Understanding Kubernetes Custom Schedulers

Author

Govardhana M K
September 25, 2025

In partnership with

TechOps Examples

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🧠 DEEP DIVE USE CASE

Understanding Kubernetes Custom Schedulers

In every Kubernetes cluster, the scheduler is one of the most critical control plane components. By default, the Kubernetes scheduler decides which worker node a newly created Pod should run on. This decision is not random. It is based on a combination of resource availability, constraints like node affinity and taints, and policies that ensure workloads are spread efficiently across nodes.

Kubernetes Architecture

For example, when you create a Pod without node preferences, the scheduler checks CPU, memory, and node conditions to place it on the best fit node. This usually works well in production, balancing workloads and optimizing resources.

But in some cases, you may want to pack workloads to cut costs, isolate sensitive apps, or place latency critical pods closer to users. These needs show the limits of the default scheduler.

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Custom schedulers let you extend or replace this logic with your own rules, giving fine grained control over workload placement.

How a Custom Scheduler Works

When a user submits a Pod, the API Server records it and persists the state in etcd. At this point, the Pod sits in a pending state, waiting to be scheduled. A custom scheduler continuously watches for such Pods and requests the latest cluster state through a state monitor.

Custom Scheduler Workflow

Based on the scheduling logic defined, the custom scheduler decides the target node and updates the Pod’s assignment. The scheduler then communicates with the Kubelet on the chosen node, which triggers the container runtime to create the container.

This flow ensures that Pods are not only scheduled but also follow the specialized placement rules you design, whether that means spreading across zones, pinning workloads to specific hardware, or optimizing for performance and cost.

With this basic understanding, let us look at the custom scheduler architecture and how to design custom logic.

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