---
id: mat-04-pod-networking-lifecycle-and-patterns
title: "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`