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2443 Full-Stack, Algorithms & System Design Interview Questions

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How does the Kubernetes API Server internally process an incoming request from authentication to admission?

The request flows through: Authentication (verify identity — bearer token, certificate, OIDC), Authorization (RBAC — can this identity do this action on this resource?), Admission Controllers (mutating — modify the object; validating — reject invalid objects), then object validation against the API schema, and finally persistence to etcd.

Dec 02, 2025 Kubernetes Expert

Why does Kubernetes use a watch mechanism instead of continuous polling for state changes?

Polling would require each component to ask the API server "any changes?" every N seconds — generating massive load and adding latency proportional to the poll interval. Watches are long-lived HTTP connections where the API server pushes events to clients instantly when objects change. Much lower API server load and near-real-time response to state changes.

Dec 02, 2025 Kubernetes Expert

How does the API Server deduplicate events during heavy watch traffic?

During heavy watch traffic, many rapid changes to the same object generate many events. The API server uses event aggregation and watch bookmarks to reduce noise. Bookmarks are synthetic events that advance the client's resourceVersion without carrying object data — helping clients stay current without receiving every intermediate state change.

Dec 02, 2025 Kubernetes Expert

Why does etcd use MVCC instead of overwriting values directly?

MVCC (Multi-Version Concurrency Control) keeps old versions of values tagged with their revision number. This enables Watches to replay history from any past revision without locking current values. It also makes atomic compare-and-swap (CAS) operations safe — etcd compares the current revision before writing. Overwriting directly would lose the history needed for watches.

Dec 02, 2025 Kubernetes Expert

How does the scheduler handle scheduling cycles to prevent race conditions in multi-scheduler setups?

Each scheduler instance runs a scheduling cycle: claim a Pod to schedule, find a node, then bind it. Without locking, two scheduler instances could both select the same node for the same Pod. Kubernetes uses optimistic binding — the scheduler writes the binding to the API server which validates against resourceVersion. Only one binding succeeds; the other is rejected and retried.

Dec 02, 2025 Kubernetes Expert

What is preemption toxicity and how does Kubernetes mitigate it?

Preemption toxicity is when preempting lower-priority Pods to make room for a high-priority Pod causes so many Pod disruptions that the cluster destabilizes. Kubernetes mitigates this with PDB (prevents preempting too many replicas), a minimum preemption threshold, and grace periods — preemption is tried only after no available node is found.

Dec 02, 2025 Kubernetes Expert

How does CRI allow Kubernetes to remain runtime-agnostic?

CRI (Container Runtime Interface) is a standard gRPC API between kubelet and the container runtime. Any runtime implementing CRI (containerd, CRI-O) can be used. Kubernetes communicates via CRI calls (RunPodSandbox, CreateContainer, StartContainer) — the runtime handles the implementation details. This lets Kubernetes run on any OCI-compatible runtime without code changes.

Dec 02, 2025 Kubernetes Expert

Why does kubelet maintain a pod sandbox even when containers crash?

The pod sandbox is the network namespace and cgroup hierarchy for the Pod — created once when the Pod is scheduled. It persists across container restarts. Even if all containers in the Pod crash and restart, the sandbox (Pod IP, network, volumes) stays stable. This is why Pod IP doesn't change when a container restarts inside it.

Dec 02, 2025 Kubernetes Expert

How does Kubernetes prevent deadlocks in the node drain process?

kubectl drain cordon the node (stops new scheduling) and evicts Pods. If a Pod has a finalizer that never gets removed (deadlocked controller), it blocks the eviction — the drain hangs. Kubernetes uses a configurable eviction timeout; if exceeded, drain fails. The operator must manually intervene — remove the finalizer or force-delete the stuck Pod.

Dec 02, 2025 Kubernetes Expert

Why are long-lived connections tricky behind Kubernetes Services?

HTTP/1.1 long-lived connections (WebSocket, gRPC streams, database persistent connections) don't benefit from Service load balancing because one connection goes to one Pod and stays there. All requests on that connection hit the same Pod — no load balancing. Use a service mesh (Istio, Linkerd) for request-level load balancing of persistent connections.

Dec 02, 2025 Kubernetes Expert

How does Cilium replace kube-proxy using eBPF?

Cilium attaches eBPF programs to network interfaces and socket layers. When a Pod connects to a Service ClusterIP, Cilium's eBPF socket-level program intercepts the connect() syscall and redirects it directly to a backend Pod's IP — without going through the kernel network stack, without iptables, and without needing kube-proxy at all. L7 policies work similarly.

Dec 02, 2025 Kubernetes Expert

What are API Priority and Fairness queues and why do they matter?

API Priority and Fairness (APF) classifies incoming API requests into priority levels (leader-election, cluster-critical, system, workload-low). Each level has a queue with concurrency limits. This prevents a burst of low-priority requests (like a misconfigured controller) from blocking critical operations (kubelet heartbeats, leader election). Every request gets fair access within its priority class.

Dec 02, 2025 Kubernetes Expert

How does Kubernetes ensure strict ordering of writes to a single object under high concurrency?

All writes to a single etcd key are serialized by etcd's Raft protocol. The API server includes the resourceVersion in writes — etcd rejects writes where the version doesn't match current (optimistic locking). The Raft leader serializes competing writes to the same key — only one succeeds per revision. This provides single-object write linearizability.

Dec 02, 2025 Kubernetes Expert

Why do multi-cluster architectures require federation or service meshes?

Each cluster is an isolated control plane — Pods in cluster A can't directly discover or communicate with Services in cluster B. Multi-cluster networking (Submariner, Cilium Cluster Mesh) creates cross-cluster service discovery and connectivity. Service meshes (Istio multi-cluster) extend traffic management across clusters. Federation (KubeFed) synchronizes resources across clusters.

Dec 02, 2025 Kubernetes Expert

How does kube-controller-manager prevent infinite reconciliation loops?

Controllers use the "level-triggered" model — they don't remember the sequence of events, just the current desired and actual state. After any reconciliation attempt, they re-read the current state from the API server and compare to desired. If already at desired state, they do nothing. This idempotent design prevents infinite loops — reconciliation converges.

Dec 02, 2025 Kubernetes Expert
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