devops/materials/04-pod-networking-lifecycle-and-patterns.md

id	title
mat-04-pod-networking-lifecycle-and-patterns	Pod Networking, Lifecycle, and Multi-Container Patterns

Pod networking, lifecycle, and patterns

Anatomy of a Pod (namespaces)

A Pod isolates resources via namespaces such as:

• net namespace (IP, ports, routing table)
• pid namespace (isolated process tree)
• mnt namespace (filesystems and volumes)
• UTS namespace (hostname)
• IPC namespace (Unix domain sockets and shared memory)

Pods and shared networking

Every Pod has its own network namespace:

• its own IP
• its own TCP/UDP port range
• its own routing table

Single-container Pod:

• the container uses the Pod’s network namespace directly.

Multi-container Pod:

• all containers share the Pod’s IP/ports/routing table
• container-to-container communication is via localhost:<port>

Kubernetes pod network overlay

• Each Pod gets a unique IP that is routable inside the cluster’s pod network.
• The pod network is a flat overlay network that allows Pod-to-Pod communication even across nodes on different underlay networks.

Pod lifecycle

Typical phases:

1. Pending (accepted but not yet running)
2. Running
3. Succeeded (for short-lived apps that complete)
4. Running (for long-lived apps that keep running)

Pod restart policy

Possible configs:

• Always (default)
• OnFailure
• Never

Guidance:

• short-lived apps should be OnFailure or Never
• long-lived apps can be either, but are typically managed via Controllers
• short-lived apps are often wrapped in Jobs (e.g., CronJobs)

Pod immutability

Pods are immutable. If you need to change metadata, create a new Pod.

Multi-container Pod patterns

Common patterns:

• Sidecar: performs a secondary task for the main container (logging, metrics, service mesh, …)
  • adapter (variation): reformats output (e.g., nginx logs → prometheus)
  • ambassador (variation): brokers connectivity to external systems
• Init container: guaranteed to start and finish before the main container (pull content, setup privileges, …)

Pod hostnames

• Every container in a Pod inherits its hostname from the Pod name.
• All containers in a multi-container Pod share the same hostname.
• Use DNS-safe Pod names: a-z, 0-9, -, .

Pod DNS (example format)

Pods can have DNS names in the format:

• pod-ip-address.my-namespace.pod.cluster-domain.example

Example (default namespace; cluster domain cluster.local):

• 172-17-0-3.default.pod.cluster.local