Pods, ReplicaSets & Deployments

Pods, ReplicaSets & Deployments

Step 1 of 10

›kubectl apply -f api.yaml

 

You write desired state: a Deployment that wants three replicas of api.

The Idea

A Pod is the smallest thing Kubernetes runs: one or more containers that share a network namespace and storage. Pods are disposable, so you rarely create them yourself. A ReplicaSet keeps a set number of matching Pods running; a Deployment manages ReplicaSets and gives you declarative updates and rollbacks.

Real-World Example

You apply a Deployment with replicas: 3. It creates a ReplicaSet, which creates three Pods. Delete one, or lose its node, and the ReplicaSet makes a replacement — a new Pod with a new UID, not the old one moved.

The Tradeoff

Everything hangs on labels: the selector decides which Pods a ReplicaSet counts, so a careless label can make it adopt Pods it should not. Never edit a Deployment's ReplicaSets by hand; change the Deployment and let its controller reconcile.

Hands-On

# illustrative — kubectl apply -f api.yaml, then kubectl get rs,pods -l app=api
apiVersion: apps/v1
kind: Deployment
metadata: { name: api }
spec:
  replicas: 3
  selector: { matchLabels: { app: api } }
  template:
    metadata: { labels: { app: api } }
    spec:
      containers:
        - name: api
          image: registry.example.com/api:1.4.2
          ports: [{ containerPort: 8000 }]

Your turn

Put the steps in the right order.

1. 1The ReplicaSet creates three Pods from the template↑↓
2. 2kubectl apply sends the Deployment to the API server↑↓
3. 3The scheduler assigns each Pod to a node↑↓
4. 4The Deployment controller creates a ReplicaSet↑↓

Check order

Mini quiz

1 / 3

Containers in the same Pod can talk to each other over:

• AA Service only
• BThe node's public IP
• Clocalhost
• DNothing — a Pod holds one container

Sources

• Pods — Kubernetes Documentation
• ReplicaSet — Kubernetes Documentation
• Deployments — Kubernetes Documentation
• Controllers — Kubernetes Documentation