What is the difference between Horizontal and Vertical scaling?

Horizontal scaling (HPA) means adding *more* identical pods. Vertical scaling (VPA) means adding more physical CPU/RAM to an *existing* pod. Kubernetes is heavily optimized for Horizontal scaling because it is faster and allows for true high availability without downtime.

If the HPA scales my pods to 50, but my physical cluster only has enough servers to run 30, what happens?

The HPA will change the Deployment's desired state to 50. The ReplicaSet will create 50 pod objects. However, the Kubernetes Scheduler will only be able to place 30 of them. The remaining 20 pods will sit permanently in a `Pending` state. To fix this, you must also use the 'Cluster Autoscaler', which provisions entirely new physical virtual machines from your cloud provider.

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Horizontal Pod Autoscaling

Master Kubernetes autoscaling mechanics. Understand the relationship between the HPA and Resource Requests, how utilization targets are calculated, the importance of cooldown periods, and the Metrics Server dependency.

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