why we need kube-scheduler in Kubernetes (k8s)

The Kubernetes kube-scheduler is a crucial component responsible for making decisions about where to place Pods within the cluster. Its primary role is to ensure efficient resource utilization, high availability, and optimal performance by determining which node should run each Pod based on various factors. Here’s why kube-scheduler is essential in Kubernetes:

Key Responsibilities of kube-scheduler:

1. Pod Scheduling:

   • Node Selection: kube-scheduler selects an appropriate node for a newly created Pod based on factors like resource requirements (CPU, memory), node affinity/anti-affinity rules, and taints/tolerations.
   • Bin Packing: It aims to optimize resource utilization by packing Pods onto nodes to avoid resource wastage and balance the workload across the cluster.

2. Interacts with API Server:

   • kube-scheduler interacts with the Kubernetes API server to watch for newly created Pods that need scheduling.
   • It retrieves Pod specifications and evaluates them against the current state of the cluster (node availability, resource usage) to make informed placement decisions.

3. Node Affinity and Anti-affinity:

   • kube-scheduler respects Pod affinity and anti-affinity rules specified in Pod definitions. These rules dictate preferences or constraints for scheduling Pods based on labels assigned to nodes.

4. Resource Constraints:

   • Ensures that Pods are only scheduled on nodes that have sufficient resources (CPU, memory) to meet their requirements.
   • It considers both resource requests (amount of resource a Pod claims) and limits (maximum amount a Pod can consume).

5. Pod Priority and Preemption:

   • Supports Pod priority and preemption policies to prioritize scheduling of critical Pods (e.g., high-priority applications) over less critical ones.
   • If resources are scarce, kube-scheduler may evict lower-priority Pods to make room for higher-priority Pods.

6. Integration with Persistent Volumes:

   • kube-scheduler takes into account Pod requirements for persistent storage (PersistentVolumes) and schedules Pods on nodes with appropriate storage capabilities.

Example Scenario:

Consider a scenario where a new Pod needs to be scheduled:

1. Pod Creation:

   • An application developer creates a new Pod specification and submits it to the Kubernetes API server.

2. Pod Scheduling Request:

   • The kube-scheduler watches for new Pod specifications and receives a scheduling request for the newly created Pod.

3. Scheduling Decision:

   • Based on node availability, resource requirements (CPU, memory), node affinity/anti-affinity rules, and any other constraints, kube-scheduler selects an appropriate node.

4. Node Assignment:

   • The selected node is assigned to run the Pod, and kube-scheduler updates the Kubernetes API server with the scheduling decision.

5. Pod Initialization:

   • The kubelet on the chosen node initializes the Pod by pulling the container images, setting up networking, and starting the containers.

6. Monitoring and Rescheduling:

   • kube-scheduler continuously monitors the cluster state and may reschedule Pods if nodes become unavailable or if there are changes in resource availability that affect scheduling decisions.