Q14: Kube-Scheduler Deep-Dive: How Filtering, Scoring, Node Affinity, Taints, and Tolerations place Pods. - DevOps & Kubernetes (50 Qs)

Scheduling Pipeline & Algorithms

When a Pod is submitted with spec.nodeName == "", the kube-scheduler selects the best node in two primary phases: Filtering (Predicates) and Scoring (Priorities).

scss

[ Unschedulable Pod ]
         |
         v
+---------------------------------------------------+
| 1. Filtering Phase (Predicates)                   |
| - NodeResourcesFit (CPU, RAM available)           |
| - NodeName matches (if specified)                 |
| - PodFitsHostPorts                                |
| - NodeAffinity & NodeSelector                     |
| - Taints & Tolerations matching                   |
| - PodTopologySpreadConstraints                    |
+---------------------------------------------------+
         | (List of Feasible Candidate Nodes)
         v
+---------------------------------------------------+
| 2. Scoring Phase (Priorities)                     |
| - LeastAllocated (Balance load)                   |
| - MostAllocated (Bin-packing for cost saving)     |
| - ImageLocalityPriority (Image already cached)    |
| - InterPodAffinityPriority                        |
+---------------------------------------------------+
         | (Highest Scoring Node)
         v
[ Bind Pod to Node via API Server ]

Taints vs. Tolerations vs. Node Affinity

Mechanism	Where it is set	Purpose	Rule Syntax
Taint	On the Node	Repels Pods from being scheduled unless the Pod tolerates the taint.	`kubectl taint nodes node-1 dedicated=gpu:NoSchedule`
Toleration	In the Pod spec	Allows (but does not force) a Pod to schedule onto a tainted node.	`tolerations: [{key: "dedicated", value: "gpu", effect: "NoSchedule"}]`
Node Affinity	In the Pod spec	Attracts a Pod to specific nodes based on node labels.	`requiredDuringSchedulingIgnoredDuringExecution` (hard) vs `preferredDuringScheduling...` (soft)

Production Topology Spread Example

Ensures Pod replicas are distributed evenly across Availability Zones:

yaml

topologySpreadConstraints:
  - maxSkew: 1
    topologyKey: topology.kubernetes.io/zone
    whenUnsatisfiable: DoNotSchedule
    labelSelector:
      matchLabels:
        app: payment-api