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Kubernetes Fundamentals
Learn Kubernetes through Pods, Deployments, Services, probes, resources, rollouts, and an official-docs-based troubleshooting workflow.
What is the difference between a Pod, a Deployment, and a Service?
A Pod is the smallest deployable unit, one or more containers that share network and storage, scheduled together onto a node. Pods are ephemeral and disposable; Kubernetes recreates them freely and their IPs change every time. A Deployment manages a set of identical Pods (a ReplicaSet under the hood), handling rolling updates, rollbacks, and keeping the desired replica count running even as individual Pods die. A Service gives that changing set of Pods a stable network identity, a fixed virtual IP and DNS name, so other things in the cluster don't need to track individual Pod IPs, which change constantly.
What does the Kubernetes control loop actually do?
Every Kubernetes controller (Deployment, ReplicaSet, etc.) runs a reconciliation loop: it continuously compares the desired state (what you declared in a manifest, stored in etcd) against the observed actual state of the cluster, and takes action to close any gap. If you declared 3 replicas and only 2 Pods are running, the ReplicaSet controller creates one more. This is the same declarative, converge-toward-desired-state model as Terraform, but running continuously and automatically rather than on-demand.
Why can't you rely on a Pod's IP address for service discovery?
Pods are ephemeral by design, Kubernetes kills and recreates them constantly (failed health checks, node drains, rolling deployments, autoscaling), and every new Pod gets a brand-new IP address. Hardcoding or caching a Pod IP breaks the moment that Pod is replaced. A Service solves this by providing a stable virtual IP and DNS name that always routes to whichever Pods currently match its label selector, regardless of how many times the underlying Pods have been replaced.
What is the difference between readiness, liveness, and startup probes?
Readiness decides whether a Pod should receive Service traffic; a failed readiness probe removes the Pod from eligible backends without restarting it. Liveness decides whether a stuck container should be restarted. A startup probe protects slow-starting containers by delaying readiness and liveness checks until startup succeeds. Reusing one strict check for all three can create restart loops or route traffic too early.
How would you troubleshoot a Service that exists but returns no response?
Work from the application outward: confirm the selected Pods are Ready and serving on the expected container port, compare the Service selector with Pod labels, inspect EndpointSlices to verify Kubernetes discovered backends, confirm port and targetPort, then test Service DNS and IP from inside the cluster. An empty EndpointSlice usually points to a selector/readiness mismatch; healthy endpoints with failed DNS or routing move the investigation to cluster networking.