Retrieve current environment in code (Kotlin, Docker)

Question

I would like to retrieve info about where the application is running.

So let's say I have a DEV and PROD environment. In my code, I would like to retrieve the link of the environment, where the application is running, e.g.:

• if in DEV environment: https://dev.mycompany.com/myproduct/app
• if in PROD environment: https://mycompany.com/myproduct/app

Any hints?

Answer 1

Just pass it as environment variable (e.g. APP_ENV) to you app and use System.getenv("APP_ENV") to access it. It's one of the twelve-factor app best practices, btw.

The way you pass to you container differs depending on the way you start it.

Docker run

Pass the variables directly via --env / -e options for docker run command, or store them in a file and pass the whole file via --env-file:

docker run -e VAR1 --env APP_ENV=dev --env-file ./env your_image:latest

Docker compose

Use environment or env_file (note that they set NODE_ENV variable in the docs, just like in your case!):

version: '3'
services:
  app:
    image: 'your_image:latest'
    env_file:
     - ./env
    environment:
     - APP_ENV=dev

Kubernetes

Use env:

spec:
  containers:
  - name: app
    image: your_app:latest
    env:
    - name: APP_ENV
      value: "dev"

In k8s you can reference another values in environment variables, so the environment can be stored in the metadata.

---

There are, of course, other ways to provide values to the app, like:

• Code generation and build-time substitutions. It can be used to provide, for example, Git revision: the output of git rev-parse is just substituted somewhere in your code. Requires extra build configuration + you'll need different builds for different envs.
• Program arguments. It's as simple as running app arg1 arg2 ... dev ... argn. Pretty similar to environment variables, but, IMHO, has a flaw: program args are accessible only in your program's entry point (main), so you'll need to parse them and pass to the other parts of the app. I can be good for some dynamic inputs, but inconvenient for a static data, like app's environment.
• Vendor metadata, like EC2 Instance Metadata and User Data. Requires specific APIs to access.

As you see none of them provides a combination of flexibility, ubiquity and simpleness of environment variables.