# Understanding Kubernetes: Part 26 Preemption and Priority

***

<figure><img src="https://cdn-images-1.medium.com/max/800/1*vxxieTmeNJRiUwOeBQMabQ.png" alt="Preemption and Priority"><figcaption></figcaption></figure>

#### What is a Pod Priority?

A **Pod Priority** defines the importance of a Pod relative to others. Higher-priority Pods get scheduled before lower-priority ones. If resources are insufficient, Kubernetes may preempt (evict) lower-priority Pods to make room for higher-priority ones.

#### Defining a Pod Priority Class

Pod priorities are defined using **PriorityClasses**, which assign numerical values to indicate importance.

**Example: Creating a Priority Class**

```
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000
preemptionPolicy: PreemptLowerPriority
globalDefault: false
```

* `value`: Determines the priority level; higher values indicate higher priority.
* `preemptionPolicy`: Defines whether the Pod can preempt lower-priority Pods (`PreemptLowerPriority`).
* `globalDefault`: If `true`, this priority applies to all Pods by default.

#### Assigning a Priority to a Pod

After defining a PriorityClass, assign it to a Pod:

```
apiVersion: v1
kind: Pod
metadata:
  name: critical-app
spec:
  priorityClassName: high-priority
  containers:
    - name: app
      image: my-app
```

This Pod is now considered high-priority and may preempt lower-priority Pods if needed.

***

#### What is Preemption?

**Preemption** occurs when a high-priority Pod cannot be scheduled due to resource constraints. Kubernetes then evicts lower-priority Pods to free up resources for the high-priority Pod.

#### Preemption Process

1. A high-priority Pod cannot be scheduled due to lack of resources.
2. Kubernetes identifies lower-priority Pods that can be evicted.
3. The lower-priority Pods are terminated.
4. The high-priority Pod is scheduled.

#### Controlling Preemption

By setting `preemptionPolicy: Never`, you can prevent a Pod from evicting other workloads:

```
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: non-preempting-priority
value: 500
preemptionPolicy: Never
```

This ensures that Pods with this priority will not preempt lower-priority Pods.

***

#### Use Cases

#### 1. Ensuring Critical Workloads Run First

* Assign higher priority to critical applications to guarantee their availability.

#### 2. Protecting Low-Priority Batch Jobs

* Set low priority for batch processing jobs to avoid interference with real-time applications.

#### 3. Controlled Eviction of Non-Essential Workloads

* Preempt lower-priority workloads during resource shortages, ensuring vital services remain operational.

***

#### Removing a Priority Class

If a PriorityClass is no longer needed, remove it using:

```
kubectl delete priorityclass high-priority
```

This prevents future Pods from using this priority but does not affect running Pods.

***

#### Conclusion

**Kubernetes Preemption and Priority** allow fine-grained control over workload scheduling, ensuring that essential applications always have the resources they need. By assigning priorities wisely, you can optimize cluster stability and resource allocation efficiently.

***

#### In My Previous Role

As a Senior DevOps Engineer, I leveraged Kubernetes Priority and Preemption to ensure efficient workload scheduling:

* **Critical Services**: Assigned high-priority classes to essential applications to prevent downtime.
* **Batch Jobs**: Set lower priorities for non-essential batch jobs to minimize their impact on critical workloads.
* **Resource Optimization**: Used preemption to dynamically allocate resources based on workload demand, ensuring efficient cluster utilization.

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