StatefulSets

StatefulSets are a Kubernetes workload API object designed to manage stateful applications. They provide guarantees about the ordering and uniqueness of pod identities and stable, persistent storage across rescheduling, which makes them well suited for databases and other stateful services.

Key features include stable network identities, stable storage, and ordered deployment, scaling, and termination. Each Pod in a StatefulSet has a predictable, DNS-resolvable hostname, and the cluster uses a headless service (a service with clusterIP: None) to enable stable DNS for the pods. Stable, unique storage is achieved through volumeClaimTemplates, which create a per-pod persistent volume that persists across restarts and rescheduling. Pods receive a unique ordinal index (for example, 0, 1, 2), which contributes to their stable identity and storage association.

The StatefulSet controller manages pod lifecycle with an emphasis on ordering. Pods are created, scaled, updated, and terminated in ordinal order, and the system waits for each Pod to become ready before proceeding to the next. This ordering also applies to scaling down, where higher ordinals are terminated first.

Update strategies include RollingUpdate and OnDelete. The default RollingUpdate proceeds in order, allowing controlled upgrades, while OnDelete defers updates until a Pod is manually deleted, offering fine-grained control over changes.

Typical use cases include databases (such as MySQL, PostgreSQL), distributed systems (like etcd, Zookeeper, Cassandra), and other services requiring stable network identity and durable storage. Limitations include the need for persistent storage provisioning, potential performance considerations for large-scale deployments, and the inherent sequential nature of operations which can affect upgrade and scale-out speed.