Env. Variables & Probes

Kubernetes allows you to configure runtime behavior of containers using environment variables, and to monitor their health using liveness and readiness probes. These features are essential for building reliable, configurable, and observable applications in the cluster.

Environment Variables

You can pass key-value pairs into containers using environment variables. These can be hardcoded, referenced from ConfigMaps, Secrets, or even dynamically derived from field references.

Static Environment Variables

env:
  - name: LOG_LEVEL
    value: "debug"

From ConfigMap

envFrom:
  - configMapRef:
      name: app-config

Or individual keys:

env:
  - name: APP_PORT
    valueFrom:
      configMapKeyRef:
        name: app-config
        key: port

From Secret

env:
  - name: DB_PASSWORD
    valueFrom:
      secretKeyRef:
        name: db-secret
        key: password

From Pod Metadata

env:
  - name: POD_NAME
    valueFrom:
      fieldRef:
        fieldPath: metadata.name

Probes Overview

Kubernetes uses probes to check if a container is:

  • Alive (liveness probe): Whether the app should be restarted
  • Ready (readiness probe): Whether the app is ready to receive traffic
  • Started (startup probe): Whether the app has finished starting up

Each probe runs a check (HTTP request, TCP socket, or command) and takes action based on success or failure.

Liveness Probe

Restarts the container if the probe fails repeatedly.

livenessProbe:
  httpGet:
    path: /healthz
    port: 8080
  initialDelaySeconds: 5
  periodSeconds: 10

Readiness Probe

Used to signal when the container is ready to receive traffic. If the probe fails, the Pod is removed from Service endpoints.

readinessProbe:
  httpGet:
    path: /ready
    port: 8080
  initialDelaySeconds: 5
  periodSeconds: 10

Startup Probe

Useful for applications that take a long time to initialize. Prevents premature liveness failures during startup.

startupProbe:
  httpGet:
    path: /startup
    port: 8080
  failureThreshold: 30
  periodSeconds: 10

Best Practices

  • Use readiness probes to avoid routing traffic to unready pods.
  • Use liveness probes for self-healing on deadlocks or hung apps.
  • Use startup probes for slow-starting applications.
  • Avoid setting initialDelaySeconds too low — allow the app to start first.
  • Prefer HTTP or command probes for rich diagnostics.
  • Avoid hardcoding values inside your images. Use environment variables, ConfigMaps, and Secrets for maximum flexibility and security.

Summary

  • Environment variables make your containers configurable and portable.
  • Use ConfigMaps and Secrets for dynamic or sensitive values.
  • Probes help keep your apps healthy and ready for traffic.