When the cluster is full, priority decides who yields, and a PDB protects against voluntary loss.

They look similar but protect against different events. A PDB does not fully protect against preemption.

PriorityClass + PDB

kind: PriorityClass
metadata: {name: business-critical}
value: 1000000
---
kind: PodDisruptionBudget
spec:
  minAvailable: 2
  selector: {matchLabels: {app: checkout}}

PDB limits voluntary disruption

Drains and upgrades wait rather than drop below minAvailable. Preemption tries to respect it but can still override it.

Use priority sparingly

A handful of classes for genuinely critical services. Too many high priorities cause surprising evictions.

Priority, preemption and disruption budgets

PriorityClass gives Pods a numeric importance. When a high-priority Pod cannot be scheduled because the cluster is full, the scheduler's PostFilter step checks whether evicting lower-priority Pods on some node would make room, and if so it evicts them (they get their normal graceful termination) and schedules the high-priority Pod there. Reveal the four steps with the right arrow.

A PodDisruptionBudget solves a different problem: voluntary disruptions such as kubectl drain, node upgrades and autoscaler scale-down. It states how many Pods of an application must remain available (minAvailable) or may be unavailable (maxUnavailable). The eviction API refuses to remove a Pod if that would violate the budget, so rolling node upgrades proceed at a safe pace.

The precise interaction to remember: the scheduler's preemption tries to honour PDBs when choosing victims, but PDB protection is not absolute against preemption. If there is no victim set that respects the budgets, preemption can still proceed to place a more important Pod. PDBs guarantee safety during voluntary operations; they are not a shield from priority.

Gotchas: a PDB that sets minAvailable equal to the replica count blocks all drains and stalls node upgrades; and a PDB on a single-replica Deployment means that Pod can never be evicted. Review PDBs before every upgrade, which Deck 6 covers in detail.