An Introduction To Docker

This tutorial contains examples to become familiar with docker's basics.

Requirements

Well, you must have docker up and running on your PC.
Follow the guide at Configure local environment if you are still out of luck.

NOTE: Mind that if you are on MacOSX, you must make sure that the DOCKER_MACHINE_IP environment variable is configured on your host system

Check for the following two lines:

eval "$(docker-machine env dev)"
export DOCKER_MACHINE_IP=$(docker-machine ip dev)

to be found in .zshrc, .bashrc or .bash_profile (depending on what shell you are using).
To check if for the variable to be set, issue the following command in a terminal:

echo $DOCKER_MACHINE_IP

The docker-machine (aka docker-host or docker server) is the server on which we'll start our containers. MacOSX's kernel is unable to provide the container technology docker relies on (which is built-in on Linux kernel), so in the guide linked above, we provisioned a virtual machine with VirtualBox, running a Linux distro.

If you have

The docker-machine command

docker-machine is also a command-line tool able to interact with the docker-host (the vm), so that we can control it by remote. Sorry for the confusion: you can easily tell the meaning of the term "docker-machine" (command or vm) by the context.

Run this command to see the options of docker-machine tool:

docker-machine -h

The docker-host we just provisioned is called dev, so issuing

docker-machine ls

you should find you dev docker-machine up and running.

In the best Unix tradition, the ls command lists all available and running docker-hosts.
The DOCKER_MACHINE_IP environment variable provides thus an host that the docker command (not `docker-machine command!) will use as default target for all commands we'll give.

The docker command

docker is a command-line client for the docker-engine, the service which manage the containers on a docker-enabled maching.
You'll use it to control, interact with and inspect all containers running of your beloved docker-host.

Starting with the basics, let's learn which containers we have started:

docker ps

as the Unix ps command shows all running processes, this command lists all running containers on the target machine. Mind the "running" keyword: a container is often identified by the process in it. Stop the process and the container will result stopped.

But wait: you have no running containers? Here is how to create and run one.

First of all let's download a container image:

docker pull php:7.0.4

or maybe you don't like PHP?

docker pull busybox

What is happening here is that you are asking Docker to download an image (much like one of those you can use in a virtual machine or an ISO file) on your machine.
From where are those images coming? If not otherwise instructed, Docker will search for those images on the DockerHub (https://hub.docker.com), a huge online public repository similar to what GitHub is for code.

Spoiler: you can have private images on DockerHub attending a payment plan.

Now you docker-host (remember it is your OS if you are running Linux natively) has the container image in its portfolio. Let's check listing for all locally available images:

docker images

You can visit the DockerHub to find all available public images. There are a ton of them to choose from and to kickstart your own image (read along).

Containers - a basic example

As an exercise we'll now create and run an image. We'll use the official busybox one. Busybox is a tiny Linux distribution.
Issuing the following command we'll run a process inside a container:

docker run -it --rm busybox ash

Let's dissect this command to understand what's going on:

  1. docker: we are invoking the Docker client (pretty dull, ok)
  2. run: this is a command to the docker-engine. We are asking to run a process to be found in an image. This translates into "running a container", since docker will instantiate a container in which to run a process and use a given image for that container. This also means that if you stop that process in the container the container itself will "stop": it will still exists but nothing will be running in it.
  3. -it: these parameters can be a bit tricky, but basically means: assign a TTY to the container and make me able to interact with the container by my console (more here: https://docs.docker.com/engine/tutorials/dockerizing/#/run-an-interactive-container). So far let's state that if you want to run a process that will run in background you won't need this.
  4. --rm: this parameters make docker remove the container filesystem from the host when the container is stopped (good for an example so the garbage won't take space from the host - more at https://docs.docker.com/engine/reference/run/#clean-up-rm)
  5. busybox is the image from which the container is created (we downloaded it earlier but if not docker will download it for you)
  6. ash is the process to start inside the container. It should obviously be available in the image or docker won't be able to start it. In this case ash is a simple shell

Running the command you will end up executing a shell inside your busybox-based container.
If you stop the shell process (issuing exit or ctrl-D), the filesystem for that container will be destroyed, so don't write your next fiction blockbuster in it! :D

Look here https://docs.docker.com/engine/reference/run/ for a complete overview of the run command.

Containers - a more advanced example

OK, executing a shell in a volatile container is not something that makes us drool. Let's try something more juicy.

Say that we want to test our next killer PHP script (which surely greet the world from inside a container), and we don't want to bother installing apache and PHP on our host.
Create a demo folder and your script in it:

mkdir demo
cd demo
echo ‘<?php print “Hello from a docker container, world!”;' > index.php      ## we kept this short, you can use your editor for a more complex test

Now we want a php-powered apache instance to be available to test our code. Instead of installing the whole stack, let's try this (from inside the demo folder of course):

docker run -d -v $PWD:/var/www/html -p 80 --name myphpapache php:7.0.4-apache

Another walkthrough:

  1. docker run: execute a process in the container
  2. -d: stays for "detached mode", which is a fancy way to say "in background". You can read this as the opposite of -it that we saw before
  3. -v: creaite a volume in the container, which mounts,,,
  4. $PWD:/var/www/html: ...this directory! $PWD is the current directory we are while issuing the command: we'll mount it in the container in the volume at the position /var/www/html. Let's make it even simpler: what's inside the directory you are while issuing the command will be available "live" in the container at /var/www/html, so if you change your script in your host it will be changed in the container also!
  5. -p 80: expose the TCP/UDP port 80 in the container on the "public" network interface automatically assigned by docker to the new container. Again, let's state it clear: the new container will have an assigned IP, say 172.17.0.15. If you connect to an external port on this IP, any service in the container bound to the port 80 will reply. Which port? Read on.
  6. --name myphpapache: we are naming our container so we can refer to it with a mnemonic label instead that an awful string like abfe13e3f12da122cc212
  7. php:7.0.4-apache: the image we want the container to be based upon: in this case an installation of Apache with PHP7 support

Notice that in the previous example we specified a command to run in the image (ash in that case). This has not been the case in this example? Why?
To make it easy, when you build an image you can specify an "entry point" that is a command to run as default when a new container is created and run by that image. php:7.0.4-apache official image has been built so that executing the container automatically runs apache and all necessary modules.
This is a common pattern in the Docker industry, since Docker is often used to package and distribute application with all necessary dependencies to fulfill a specific purpose: in this case provide a php web stack with no fuss!

So we should now see our apache process running in a docker container:

> docker ps

In the list of running containers we'll see something like this in the row of myphpapache

0.0.0.0:<PORT NUMBER>->80/tcp

<PORT NUMBER> is automatically generated when you use the -p option. Let's say it's 32768.

so

open http://$DOCKER_MACHINE_IP:<PORT NUMBER>

or say

open http://$DOCKER_MACHINE_IP:32768

Hurray!!! :)

OK, this sucks! We don't want to chase the port every time we start the container, so let's specify which public port to expose the internal port onto:

docker run -d -v $PWD:/var/www/html -p 80:80 --name myphpapache php:7.0.4-apache

Can you spot the difference? In this case we are using 80:80 as value for the -p parameter, so that we'll be able to reach apache in our container querying http://$DOCKER_MACHINE_IP:80.
Mind that the first number is the public port and the second is the internal port, so -p 90:80 will expose the interal apache port on external port 90.

Containers - Network of containers

What's a PHP script worth without a database to query?
We now want to start a MySQL database alongside Apache, so that our PHP script can connect to the database and rock a lot.

Let's destroy the former Apache container (named myphpapache, as we can tell issuing docker ps):

docker rm -vf myphpapache

Again, here is the command dissection:

  • rm: remove a container (spoiler: to remove an image instead the command is mri, but don't do it now)
  • -vf:
  • myphpapache: is the name we gave to the container (spoiler: you can refer to the container by its container-id also. Find it with docker ps, beginning of the line)

For further details on rm command visit: https://docs.docker.com/engine/reference/commandline/rm/

OK, we made clean! Now pull a proper MySQL container image from the docker hub:

docker pull mysql:5.6

start the container mounting a local folder as a volume as we already did before, so that database data are outside the container (they will persist if the container is destroyed):

docker run --name mydatabase -d -v $PWD:/home/data -e MYSQL_ROOT_PASSWORD=root mysql:5.6

Note that we mounted the local directory to /home/data: to learn where the internal process expects config files, data, etc you must refer to the documentation of the image itself! There is no a magic catchall solution or standard!

Check MySQL is running with

docker ps

OK, the server is there!

No we want some data, so we'll add a database into our fresh server, creating a table with dummy data in it.
To do so, let's save the following in a file named test_data.sh (which is a shell script).

#!/bin/sh
mysql -uroot -proot << END
create database IF NOT EXISTS dockertest;
use dockertest;
CREATE TABLE IF NOT EXISTS test_table (
  id int(10) unsigned NOT NULL AUTO_INCREMENT,
  title varchar(255) DEFAULT NULL,
  PRIMARY KEY (id)
);
INSERT INTO test_table (title) VALUES ('titolone 1');
END

This, when executed into the container, will use the MySQL command line client to fire a query that does the job.

Execute this bash script invoking bash inside the container:

docker exec -it mydatabase /bin/bash -c /home/data/test-data.sh
chmod a+rx ./test-data.sh

Look what we have done here:

  • docker exec: exectues a command in a container
  • -it mydatabase: do it interactively in the running container named mydatabase
  • /bin/bash -c /home/data/test-data.sh: this is the command to execute, which basically is "run an instance of the bash shell, which reads the commands found in the file /home/data/test-data.sh"

Note: This is interesting: since we mounted $PWD - which is the directory we are into - as a volume in /home/data inside the container, our script is "already there". Nice uh?

Now we have a MySQL database. The server is running in a container and the data are kept separated, in the local folder on the host system. Great!
Next step: we need an apache+php container able to connect to MySQL. We'll do this starting a new container like we already did, with the --link option.

docker run --name myphpapache -d -v $PWD:/var/www/html -p 80:80 --link mydatabase:mysql php:7.0.4-apache

Can you spot the --link mydatabase:mysql part? Here is where we are instructing docker to configure the inner network stack of the two containers to create a functional routing, so they can exchange information over TCP/IP.

But what the :mysql part means? It's an hostname that will automatically be available in the Apache container to resolve the MySQL container. Pure awesomeness!

Now we can edit our previous script to make it more interesting (let's fake interest for this stuff at least!):

try {
  $dbh = new PDO("mysql:host=mysql;dbname=dockertest", 'root', 'root');
  $sql = 'SELECT * FROM test_table';
  print '<pre>';
  foreach ($dbh->query($sql) as $row) {
    print_r($row);
  }
  print '</pre>';
}
catch(PDOException $e) {
    echo $e->getMessage();
}

Note: can you see that we just mentioned the MySQL container hostname specified in the --link option? Yay!

Reach back to our browser and visit our container URL

> open http://$DOCKER_MACHINE_IP

Nah! We got an error! :/

could not find driver

php-mysql extension is not in the container! This is true since the php image in the Docker Hub only contains essential modules and has no PDO extensions in it.
We need an image with such and extension!i Instead of hunting for it in the Docker Hub, why not building one by ourselves? We'll extend the default one with more useful options.

Images - How to build one

To build a custom image, we have to describe to docker what we want inside it. It suffice we create a file with a list of directives. It will act as a recipe that docker can follow to bake a delicious cookie.

This file is called Dockerfile and its syntax is really straightforward. Here is a simple example that fulfills our needs:

FROM php:7.0.4-apache
RUN docker-php-ext-install pdo pdo_mysql && docker-php-ext-enable pdo pdo_mysql

each row in a Dockerfile starts with a command that takes parameters or information on what to do.
In this case we are using:

  • FROM command to tell docker to build an image taking another contributed one as a starting point. In our case it's the image we are already using, since we want to add PHP modules to it!
  • RUN command to tell docker we want to execute a command inside the container. The command is docker-php-ext-install pdo pdo_mysql && docker-php-ext-enable pdo pdo_mysql which installs the extensions we need. (NOTE: don't be fooled by the docker-php-ext-enable command, it is not a docker internal! Instead it is an helper script that the base image provide and the docker- part of the name is simply a choice of the script developer).

Now let's destroy the old container, which is useless as-is:

docker rm -vf myphpapache

and build a new image for our next container from our brand new Dockerfile recipe:

docker build -t spark/php .

what's happening here is:

  • build is the command to bake a new image from a local Dockerfile (you don't have to specify the file, docker will search for it in ./)
  • -t allows us to assign a name to the newly created image (is stands for tag, so we often refer to an image as tagged, not named)
  • spark/php is the name of our custom image.

It is considered good practice to namespace images with the owner's profile name, so that no collision will happen on dockerhub. This is the reason for the spark/ prefix`.

Learn more on the build command here: https://docs.docker.com/engine/reference/commandline/build/

We can now list all your local images (local! we haven't pushed our image anywhere yet so we and we only can start a container from that image!)

docker images

Learn more on the images command here: https://docs.docker.com/engine/reference/commandline/images/

Is the spark/php image there? Well, we can start our container then! :)

docker run --name myphpapache -d -v $PWD:/var/www/html -p 80 --link mydatabase:mysql spark/php

let's discover the exposed port for apache on this container:

docker ps

In our case it is 32770/tcp. Head back to the browser!

open http://$DOCKER_MACHINE_IP:32770

et voilà! Le site est servì. :)

Happy with the ephemeral nature of our work of art, let clean the slate for a new, creative experience :)

docker rm -vf myphpapache
docker rm -vf mydatabase

(At this point you should be aware of what you are doing)

This closes the curtain on the docker basics we need to work. In the next chapter we'll discover of a useful tool to avoid having to start and stop containers by hand over and over.

Orchestrating containers wirh docker-compose

Now that we have the tools to create complex infrastructures with Docker, we can go fancy with complex networks of containers and services to build our projects upon.
That's great, but it is a bit tiresome to manage all of this by hand. After all, we don't want to start all our containers with the --restart policy to stop worring about it. What if you run tens of sites locally, but work on just one of them at a time? All that system resources occupied with no good.

So we need something that allows us to control and orchestrate the containers for an infrastructure in a single place. This tool exists and it's docker-compose.

Basically docker-compose makes use of a YAML file that, with a simple syntax, describes a set of containers, their start options and the links (network connections) between them. In addition it can interact with other pieces of the system, like dnsdock or nginx-proxy to inform them of newly created containers and automatically set domains or vhosts for us.

In addition the declarative nature of the docker-compose.yml file makes really, really easy for anyone to get a grasp on a multicontainer architecture at a glance!
Look at this:

data:
  image: busybox
  volumes:
    - ./:/home/data
    - ./:/var/www/html
apache:
  build: .
  dockerfile: Dockerfile
  environment:
    - DNSDOCK_ALIAS=myphpapache.docker.sparkfabrik.loc
  volumes_from:
    - data
  links:
    - mysql
mysql:
  image: mysql:5.6
  environment:
    - MYSQL_ROOT_PASSWORD=root
    - DNSDOCK_ALIAS=mydatabase.docker.sparkfabrik.loc
  volumes_from:
    - data

Pretty straightforward, isn't it? This adds a data container to the set, which is a container which mounts all volumes, so that every other container in the set can see them without redeclaring them over and over.

Saving this snippet in our demo directory as docker-compose.yml will suffice to effectively run:

docker-compose up

or better yet

> docker-compose up -d

so all the containers' services will run in background without cluttering the screen.

Running the command will automatically instantiate all parts of the infrastructure we already seen in previous chapter.
Notice how, populating the DNSDOCK_ALIAS variable for each container, we can inform the dnsdock container of new resolution rules for that specific container. But not only this: see that we are not binding any port? That's another automagic of dnsdock: the port will be exposed to match the resolver, so we can simply reach the container from outside using a canonical name.

open http://myphpapache.docker.sparkfabrik.loc/

Learn more on docker-compose here: https://docs.docker.com/compose/

The only missing thing is the execution of /home/data/test-data.sh after the database server startup.
This can't be done by docker-compose, which by itself is not meant to execute commands. Instead we can put this in (wait for it...) a Dockerfile to build a database server image which runs the command once and for all.

Stop the execution of containers with ctrl-C (if you didn't use the -d option) or issuing:

docker-compose down

Rename the spark/php container in Dockerfile.apache for convenience and create a new file named Dockerfile.mysql; then populate it as follow:

FROM mysql:5.6
COPY test-data.sql /docker-entrypoint-initdb.d/test-data.sql

The test-data.sql file will contain this SQL instructions:

create database IF NOT EXISTS dockertest;
use dockertest;
CREATE TABLE IF NOT EXISTS test_table (
  id int(10) unsigned NOT NULL AUTO_INCREMENT,
  title varchar(255) DEFAULT NULL,
  PRIMARY KEY (id)
);
INSERT INTO test_table (title) VALUES ('titolone 1');
INSERT INTO test_table (title) VALUES ('titolone 2');

Now we have to edit docker-compose.yml in accordance, so that it will use the new Dockerfiles to start the containers instead of some contributed image.
See how the image: property is changed into dockerfile: one for apache and mysql blocks:

data:
  image: busybox
  volumes:
    - ./:/home/data
    - ./:/var/www/html
apache:
  build: .
  dockerfile: Dockerfile.apache
  environment:
    - DNSDOCK_ALIAS=myphpapache.docker.sparkfabrik.loc
  volumes_from:
    - data
  links:
    - mysql
mysql:
  build: .
  dockerfile: Dockerfile.mysql
  environment:
    - MYSQL_ROOT_PASSWORD=root
    - MYSQL_DATABASE=dockertest
    - DNSDOCK_ALIAS=mydatabase.docker.sparkfabrik.loc
  volumes_from:
    - data

Issuing now a restart of the whole containers set (in background unless you don't want to inspect what's happening at startup)

docker-compose up -d

Head to the browser

open http://myphpapache.docker.sparkfabrik.loc/

And once again: Voilà! Le site est servì.

Wrap up

We have took a good understanting on the concepts of images and containers, being the first a sort of blueprint for the latter, much like an ISO image can be a blueprint for a set of perfectly identical DVDs.

We have learned that there is a place where public images can be hosted freely (privates at a price) and retrieved from everywhere, so we can leverage them to build new images or to start our containers locally.

We have learned we can build upon the work of others, using a Dockerfile to bake new images.

We have seen how a container can run a service in isolation, with alle dependencies, configurations, bells and whistles attached.

We have learned how to inspect a running container and reach for the service running in it, exposing the service port on the public-facing container's IP (bonus: and to map the outermost port on that container so we can head firmly to that port, not relying on a random one assigned by Docker to 0.0.0.0).

We have learned that those services can be networked together to form an isolated infrastructure mimicing a real-world one, with high decoupling (and thus high modularity!) and great efficiency.

We have found a way to manage all that complexity in a single place (a docker-compose.yml file), so that we can easily understand, describe and contribute our containers set.

Gotchas

How we put this to use

Working on different projects, with different versions of Drupal and production machines not always managed by us always put a great delta between our local environment and the various other ones we had to address.
Docker allows us to build our projects on services which versions and options are the same of the target production environment. To achieve this we have a different set of containers for each and every project. We also use the same containers on stage, test and integration envs, so we are not running those environments on the host OS or bare metal.

With docker-compose this is a breeze. What we basically did was:

  • creating a set of docker images for the base services (apache, mysql, solr, etc) ready for our projects
  • setting a docker-compose.yml template we can inherit in every project we start, describing that project's infrastructure
  • committing the docker-compose file and every additional Dockerfile needed by the application within the application repository

Thus all apps will have their own local infrastructure, you can start it with a single command and only when needed. The only persistent (i.e. --restart=always) container on the local machine is the dnsdock one.

To learn all you need on our local environment read our Local Environment setup guide.

MacOSX Resolver's issues

If for some reason you can't seem to resolve the dnsdock hostnames on MacOSX, screw it the hard way:

sudo dscacheutil -flushcache
sudo killall -HUP mDNSResponder
Last updated on 2018-11-23T16:08:06+00:00