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StatefulSetによるステートフルアプリケーション管理

StatefulSetを使ってデータベースなどのステートフルアプリケーションをデプロイ・管理し、安定したネットワークIDと永続ストレージを確保します。

「StatefulSetによるステートフルアプリケーション管理」はCoddyKit上の無料Docker & Kubernetes for Developersレッスンです。 これはレッスン2/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはDocker & Kubernetes for Developers学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Docker & Kubernetes for Developersコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Stateful Applications in K8s

When deploying applications in Kubernetes, some applications are stateless, meaning they don't store data locally and can be easily scaled or replaced.

However, many critical applications, like databases or message queues, are stateful. They need persistent storage and stable network identities.

This lesson introduces StatefulSets, a Kubernetes API object designed specifically to manage these stateful workloads.

Why StatefulSets?

Traditional Kubernetes Deployments are great for stateless apps. They create Pods with arbitrary names and can replace them freely.

Stateful applications, however, require:

  • Stable, unique network identities: Each instance needs a consistent name.
  • Stable, persistent storage: Data must survive Pod restarts or rescheduling.
  • Ordered deployment and scaling: Sometimes, instances need to start or stop in a specific sequence.

StatefulSets provide these crucial capabilities.

Key Features of StatefulSets

StatefulSets offer several powerful features for managing stateful applications:

  • Stable Network ID: Each Pod gets a predictable name (e.g., web-0, web-1) and DNS hostname.
  • Stable Persistent Storage: Integrates with Persistent Volumes (PVs) and Persistent Volume Claims (PVCs) to give each Pod its own persistent storage.
  • Ordered Guarantees: Ensures Pods are created, scaled, and deleted in a strict, ordinal order.
  • Graceful Deployment: Pods are only created after their storage is ready, and only deleted after they've gracefully shut down.

StatefulSet Components

A StatefulSet typically works in conjunction with other Kubernetes resources:

  • Headless Service: Provides stable network identities for the Pods. It doesn't load-balance traffic, but creates unique DNS entries for each Pod.
  • Pod Template: Defines the specification for the Pods, including containers, resources, etc.
  • Volume Claim Templates: Generates a unique Persistent Volume Claim (PVC) for each Pod, ensuring dedicated storage.

Defining a Headless Service

Before creating a StatefulSet, you often define a Headless Service. This service is crucial for assigning stable network identities to your stateful Pods.

Notice clusterIP: None, which tells Kubernetes not to assign a cluster IP, making it 'headless'.

apiVersion: v1
kind: Service
metadata:
  name: my-app-service
  labels:
    app: my-app
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None # This makes it a Headless Service
  selector:
    app: my-app

Creating a Basic StatefulSet

Now, let's look at a basic StatefulSet definition. It links to our Headless Service using serviceName and defines a volumeClaimTemplates for persistent storage.

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: my-app
spec:
  serviceName: "my-app-service" # Link to Headless Service
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
      - name: my-container
        image: nginx
        ports:
        - containerPort: 80
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
  - metadata:
      name: www
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi

Understanding Pod Identity

When the StatefulSet from the previous scene is deployed, it will create three Pods named my-app-0, my-app-1, and my-app-2.

Each Pod also gets its own stable DNS entry, like my-app-0.my-app-service.default.svc.cluster.local. This allows other services to reliably connect to specific instances.

Crucially, each Pod will also get its own unique Persistent Volume Claim (e.g., www-my-app-0), ensuring dedicated storage.

Ordered Deployment & Scaling

StatefulSets enforce a strict order for Pod creation, scaling, and deletion. This is vital for applications where sequence matters (e.g., a primary database and its replicas).

  • Creation: Pods are created in ascending ordinal order (my-app-0, then my-app-1, etc.). Each Pod is fully running and ready before the next is created.
  • Deletion: Pods are terminated in descending ordinal order (my-app-2, then my-app-1, etc.). Each Pod is fully shut down before the next is deleted.
  • Scaling: Similarly, scaling up adds Pods in order, and scaling down removes them in reverse order.

Updating StatefulSets

StatefulSets support controlled updates to their Pods, typically used for rolling out new versions of your application.

  • RollingUpdate (default): Pods are updated in reverse ordinal order (my-app-2, then my-app-1, etc.). Each Pod is updated and ready before the next one starts.
  • OnDelete: This strategy requires manual intervention. The StatefulSet controller will not automatically update Pods. You must manually delete Pods for the new template to take effect.

Rolling updates ensure minimal downtime and maintain the application's state during upgrades.

StatefulSet Features Check

Which of the following are key characteristics or requirements for Kubernetes StatefulSets?

StatefulSets Recap

Congratulations! You've learned about Kubernetes StatefulSets.

  • StatefulSets manage stateful applications, ensuring stable identities and persistent storage.
  • They provide stable network IDs, ordered operations, and leverage Volume Claim Templates for dedicated storage.
  • A Headless Service is often used to provide stable DNS entries for StatefulSet Pods.
  • They support controlled updates with strategies like RollingUpdate.

StatefulSets are essential for running databases, message queues, and other stateful services reliably on Kubernetes.

よくある質問

「StatefulSetによるステートフルアプリケーション管理」レッスンは無料ですか？

はい。「StatefulSetによるステートフルアプリケーション管理」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Docker & Kubernetes for Developersコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Docker & Kubernetes for Developersコースには全4レッスンが含まれています。

「StatefulSetによるステートフルアプリケーション管理」で何を学びますか？

StatefulSetを使ってデータベースなどのステートフルアプリケーションをデプロイ・管理し、安定したネットワークIDと永続ストレージを確保します。 ブラウザで直接実行するハンズオンコードでDocker & Kubernetes for Developersを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Docker & Kubernetes for Developersを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのDocker & Kubernetes for Developersは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン2/4です。

「StatefulSetによるステートフルアプリケーション管理」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このDocker & Kubernetes for Developersレッスンでコードを書いて実行できますか？

はい。すべてのDocker & Kubernetes for Developersレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. Persistent VolumeとPersistent Volume Claim
  2. StatefulSetによるステートフルアプリケーション管理
  3. 設定用ConfigMapとSecret
  4. ストレージクラスと動的プロビジョニング
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