[Kubernetes] Scheduling(2)

source : kubecost

☣️ What is a Taint?

A taint is applied to a node to mark it as unsuitable for certain pods, unless those pods explicitly tolerate the taint.

It works like this:

🛑 "This node doesn't want your pod... unless you're okay with my taint."

 

 

🔧 Taint Syntax

kubectl taint nodes <node-name> key=value:effect

 

🧪 Example

kubectl taint nodes node1 key=env:NoSchedule

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🛡️ What is a Toleration?

A toleration is added to a pod spec. It tells Kubernetes:

✅ "I'm okay with this taint, let me run on that node."

 

🔧 Toleration YAML Example

tolerations:
  - key: "env"
    operator: "Equal"
    value: "production"
    effect: "NoSchedule"

 

This allows the pod to be scheduled on a node that has this taint:

env=production:NoSchedule

 

 

💥 Taint Effects

Effect	Description	Real-World Use case
NoSchedule	Don't schedule pods unless they tolerate the taint.	Run only special workloads
PreferNoSchedule	Avoid scheduling, but not strictly enforced.	Isolate production environments
NoExecute	Evict existing pods that don't tolerate the taint. Also prevents new ones.	Node maintenance (drain-like)

 

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source : kubecost

🔹 1. Node Selector (Simple)

`nodeSelector` is the simplest way to schedule a Pod onto a specific node based on exact label match.

 

✅ Example:

spec:
  nodeSelector:
    disktype: ssd

 

This Pod will only be scheduled on a node with:

kubectl label node <node-name> disktype=ssd

 

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🔸 2. Node Affinity (Advanced)

`nodeAffinity` is a more powerful and flexible way to do what nodeSelector does — with expressions, preferred vs required, and complex logic.

 

💬 Two Types:

• requiredDuringSchedulingIgnoredDuringExecution (💥 must match)
• preferredDuringSchedulingIgnoredDuringExecution (👌 best effort)

✅ Example: Required Node Affinity

spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
          - matchExpressions:
              - key: disktype
                operator: In
                values:
                  - ssd
                  - nvme

This only schedules on nodes where `disktype` is ssd or nvme

 

✅ Example: Preferred Node Affinity

spec:
  affinity:
    nodeAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
        - weight: 1
          preference:
            matchExpressions:
              - key: zone
                operator: In
                values:
                  - us-east-1a

This tries to schedule in `us-east-1a`, but fall abck to others if needed.

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🧠 Node Selector vs Node Affinity

 

Feature	NodeSelector	NodeAffinity
Match type	Exact	Expressions (In, NotIn, Exists, etc.)
Logic flexibility	Low	High
Required/Preferred separation	❌ No	✅ Yes
Common usage	Simple pinning	Advanced placement logic

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🔁 Quick Overview

 

Feature	Node Affinity	Taints & Tolerations
Who defines it?	Pod (chooses preferred nodes)	Node (repels Pods unless tolerated)
Direction	Pod ➡️ Node	Node ➡️ Pod
Behavior	"I want to run on this kind of node"	"I don’t want Pods on me unless allowed"
Use case	Attraction – guide scheduling	Repulsion – restrict scheduling
Hard/Soft control	Has required (hard) and preferred (soft) rules	Mostly hard rules unless PreferNoSchedule
Node setup needed?	No (but nodes must have labels)	Yes (nodes must be tainted)