One challenge when creating Dockerized apps is:
How do you create images that retain their immutability while remaining configurable across environments?
The solution I like to propose is based on the following key ideas:
Dockerized apps are artifacts just like the application artifact packaged within them.
For your application’s artifact (e.g. petclinic.jar) you would use strategies such as build-once-deploy-anywhere. This means you avoid building the application more than once and deploy the same artifact across environments.
You need to treat Docker images as artifacts as well - they need to built only once.
The app packaging, JRE version, libraries and other dependencies form the “immutable” aspect of a Docker image. The mutable component is the configuration.
- We would create one Docker Image for our sample application, but externalize its configuration.
Notes
- I use Docker Compose to demonstrate this idea but obviously, there are many ways to achieve the same thing.