The model
Kubernetes controllers continuously compare observed cluster state with a declared desired state. A Deployment describes a target number and version of replicated Pods; its controller creates or replaces ReplicaSets and Pods to move the system toward that target.
A concrete walk-through
When a container image changes, a Deployment can create a new ReplicaSet and gradually shift replicas according to rollout settings. Readiness probes influence whether a Pod is considered ready to receive traffic; liveness probes can trigger a restart when a process is unhealthy under the configured test.
Costs and failure cases
A declarative manifest is a desired-state request, not proof the change succeeded. A rollout may stall because images cannot be pulled, capacity is unavailable, or readiness never passes. Probes should test the intended health contract; overly aggressive probes can restart healthy but slow-starting processes.
Check your understanding
A Deployment requests six replicas but only four are ready. List the cluster and application signals you would inspect before increasing the replica count.