Kubernetes Priority Classes: Keeping Critical Pods First in Line

In a busy Kubernetes cluster, everyone wants resources – but not every Pod is equally important. That’s where Kubernetes Priority Classes step in, acting like a VIP pass for your workloads.

What Are Kubernetes Priority Classes?

A PriorityClass in Kubernetes assigns a priority value to Pods. When the cluster runs out of resources, the scheduler evicts lower-priority Pods first, ensuring your mission-critical workloads always get what they need.

• Higher number → higher priority during scheduling and preemption.
• Helps Kubernetes decide which Pods to schedule first when resources are limited and which Pods to evict when the cluster is under pressure.

Think of it as:

• High Priority → Keep running no matter what.
• Low Priority → Give up resources if the cluster is stressed.

It’s not about allocating CPU/Memory – that’s still handled by Requests & Limits – but it influences which Pods get the chance to run.

Why Do We Need Priority Classes?

Imagine a multi-tenant Kubernetes cluster:

• Tenant A runs business-critical apps (e.g., payment processing).
• Tenant B runs batch jobs that can be delayed.

If the cluster is full, we don’t want payment processing Pods to be evicted just because a batch job got scheduled first. Priority Classes prevent that scenario.

How Kubernetes Uses Priority Classes

• Scheduling – When new Pods arrive:
  • The kube-scheduler first tries to place all Pods.
  • If resources are insufficient, it schedules higher-priority Pods before lower-priority ones.

• Preemption – If a higher-priority Pod can’t be scheduled
  • Kubernetes finds lower-priority Pods to evict until there’s enough room.
  • Only Pods with lower priority can be preempted — never higher or equal.

How It Works

• Create an PriorityClass object with a value (higher = more important).
• Assign it to your Pod via priorityClassName.
• Kubernetes uses it during scheduling and preemption.

Creating a Priority Class

You create a PriorityClass object once, then reference it in Pods.

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: critical-apps
value: 100000
globalDefault: false
description: "Used for mission-critical workloads"

Apply it to a Pod:

apiVersion: v1
kind: Pod
metadata:
  name: payment-service
spec:
  priorityClassName: critical-apps
  containers:
  - name: app
    image: myapp:1.0
    resources:
      requests:
        cpu: "500m"
        memory: "512Mi"

Key Parameters in Priority Classes

Best Practices

• Reserve highest priorities for essential workloads (monitoring, logging, core services).
• Avoid making everything high-priority — it defeats the purpose.
• Combine with PodDisruptionBudgets to avoid unwanted evictions.
• For batch workloads, use low-priority classes so they yield under pressure.

When to Use Priority Classes

• Critical system components (e.g., monitoring, ingress controllers)
• Business-critical applications during peak load
• Multi-tenant clusters to prevent resource starvation

Kubernetes Priority Classes keep your most important workloads running when resources get tight — because in the cluster world, priority matters.