This is a simple guide on how to deploy a multi-container ‘linked’ web app using Docker.
If you have not yet installed or set up a Docker host to run the containers on, here is my guide on setting up a basic uBuntu 16.04 Docker host VM.
The ‘web app’ we’ll be looking at how to deploy will consist of two basic components – a MySQL database for the back-end, and a simple PHP script for the ‘web front-end’ which simply connects to the MySQL container and displays some info from a database table.
For the MySQL container we’ll be using the official Docker repo ‘mysql-server’ image, and for our web front-end, we’ll be creating our own Docker image using a custom Dockerfile we’ll craft ourselves, based on an uBuntu 15.04 image.
This means we’ll be covering the following Docker basics:
- Running docker containers
- Linking docker containers (more secure than exposing ports directly)
- Creating custom docker images using a Dockerfile
- Building a custom image
Start off by creating a new directory in your home directory called ‘web01’ to create and store the Dockerfile we’ll using to build our custom web front-end image. Then create an emtpy file called ‘Dockerfile’ in this directory and edit it using your favourite text editor. I’m using nano for this.
This is what your new Dockerfile should look like:
The commands do the following:
- FROM – tells docker build to base this image build on the ubuntu:15.04 image
- RUN – strings a few apt-get commands together to install apache, php5, and a few other tools like curl. This is important, as every RUN command in a Dockerfile creates a new image layer, and we don’t want our image to contain too many layers.
- The last RUN command grabs the content from a gist I created which is a basic PHP script, and places it in the /var/www/html directory in the container, then deletes the default index.html file that apache places there. This is the script that will connect to our MySQL container and display some basic info (our basic ‘web app’).
- EXPOSE – exposes port 80 so we can map this to our Docker host and access the website outside of the container.
- CMD – runs the apache2 service with PID 1 when the container starts.
Now you can build the Dockerfile and create your own custom image, which is what will be used to start the web container later.
Use the following build command to build the new image from your custom Dockerfile
docker build -t=”web01image” ~/web01/Dockerfile
Run ‘docker images’ after the build completes and you should see the new image listed:
Next, you’ll run a new container using the official mysql-server image from the Docker repository. You won’t yet have this image locally, but the command will automatically download the image for you.
docker run –name db01 -e MYSQL_ROOT_PASSWORD=MyRootPassword -d mysql/mysql-server:latest
Note that I’ve called my container ‘db01’ and given it a root password of ‘MyRootPassword’. The -e parameter specifies that an environment variable called MYSQL_ROOT_PASSWORD inside the container should be given the value of ‘MyRootPassword’. The MySQL container then uses this environment variable to setup the root user for MySQL when the container starts.
Now that the database container is up and running (verify by running ‘docker ps’ to check its running), you can deploy the custom web container using your image you created above. In this docker run command, you’ll also link the web container to the db01 container you previously started up using the –link parameter. This is important to link the two containers.
The web container will be given environment variables with information telling it about the networking config of the DB container. These environment variables will then be access by the simple web PHP script to tell it where to find the database server, and what credentials to use to connect.
docker run –name=web01 –link=db01:mysql -d -p=80:80 web01image
Important: notice that in the –link parameter, the name of the database/MySQL container is specified. Make sure you use the exact name you gave your MySQL database container here – this ensures that the linking of the two containers is correct. The last ‘web01image’ bit specifies to base the container you are running off of the newly built ‘web01image’.
The -p parameter maps the exposed port 80 in the container to port 80 on the docker host, so you’ll be able to access the website by using http://dockerhost:80
Check that the new web container and previously created MySQL container are running by using the ‘docker ps’ command.
Out of interest, this is what the PHP script looks like (this is what is downloaded and placed on the web container as a RUN build step in the Dockerfile you created above):
You can see the environment variables that the PHP script grabs (top of the script) to establish the database connection from the docker container. These environment variables are what are created and populated by linking the web container to the db container using the –link parameter.
Lastly, you may want to create a sample database, table and some data for the simple ‘web app’ to display after it connects to the database container. Issue the following ‘docker exec’ command, which will add the sample database, create a sample table, and add some sample data.
Make sure you change the ‘MyRootPassword’ bit to whatever root MySQL password you chose when you ran the MySQL container above, and ensure you run exec against the name of the MySQL container you chose (I used db01). Keep the database name and the rest of the command intact, as the PHP script relies on these staying the same.
docker exec db01 mysql -u root -pMyRootPassword -e “create database testdb1; use testdb1; CREATE TABLE events (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, name VARCHAR(20), signup_date DATE); INSERT INTO events (id,name,signup_date) VALUES (NULL, ‘MySpecialEvent’, ‘2016-06-11’);”
Finally, browse to http://dockerhostnameorip and you should see the simple PHP script display some basic info, stating it was able to connect to the MySQL server and display the sample data in the database.
