Управление приложениями с состоянием с помощью StatefulSets

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.

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.

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.

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.

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'.

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

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.

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.

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.

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.