LEARN. PRACTICE. GROW.Concept and mechanism
Kubernetes maintains objects describing desired state and controllers that reconcile it with observed state. A Pod groups closely related containers with shared networking context and resources. Those containers are scheduled together. Placing three API copies inside one Pod does not provide the same operational separation as three replicas managed by a Deployment. A replica may be replaced, so an instance name or IP should not act as permanent service identity. Choose the workload for the need: replaceable replicas, finite execution, or persistent identity. The controller manages that structure; it does not automatically know whether application transactions are correct.
Guided application
In a fictional APS scenario, a release changes the image in a Deployment template. The team follows desired, updated, and available replicas, events, and functional validation. ProgressDeadlineExceeded indicates that the rollout stopped progressing within its configured deadline; it does not constitute automatic Kubernetes rollback. Investigate image retrieval, readiness, capacity, and configuration before deciding how to recover. Reverting the template also does not undo an external data migration. For an application needing stable names and associated storage, consider StatefulSet while retaining explicit responsibility for database replication and backups. PVC retention is configurable: confirm effective policy before scaling down or retiring the workload.
A release may have three desired replicas and only one available; creating the object does not complete delivery.
Common pitfalls
Several copies in one Pod as high availability; deadline as rollback; StatefulSet as backup.
Related topics: Traffic and probes · Resources and scheduling · Configuration and data
Connect controller state with evidence that the application works.
Reference: Deployment rollout and progress conditions · Kubernetes v1.37 concepts; current official documentation consulted 2026-09-30; cluster versions and plugin capabilities must be confirmed