Dockerizing legacy applications with confd
This blog post is an excerpt of our book Rapid Docker on AWS.
A legacy application typically uses files to read configuration parameters. But working with configuration files is cumbersome when building Docker images for the use with ECS (EC2 or Fargate). In theory, you could copy configuration files to the EC2 instances and mount them into your containers. However, this approach is cumbersome and does not work with Fargate at all. Alternatively, you could create a Docker image for every configuration. Building and managing so many different Docker images for each application will cause a lot of headaches.
It is a best practice to use environment variables to configure an application running inside a Docker container. Doing so allows you to start the Docker image with a customized configuration on any platform.
You could rewrite your application to read configuration parameters from environment variables instead of files. However, there is a much simpler way. The tool confd generates configuration files based on templates and inserts configuration parameters handed over via environment variables.
You will learn how to dockerize your legacy application with little effort next. Let’s start with creating a
The following section installs and configures
confd. You will learn more about how to create configuration files with
confd in a second — no need to change anything here.
In the following steps, you will learn how to write configuration files on container startup based on environment variables with
The legacy application uses a configuration file to configure the database connection:
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Our goal is to use environment variables for each property. To do so with
confd, we need to create a configuration template. The following snippet shows the template
docker/confd/templates/app.ini.tmpl for our
conf/app.ini configuration file.
In the template, each value has been replaced with a placeholder.
For example, the following example references the environment variable
Besides the template you need to create a
confd configuration file as shown in the following snippet from
srcthe template file
destthe destination path for the generated configuration file
keysthe keys used within the template file
One last step, the script
custom-entrypoint is executed whenever you start a container. The script runs
confd and executes the
When dockerizing your application, you need to create your configuration files with
confd?: Create a new template file
.tmpl and store it at
docker/confd/templates/. Next, create a new configuration file
.toml and store it at
docker/confd/conf.d/. Finally, modify the template and configuration file according to this example.
Looking for the source code of an end-to-end example? Check out our book Rapid Docker on AWS.
confd, it is simple to build a Docker image reading configuration parameters from environment variables. No need for rebuilding the way your legacy application reads configuration files.
Want to learn more about how to dockerize your application and run containers on AWS? Our book Rapid Docker on AWS is designed for web developers and DevOps engineers who want to dockerize their web applications and run their Docker containers on Amazon Web Services.