Dockerfile

A Dockerfile is a simple text file that contains the commands a user could call to assemble an image.

Example, Dockerfile

FROM ubuntu:latest
MAINTAINER john doe 

RUN apt-get update
RUN apt-get install -y python python-pip wget
RUN pip install Flask

ADD hello.py /home/hello.py

WORKDIR /home

Docker Compose

Docker Compose

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

• define the services that make up your app in docker-compose.yml so they can be run together in an isolated environment.

• get an app running in one command by just running docker-compose up

Example, docker-compose.yml

version: "3"
services:
  web:
    build: .
    ports:
    - '5000:5000'
    volumes:
    - .:/code
    - logvolume01:/var/log
    links:
    - redis
  redis:
    image: redis
    volumes:
      logvolume01: {}

docker-compose exists to keep you having to write a ton of commands you would have to with docker-cli.

docker-compose also makes it easy to startup multiple containers at the same time and automatically connect them together with some form of networking.

The purpose of docker-compose is to function as docker cli but to issue multiple commands much more quickly.

To make use of docker-compose, you need to encode the commands you were running before into a docker-compose.yml file.

You are not just going to copy paste them into the yaml file, there is a special syntax.

Once created, you have to feed it to the docker-compose cli and it will be up to the cli to parse the file and create all the different containers with the correct configuration we specify.

So you will have separate containers, let's say, one is redis-server and the second one is node-app, and you want that created using the Dockerfile in your current directory.

Additionally, after making that container, you would map some port from the container to the local machine to access everything running inside of it.

So for your docker-compose.yml file, you would want to start the first line like so:

version: '3'

That tells Docker the version of docker-compose you want to use. After that, you have to add:

version: '3'
services: 
  redis-server: 
    image: 'redis'
  node-app:
    build: .

Please notice the indentation, very important. Also, notice for one service I am grabbing an image, but for another service I am telling docker-compose to look inside the current directory to build the image that will be used for the second container.

Then you want to specify all the different ports that you want open on this container.

version: '3'
services: 
  redis-server: 
    image: 'redis'
  node-app:
    build: .
    ports:
      -

Please notice the dash, a dash in a yaml file is how we specify an array. In this example, I am mapping 8081 on my local machine to 8081 on the container like so:

version: '3'
services: 
  redis-server: 
    image: 'redis'
  node-app:
    build: .
    ports:
      - "8081:8081"

So the first port is your local machine, and the other is the port on the container, you could also distinguish between the two to avoid confusion like so:

version: '3'
services:
  redis-server:
    image: 'redis'
  node-app:
    build: .
    ports:
      - "4001:8081"

By developing your docker-compose.yml file like this, it will create these containers on essentially the same network and they will have free access to communicate with each other any way they please and exchange as much information as they want.

When the two containers are created using docker-compose, we do not need any port declarations.

Now in my example, we need to do some code configuration in the Nodejs app that looks something like this:

const express = require('express');
const redis = require('redis');

const app = express();
const client = redis.createClient({
  host: 'redis-server'
});

I use this example above to make you aware that there may be some specific configuration you would have to do in addition to the docker-compose.yml file that may be specific to your project.

Now, if you ever find yourself working with a Nodejs app and redis, you want to ensure you are aware of the default port Nodejs uses, so I will add this:

const express = require('express');
const redis = require('redis');

const app = express();
const client = redis.createClient({
  host: 'redis-server',
  port: 6379
});

So Docker is going to see that the Node app is looking for redis-server and redirect that connection over to this running container.

The whole time, the Dockerfile only contains this:

FROM node:alpine

WORKDIR '/app'

COPY /package.json ./
RUN npm install
COPY . .

CMD ["npm", "start"]

So, whereas before you would have to run docker run myimage to create an instance of all the containers or services inside the file, you can instead run docker-compose up and you don't have to specify an image because Docker will look in the current working directory and look for a docker-compose.yml file inside.

Before docker-compose.yml, we had to deal with two separate commands of docker build . and docker run myimage, but in the docker-compose world, if you want to rebuild your images, you write docker-compose up --build. That tells Docker to start up the containers again but rebuild it to get the latest changes.

So docker-compose makes it easier for working with multiple containers. The next time you need to start this group of containers in the background, you can do docker-compose up -d; and to stop them, you can do docker-compose down.

Docker compose file is a way for you to declaratively orchestrate the startup of multiple containers, rather than run each Dockerfile separately with a bash script, which would be much slower to write and harder to debug.

Dockerfile and Docker Compose are two different concepts in Dockerland. When we talk about Docker, the first things that come to mind are orchestration, OS level virtualization, images, containers, etc.. I will try to explain each as follows:

Image: An image is an immutable, shareable file that is stored in a Docker-trusted registry. A Docker image is built up from a series of read-only layers. Each layer represents an instruction that is being given in the image’s Dockerfile. An image holds all the required binaries to run.

Container: An instance of an image is called a container. A container is just an executable image binary that is to be run by the host OS. A running image is a container.

Dockerfile: A Dockerfile is a text document that contains all of the commands / build instructions, a user could call on the command line to assemble an image. This will be saved as a Dockerfile. (Note the lowercase 'f'.)

Docker-Compose: 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 (containers). Then, with a single command, you create and start all the services from your configuration. The Compose file would be saved as docker-compose.yml.

In my workflow, I add a Dockerfile for each part of my system and configure it that each part could run individually. Then I add a docker-compose.yml to bring them together and link them.

Biggest advantage (in my opinion): when linking the containers, you can define a name and ping your containers with this name. Therefore your database might be accessible with the name db and no longer by its IP.

"better" is relative. It all depends on what your needs are. Docker compose is for orchestrating multiple containers. If these images already exist in the docker registry, then it's better to list them in the compose file. If these images or some other images have to be built from files on your computer, then you can describe the processes of building those images in a Dockerfile.

I understand that Dockerfiles are used in Docker Compose, but I am not sure if it is good practice to put everything in one large Dockerfile with multiple FROM commands for the different images?

Using multiple FROM in a single dockerfile is not a very good idea because there is a proposal to remove the feature. 13026

If for instance, you want to dockerize an application which uses a database and have the application files on your computer, you can use a compose file together with a dockerfile as follows

docker-compose.yml

mysql:
  image: mysql:5.7
  volumes:
    - ./db-data:/var/lib/mysql
  environment:
    - "MYSQL_ROOT_PASSWORD=secret"
    - "MYSQL_DATABASE=homestead"
    - "MYSQL_USER=homestead"
  ports:
    - "3307:3306"
app:
  build:
    context: ./path/to/Dockerfile
    dockerfile: Dockerfile
  volumes:
    - ./:/app
  working_dir: /app