A Practical Introduction to

Docker Compose

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Source Code : https://github.com/dockersamples/example-voting-app

https://hackernoon.com/practical-introduction-to-docker-compose-

In the words of Docker Inc.

Compose is a tool for defining and running multi-container Docker applications. With

Compose, you use a YAML file to configure your application’s services. Then, with a single

command, you create and start all the services from your configuration.

The Voting App

Introducing the most favourite demonstration app for the Docker community “The Voting

App”, as if it needs an introduction at all. This is a simple application based on micro-services

architecture, consisting of 5 simple services.

Docker containers opened a world of possibilities for the tech community, hassles in setting

up new software were decreased unlike old times when a mess was to be sorted by a grievous

format, it reduced the time to set up and use new software which eventually played a big part

for techies to learn new things, roll it out in a container and scrap it when done. Things

became easy, and the best thing its open source anyone and everyone can use it, comes with

a little learning curve though.

Out of the myriad possibilities was the possibility of implementing complex technology stacks

for our applications, which previously would have been the domain of experts. Today with

the help of containers software engineers with sound understanding of the underlying

systems can implement a complex stack and why not it’s the need of the hour, the figure of

speech “Jack of all trades” got a fancy upgrade; “Master of some” based on the needs of the

age. Simply put “T” shaped skills.

The possibility of defining a complex stack in a file and running it with a single command,

pretty tempting huh. The guys at Docker Inc. choose to call it Docker compose.

In this article, we will use Docker’s example Voting App and deploy it using Docker compose.

Docker Compose

Voting app architecture [https://github.com/docker/example-voting-app]

1. Voting-App: Frontend of the application written in Python, used by users to cast

their votes.

2. Redis: In-memory database, used as intermediate storage.

3. Worker: .Net service, used to fetch votes from Redis and store in Postres database.

4. DB: PostgreSQL database, used as database.

5. Result-App: Frontend of the application written in Node.js, displays the voting

results.

Cloud Computing – Docker Compose Tutorial

The Voting repo has a file called docker-compose.yml this file contains the configuration for

creating the containers, exposing ports, binding volumes and connecting containers through

networks required for the voting app to work. Sounds like a lot of pretty long docker

run and docker network create commands otherwise, docker compose allows us to put

all of that stuff in a single docker-compose file in yaml format.

Compose Time

With all of our application defined in a single compose file we can take a sigh of relief, chill

and simply run the application. The beauty of compose lies in the fact that a single command

creates all the services, wires up the networks(literally), mounts all volumes and exposes the

ports. Its time to welcome the up command, its performs all of the aforementioned tasks.

$ docker-compose up

After lots of “Pull complete”, hundreds of megabytes and few minutes (maybe more). . .

Voila, we have the voting app up and running.

Command docker ps lists all the running containers

$ docker ps -a --format="table {{.Names}}\t{{.Image}}\t{{.Ports}}"

NAMES IMAGE PORTS

voting_worker_1 voting_worker

db postgres:9.4 5432/tcp

voting_vote_1 voting_vote 0.0.0.0:5000->80/tcp

voting_result_1 voting_result 0.0.0.0:5858->5858/tcp,

0.0.0.0:5001->80/tcp

redis redis:alpine 0.0.0.0:32768->6379/tcp

The above command displays all the running containers, respective images and the exposed

port numbers.

The Voting app can be accessed on http://localhost:5000

Likewise the Voting results app can be accessed on http://localhost:5001

Each vote cast on the Voting app is first stored in the Redis in-memory database, the .Net

worker service fetches the vote and stores it in the Postgres DB which is accessed by the

Node.js frontend.

Compose Features

Compose provide the flexibility to use a project name to isolate the environments from each

other, the project name is the base name of the directory that contains the project. In our

voting app this is signified by the name of the containers voting_worker_1 where voting is

the base name of the directory. We can set a custom project name using the -p flag followed

by the custom name.

Compose preserves all volumes used by the services defined in the compose file, thus no data

is lost when the containers are recreated using docker-compose up. Another cool feature is

that only the containers which have changed are recreated, the containers whose state did

not change remain untouched.

Another cool feature is the support for variables in the compose file, we can define variables

in a .env file and use them in the docker-compose file. Here the POSTGRES_VERSION=9.4 can

defined in the environment file or can be defined in the shell. It is used in the compose file in

the following manner:

db:

image: "postgres:${POSTGRES_VERSION}"

Cloud Computing – Docker Compose Tutorial

Command Cheat Sheet

Its easy as breeze to start, stop and play around with compose.

$ docker-compose up -d

$ docker-compose down

$ docker-compose start

$ docker-compose stop

$ docker-compose build

$ docker-compose logs -f db

$ docker-compose scale db=4

$ docker-compose events

$ docker-compose exec db bash

Summary

Docker Compose is a great tool to quickly deploy and scrap containers, the compose file can

run seamlessly on any machine installed with docker-compose. Experimentation and learning

technologies is just a Compose file away ;).

I hope this article helped in the understanding of Docker Compose. I’d love to hear about how

you use Docker Compose in your projects. Clap if it increased your knowledge, help it reach

more people.

Cloud Computing – Docker Compose Tutorial