What is a pod and when would you put multiple containers in one?

Containers in a pod are always scheduled together on one node. They share an IP and port space, so they can reach each other over localhost, and they can share volumes. The pod, not the individual container, is the unit of scaling and replacement.

• One container per pod is the common default
• Sidecars suit proxies, log forwarding, metrics exporters and TLS termination
• Init containers run before app containers and must finish successfully
• Pods are ephemeral; do not rely on their identity or local disk
• Use a controller such as a Deployment rather than creating bare pods

apiVersion: v1
kind: Pod
metadata:
  name: web
spec:
  containers:
    - name: app
      image: registry.example.com/web:2.1.0
      ports:
        - containerPort: 8080
    - name: log-shipper
      image: registry.example.com/shipper:1.0.0
      volumeMounts:
        - name: logs
          mountPath: /var/log/app
  volumes:
    - name: logs
      emptyDir: {}

Multi-container pods are tightly coupled by design. If the helper can be scaled or deployed independently, it should usually be its own workload. Shared fate is the trade-off: a crashing sidecar can affect the pod and its readiness.