Chapter 16 of 47 · Configuration

Environment variables

An environment variable is a name and a value that your app reads when it starts, like PORT=4000. In Kubernetes you write them in the YAML, under the container. That is really all there is to it. This chapter shows how, slowly.

What it is

Think of it as a sticky note attached to your app when it starts:

You already used them with Docker:

$ docker run -e PORT=4000 -e LOG_LEVEL=debug micro-posts

Kubernetes does exactly the same thing. You just write it in the YAML instead of the command.

Why use them

So one image can run in different places with different settings, without rebuilding it:

Typical things to put in env vars: ports, URLs of other services, log levels, feature switches.

How to write them

Add an env: list inside the container. Each item has a name and a value. Click the highlighted parts:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: posts-depl
spec:
  replicas: 1
  selector:
    matchLabels:
      app: posts
  template:
    metadata:
      labels:
        app: posts
    spec:
      containers:
        - name: posts
          image: micro-posts:1.0
          env:
            - name: EVENT_BUS_URL
              value: http://event-bus-srv:4005
            - name: LOG_LEVEL
              value: debug
            - name: PORT
              value: "4000"

If you remember the Pod chapter: it is the same containers list, with one more key. In a bare Pod it goes in exactly the same place.

Try it

Change names and values, add or remove variables. Try a value of true or 3000 and watch the quotes appear.

Reading them in Node

// posts/index.js
const PORT = Number(process.env.PORT) || 4000;
const EVENT_BUS_URL = process.env.EVENT_BUS_URL || 'http://localhost:4005';
const DEBUG = process.env.DEBUG === 'true';

app.listen(PORT, () => console.log(`Posts listening on ${PORT}`));

• Every value arrives as text. "4000", not 4000; "true", not true. Convert it yourself.
• Give a default with ||, so the app still runs on your laptop without Kubernetes.
• A variable you did not set is undefined.

Check they arrived

$ kubectl exec deployment/posts-depl -- env
PATH=/usr/local/sbin:/usr/local/bin:...
HOSTNAME=posts-depl-7d9f8c6b5d-4kq2p
EVENT_BUS_URL=http://event-bus-srv:4005
LOG_LEVEL=debug
PORT=4000
...

$ kubectl describe pod posts-depl-7d9f8c6b5d-4kq2p
...
    Environment:
      EVENT_BUS_URL:  http://event-bus-srv:4005
      LOG_LEVEL:      debug
      PORT:           4000
...

You will also see variables you never wrote, such as HOSTNAME (the Pod name) and some ..._SERVICE_HOST ones Kubernetes adds automatically. Ignore those for now.

Changing a value

Your app reads env vars once, when it starts. A running container never sees a new value. So Kubernetes starts new Pods for you:

Three common mistakes

- name: PORT
  value: 4000
- name: DEBUG
  value: true

- name: PORT
  value: "4000"
- name: DEBUG
  value: "true"

Error: ... cannot unmarshal number into Go struct field EnvVar.value of type string

containers:
  - name: posts
    image: micro-posts:1.0
env:                       # outside the container!
  - name: PORT
    value: "4000"

containers:
  - name: posts
    image: micro-posts:1.0
    env:                   # lined up with image
      - name: PORT
        value: "4000"

- name: DB_PASSWORD
  value: "hunter2"

Recap

• An env var is a name and a value your app reads at start.
• Write them under env: inside the container: - name: X, value: "y".
• Quote numbers and true/false. Your app always receives text.
• Read them with process.env.X, with a default.
• Check with kubectl exec deployment/<name> -- env.
• Changing a value in a Deployment and applying it starts new Pods with the new value.

Quick check

Next: a whole microservices app in one file, which uses env vars to connect every service.