K8S Notes

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Scheduling

Scheduling in Kubernetes

• Definition and Evolution of Scheduling:
  • Basic Concept:
    • Scheduling is a sub-category of container orchestration.
    • Describes the automated process of selecting the appropriate worker node to run a containerized workload.
  • Historical Perspective:
    • In the past, scheduling was a manual task for system administrators.
    • Involved choosing the right server for an application based on available servers, capacity, and other properties.
• Kubernetes Scheduling Components:
  • kube-scheduler:
    • In a Kubernetes cluster, kube-scheduler is responsible for making scheduling decisions.
    • Initiates the scheduling process when a new Pod object is created.
• Declarative Approach in Kubernetes:
  • Process:
    • Kubernetes uses a declarative approach.
    • The Pod is described first, and then the scheduler selects a node where the Pod will be started by the kubelet and the container runtime.
• User Input in Scheduling:
  • Misconception:
    • Commonly misunderstood that Kubernetes uses artificial intelligence for workload analysis and dynamic Pod movement based on various factors.
    • Reality:
      • Users need to provide information about application requirements.
      • Includes CPU and memory requests, as well as node-specific properties.
• User-Specified Requirements:
  • Example:
    • Users can request specific resources such as two CPU cores, four gigabytes of memory, and express preferences for node properties (e.g., fast disks).
• Scheduling Decision Process:
  • Filtering Nodes:
    • The scheduler filters nodes that meet specified requirements.
  • Priority:
    • If multiple nodes meet requirements equally, the scheduler prioritizes the node with the fewest Pods.
  • Default Behavior:
    • If no additional requirements are specified, the default behavior is to schedule the Pod on the node with the least number of Pods.
• Handling Insufficient Resources:
  • Retry Mechanism:
    • If the desired state cannot be established (e.g., insufficient resources on worker nodes), the scheduler retries until an appropriate node is found.

Kubernetes Fundamentals