StatefulSets for stateful applications in Kubernetes - Step-by-Step Execution

Process Flow - StatefulSets for stateful applications

Define StatefulSet YAML

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kubectl apply StatefulSet

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Controller creates Pod 0

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Pod 0 gets stable network ID & storage

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Controller creates Pod 1

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Pod 1 gets stable network ID & storage

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Pods run with stable identity

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Scaling up/down maintains order & identity

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Pods terminate gracefully preserving data

Shows how Kubernetes StatefulSet creates pods one by one with stable network IDs and storage, maintaining order and identity for stateful apps.

Execution Sample

Kubernetes

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  serviceName: "nginx"
  replicas: 2
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
  - metadata:
      name: www
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi

Defines a StatefulSet named 'web' with 2 replicas, each pod gets stable network ID and storage.

Process Table

Step	Action	Pod Created	Pod Identity	Storage Volume	Status
1	Apply StatefulSet YAML	None	None	None	StatefulSet resource created
2	Controller creates first pod	web-0	web-0.nginx	PVC web-0	Pending -> Running
3	First pod ready	web-0	web-0.nginx	PVC web-0	Running
4	Controller creates second pod	web-1	web-1.nginx	PVC web-1	Pending -> Running
5	Second pod ready	web-1	web-1.nginx	PVC web-1	Running
6	Scale down to 1 replica	web-1 deleted	web-1.nginx removed	PVC web-1 retained	web-0 running
7	Scale up to 2 replicas	web-1 recreated	web-1.nginx	PVC web-1 reused	Running

💡 Pods created in order with stable network IDs and storage; scaling preserves identity and data.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 7
Pods	None	[web-0]	[web-0, web-1]	[web-0]	[web-0, web-1]
Pod Identities	None	[web-0.nginx]	[web-0.nginx, web-1.nginx]	[web-0.nginx]	[web-0.nginx, web-1.nginx]
Storage PVCs	None	[PVC web-0]	[PVC web-0, PVC web-1]	[PVC web-0, PVC web-1]	[PVC web-0, PVC web-1]

Key Moments - 3 Insights

Why does the controller create pods one at a time instead of all at once?

The controller creates pods sequentially to ensure each pod gets a stable network ID and storage before the next pod starts, as shown in steps 2 and 4 of the execution_table.

What happens to the storage volumes when a pod is deleted during scale down?

The storage volumes (PVCs) are retained even if the pod is deleted, preserving data for when the pod is recreated, as seen in step 6 where PVC web-1 remains after pod deletion.

How does the StatefulSet maintain pod identity after scaling down and up?

When scaling down, pods are deleted but their identities and storage are preserved. When scaling up, pods are recreated with the same network IDs and storage volumes, as shown in steps 6 and 7.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. Which pod is created and what is its network identity?

A Pod web-2 with network ID web-2.nginx

B Pod web-0 with network ID web-0.nginx

C Pod web-1 with network ID web-1.nginx

D No pod created at step 4

💡 Hint

Check the 'Pod Created' and 'Pod Identity' columns at step 4 in the execution_table.

At which step does the StatefulSet controller delete a pod during scaling down?

A Step 6

B Step 5

C Step 3

D Step 7

💡 Hint

Look for the row mentioning pod deletion in the 'Action' column in the execution_table.

If the number of replicas was increased to 3 initially, how would the variable_tracker's 'Pods' row change after step 4?

A [web-0, web-1, web-2]

B [web-0, web-1]

C [web-0]

D None

💡 Hint

The controller creates pods one at a time; after step 4 only two pods exist as per execution_table.

Concept Snapshot

StatefulSets manage stateful apps by creating pods in order.
Each pod gets a stable network ID and persistent storage.
Pods start one by one; scaling preserves pod identity and data.
Storage volumes (PVCs) are retained even if pods are deleted.
Use StatefulSets for apps needing stable network and storage.

Full Transcript

StatefulSets in Kubernetes help run stateful applications by creating pods one at a time. Each pod gets a stable network name and persistent storage volume. The controller applies the StatefulSet YAML, then creates pod web-0 with identity web-0.nginx and storage PVC web-0. Once web-0 is running, it creates web-1 similarly. When scaling down, pods are deleted but their storage volumes remain. Scaling up recreates pods with the same identity and storage. This ensures data is preserved and pods have stable network IDs, which is important for databases and other stateful apps.