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#90DaysOfDevOps - Docker Compose - Day 46

Docker Compose

The ability to run one container could be great if you have a self-contained image that has everything you need for your single use case, where things get interesting is when you are looking to build multiple applications between different container images. For example, if I had a website front end but required a backend database I could put everything in one container but better and more efficient would be to have its container for the database.

This is where Docker compose comes in which is a tool that allows you to run more complex apps over multiple containers. With the benefit of being able to use a single file and command to spin up your application. The example I am going to the walkthrough in this post is from the Docker QuickStart sample apps (Quickstart: Compose and WordPress).

In this first example we are going to:

  • Use Docker compose to bring up WordPress and a separate MySQL instance.
  • Use a YAML file which will be called docker-compose.yml
  • Build the project
  • Configure WordPress via a Browser
  • Shutdown and Clean up

Install Docker Compose

As mentioned Docker Compose is a tool, If you are on macOS or Windows then compose is included in your Docker Desktop installation. However, you might be wanting to run your containers on a Windows server host or Linux server and in which case you can install using these instructions Install Docker Compose

To confirm we have docker-compose installed on our system we can open a terminal and simply type the above command.

Docker-Compose.yml (YAML)

The next thing to talk about is the docker-compose.yml which you can find in the container folder of the repository. But more importantly, we need to discuss YAML, in general, a little.

YAML could almost have its session as you are going to find it in so many different places. But for the most part

"YAML is a human-friendly data serialization language for all programming languages."

It is commonly used for configuration files and in some applications where data is being stored or transmitted. You have no doubt come across XML files that tend to offer that same configuration file. YAML provides a minimal syntax but is aimed at those same use cases.

YAML Ain't Markup Language (YAML) is a serialisation language that has steadily increased in popularity over the last few years. The object serialisation abilities make it a viable replacement for languages like JSON.

The YAML acronym was shorthand for Yet Another Markup Language. But the maintainers renamed it to YAML Ain't Markup Language to place more emphasis on its data-oriented features.

Anyway, back to the docker-compose.yml file. This is a configuration file of what we want to do when it comes to multiple containers being deployed on our single system.

Straight from the tutorial linked above you can see the contents of the file looks like this:

version: "3.9"

    image: mysql:5.7
      - db_data:/var/lib/mysql
    restart: always
      MYSQL_ROOT_PASSWORD: somewordpress
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wordpress
      MYSQL_PASSWORD: wordpress

      - db
    image: wordpress:latest
      - wordpress_data:/var/www/html
      - "8000:80"
    restart: always
      WORDPRESS_DB_USER: wordpress
      WORDPRESS_DB_PASSWORD: wordpress
      WORDPRESS_DB_NAME: wordpress
  db_data: {}
  wordpress_data: {}

We declare a version and then a large part of this docker-compose.yml file is made up of our services, we have a DB service and a WordPress service. You can see each of those has an image defined with a version tag associated. We are now also introducing state into our configuration unlike our first walkthroughs, but now we are going to create volumes so we can store our databases there.

We then have some environmental variables such as passwords and usernames. These files can get very complicated but the YAML configuration file simplifies what these look like overall.

Build the project

Next up we can head back into our terminal and we can use some commands with our docker-compose tool. Navigate to your directory, where your docker-compose.yml file is located.

From the terminal, we can simply run docker-compose up -d this will start the process of pulling those images and standing up your multi-container application.

The -d in this command means detached mode, which means that the Run command is or will be in the background.

If we now run the docker ps command, you can see we have 2 containers running, one being WordPress and the other being MySQL.

Next, we can validate that we have WordPress up and running by opening a browser and going to http://localhost:8000 and you should see the WordPress set-up page.

We can run through the setup of WordPress, and then we can start building our website as we see fit in the console below.

If we then open a new tab and navigate to that same address we did before http://localhost:8000 we will now see a simple default theme with our site title "90DaysOfDevOps" and then a sample post.

Before we make any changes, open Docker Desktop and navigate to the volumes tab and here you will see two volumes associated with our containers, one for WordPress and one for DB.

My Current wordpress theme is "Twenty Twenty-Two" and I want to change this to "Twenty Twenty" Back in the dashboard we can make those changes.

I am also going to add a new post to my site, and here below you see the latest version of our new site.

Clean Up or not

If we were now to use the command docker-compose down this would bring down our containers. But will leave our volumes in place.

We can just confirm in Docker Desktop that our volumes are still there though.

If we then want to bring things back up then we can issue the docker up -d command from within the same directory and we have our application back up and running.

We then navigate in our browser to that same address of http://localhost:8000 and notice that our new post and our theme change are all still in place.

If we want to get rid of the containers and those volumes then issuing the docker-compose down --volumes will also destroy the volumes.

Now when we use docker-compose up -d again we will be starting, however, the images will still be local on our system so you won't need to re-pull them from the DockerHub repository.

I know that when I started diving into docker-compose and its capabilities I was then confused as to where this sits alongside or with Container Orchestration tools such as Kubernetes, well everything we have done here in this short demo is focused on one host we have WordPress and DB running on the local desktop machine. We don't have multiple virtual machines or multiple physical machines, we also can't easily scale up and down the requirements of our application.

Our next section is going to cover Kubernetes but we have a few more days of Containers in general first.

This is also a great resource for samples of docker-compose applications with multiple integrations. Awesome-Compose

In the above repository, there is a great example which will deploy an Elasticsearch, Logstash, and Kibana (ELK) in single-node.

I have uploaded the files to the Containers folder When you have this folder locally, navigate there and you can simply use docker-compose up -d

We can then check we have those running containers with docker ps

Now we can open a browser for each of the containers:

To remove everything we can use the docker-compose down command.


See you on Day 47