HPA

Hpa is used to scale up or scale down the service based on the usage of CPU or Memory. It will based on the condition to scale up or down the pod

RollingUpdte

RollingUpdate strategy is used to control how to scale up or scale down, such as first scale up two pods, and then stop the old pods.

PDB

For HPA or RollingUpdate, even RollingUpdate may start pod first and then stop to keep the service alive. But if the cluster is scale down or any other actions occur, it may cause the service not avaliable. PDB is just to ensure there alway have minValaible pod number. Even the cluster is upgrade.

Summary

HPA shows when to trigger scale, RollingUpdate shows how to scale service, PDB ensure the service avaliable even upgrade cluster or scale clsuter.