The kube-scheduler is a critical control plane component responsible for deciding where new pods should run in your Kubernetes cluster.
Here's how it works:
- When you create a new pod with no assigned node, the scheduler notices this unscheduled pod
- It evaluates all available nodes in your cluster to find the best fit
- It considers multiple factors in its decision:
- Resource requirements (CPU/memory requested by the pod)
- Hardware/software constraints (node selectors, taints and tolerations)
- Affinity/anti-affinity rules (keeping pods together or apart)
- Data locality (placing pods near their data)
- Deadlines
The scheduling process happens in two phases:
- Filtering: Eliminates nodes that can't run the pod (insufficient resources, doesn't match node selectors, etc.)
- Scoring: Ranks the remaining nodes to find the best fit
Once it finds the optimal node, it updates the pod definition with the node name, and the kubelet on that node takes over to actually create the containers.
The scheduler doesn't directly create pods - it just decides where they should go, which is a sophisticated optimization problem, especially in large clusters.
You can influence scheduling decisions with various Pod specs like resource requests, node selectors, affinity rules, and tolerations - giving you fine-grained control over pod placement.