build: ./node-server-container
    tty: true
    environment:
      - 'MESSAGE=no. 2'
    volumes:
      - './simple-server/data:/docker-vol'
  # Build nginx Container
  nginx:
    build: ./nginx-container
    tty: true
    links:
      - server1
      - server2
    ports:
      - '8080:8080'
docker-compose up --build

Adding Rails

Rails

Without getting into building a Rails app (I have some separate posts about that), get a barebones Rails API connected to postgres. Add models, data persistence and seeders later.

Gemfile

# Gemfile
ruby '2.5.1'
gem 'rails', '~> 5.2.2'
gem 'puma', '~> 3.11'
touch Gemfile.lock

The database host field needs to be the same name as its container, which gets defined in docker-compose.yml. We will use postgres to keep it simple.

# config/database.yml
default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  host: postgres
  username: worker
  password: <%= ENV['PG_PASSWORD'] %>

development:
  <<: *default
  database: base

test:
  <<: *default
  database: base

production:
  <<: *default
  database: base
# Dockerfile
FROM ruby:2.5.1
RUN apt-get update -qq && apt-get install -y build-essential postgresql
RUN mkdir /rails-app
WORKDIR /rails-app
ADD ./Gemfile /rails-app/Gemfile
ADD ./Gemfile.lock /rails-app/Gemfile.lock
RUN bundle install
Postgres

We can use a custom image to spin up multiple databases in the same container (for rails' dev, test, and prod), but for now will just use Docker's default image.

In the docker-compose.yml file, add:

version: '3.2'

services:
  # Build Rails Container
  rails:
    build: ./rails
    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
    environment:
      PG_PASSWORD: foobarbat
    ports:
      - '3000:3000'
    depends_on:
      - postgres
    volumes:
        - .:/rails/docker-vol

  # Build Postgres Container
  # (Use the same name as defined in Rails' Database config host)
  postgres:
    image: postgres
    restart: always
    environment:
      POSTGRES_PASSWORD: foobarbat
      POSTGRES_USER: worker
      POSTGRES_DB: base
    ports:
      - '5432:5432'

volumes:
    backend:


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