9 — Application Health Patterns

Kubernetes

A book-shaped table of contents for Kubernetes: cloud-native foundations, the CKAD/CKA/CKS certification tracks, control-plane internals, platform tooling, multi-cluster architecture, and MAANG-level system design and interview prep — cross-linking the existing Prometheus, Observability, and Platform Engineering chapters instead of duplicating them.

1 — ConfigMaps

ConfigMaps mounted as volumes update live on the filesystem when the source changes, but env vars sourced from a ConfigMap are frozen at container start until the pod restarts.

3 — Downward API

The Downward API lets a container read its own pod's metadata — labels, annotations, resource limits, IP — as env vars or files, avoiding an API server round trip and the RBAC permissions that would require.

4 — Environment Variables

Env vars are resolved once when the container process starts, so a downstream ConfigMap or Secret edit has no effect until the pod is recreated — unlike a mounted volume, which the kubelet syncs live.

6 — Init Containers

Init containers run sequentially to completion before any app container starts, making them the natural place for one-time setup like schema migrations or dependency wait-checks that shouldn't re-run on every app-container restart.