Pod Disruption Budgets in Kubernetes - Step-by-Step Execution

Process Flow - Pod Disruption Budgets

Define PDB YAML

↓

Apply PDB to Cluster

↓

Kubernetes Controller Watches PDB

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Pod Eviction Request

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Check PDB Conditions

↓

Allow Eviction

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Pod Removed

Shows how a Pod Disruption Budget (PDB) is defined, applied, and used by Kubernetes to allow or block pod evictions during disruptions.

Execution Sample

Kubernetes

apiVersion: policy/v1
kind: PodDisruptionBudget
metadata:
  name: my-pdb
spec:
  minAvailable: 2
  selector:
    matchLabels:
      app: myapp

Defines a PDB that requires at least 2 pods with label app=myapp to be available during disruptions.

Process Table

Step	Event	Pods Available	minAvailable	Eviction Allowed?	Reason
1	Initial state	3	2	Yes	Pods meet minAvailable requirement
2	Eviction request for 1 pod	3	2	Yes	Eviction keeps pods >= minAvailable
3	After eviction, pods available	2	2	N/A	Pods still meet minAvailable
4	Eviction request for another pod	2	2	No	Eviction would reduce pods below minAvailable
5	Eviction blocked	2	2	N/A	PDB prevents disruption

💡 Eviction blocked when pods available would drop below minAvailable

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Pods Available	3	3	2	2	2
Eviction Allowed	Yes	Yes	N/A	No	No

Key Moments - 2 Insights

Why is eviction allowed at step 2 but blocked at step 4?

At step 2, evicting one pod keeps the available pods at 2, which meets the minAvailable requirement (see execution_table rows 2 and 3). At step 4, evicting another pod would reduce pods below minAvailable, so eviction is blocked (execution_table row 4).

What does minAvailable mean in a Pod Disruption Budget?

minAvailable is the minimum number of pods that must be running during voluntary disruptions. Kubernetes uses this to decide if it can allow pod evictions without hurting availability (refer to execution_table column 'minAvailable').

Visual Quiz - 3 Questions

Look at the execution table, how many pods are available after the first eviction?

A 1

B 2

C 3

D 0

💡 Hint

Check the 'Pods Available' column at Step 3 in the execution_table.

At which step does Kubernetes block pod eviction due to PDB?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Eviction Allowed?' column in execution_table to find where eviction is 'No'.

If minAvailable was set to 1 instead of 2, what would happen at step 4?

A Eviction would be allowed

B Eviction would still be blocked

C Pods available would increase

D PDB would be ignored

💡 Hint

Compare minAvailable value and pods available at step 4 in variable_tracker and execution_table.

Concept Snapshot

Pod Disruption Budget (PDB) controls voluntary pod evictions.
Define PDB with minAvailable or maxUnavailable.
Kubernetes checks PDB before evicting pods.
Eviction allowed only if availability stays above minAvailable.
Prevents too many pods going down at once.
Use PDB to keep app stable during maintenance.

Full Transcript

Pod Disruption Budgets help keep your app running smoothly by limiting how many pods can be taken down at once. You write a PDB YAML file specifying minAvailable pods. Kubernetes watches this and blocks pod evictions if removing a pod would drop availability below that number. For example, if you have 3 pods and minAvailable is 2, Kubernetes allows evicting one pod but blocks evicting a second pod until one comes back. This way, your app stays healthy during updates or node maintenance.

Practice

1. What is the main purpose of a Pod Disruption Budget in Kubernetes?

easy

A. To ensure a minimum number of pods stay available during planned disruptions

B. To automatically restart pods when they crash

C. To scale pods up or down based on CPU usage

D. To backup pod data before deletion

Solution

1. Step 1: Understand Pod Disruption Budget purpose

   A Pod Disruption Budget (PDB) is designed to keep your app available during planned changes by limiting voluntary disruptions.

2. Step 2: Match purpose to options

   To ensure a minimum number of pods stay available during planned disruptions correctly states that PDB ensures a minimum number of pods remain running during disruptions, unlike other options which describe unrelated features.

3. Final Answer:

   To ensure a minimum number of pods stay available during planned disruptions -> Option A

4. Quick Check:

   Pod Disruption Budget = availability during disruptions [OK]

Hint: PDB = minimum pods running during planned changes [OK]

Common Mistakes:

• Confusing PDB with auto-scaling
• Thinking PDB restarts crashed pods
• Assuming PDB backs up pod data

2. Which of the following is the correct way to specify a Pod Disruption Budget that allows at most 1 pod disruption at a time for pods with label app=web?

easy

A. apiVersion: policy/v1
kind: PodDisruptionBudget
metadata:
name: pdb-web
spec:
maxUnavailable: 1
selector:
matchLabels:
app: web
minReadySeconds: 10

B. apiVersion: v1
kind: PodDisruptionBudget
metadata:
name: pdb-web
spec:
minAvailable: 1
selector:
matchLabels:
app: web

C. apiVersion: policy/v1
kind: PodDisruptionBudget
metadata:
name: pdb-web
spec:
maxUnavailable: 1
selector:
matchLabels:
app: web
strategy: RollingUpdate

D. apiVersion: policy/v1
kind: PodDisruptionBudget
metadata:
name: pdb-web
spec:
maxUnavailable: 1
selector:
matchLabels:
app: web

Solution

1. Step 1: Check API version and kind

   The correct API version for PDB is policy/v1 and kind is PodDisruptionBudget.

2. Step 2: Validate spec fields for maxUnavailable and selector

   apiVersion: policy/v1 kind: PodDisruptionBudget metadata: name: pdb-web spec: maxUnavailable: 1 selector: matchLabels: app: web correctly uses maxUnavailable: 1 and a selector matching label app: web. Other options either use wrong API version, extra unsupported fields, or wrong spec keys.

3. Final Answer:

   apiVersion: policy/v1 kind: PodDisruptionBudget metadata: name: pdb-web spec: maxUnavailable: 1 selector: matchLabels: app: web -> Option D

4. Quick Check:

   Correct API and maxUnavailable syntax = apiVersion: policy/v1 kind: PodDisruptionBudget metadata: name: pdb-web spec: maxUnavailable: 1 selector: matchLabels: app: web [OK]

Hint: Use policy/v1 and maxUnavailable with correct selector [OK]

Common Mistakes:

• Using wrong API version like v1
• Adding unsupported fields like strategy
• Confusing minAvailable with maxUnavailable

3. Given this Pod Disruption Budget YAML snippet:

spec:
  minAvailable: 3
  selector:
    matchLabels:
      app: backend

If there are 5 pods with label app=backend, how many pods can be disrupted at once without violating the PDB?

medium

A. 3 pods

B. 2 pods

C. 5 pods

D. 0 pods

Solution

1. Step 1: Understand minAvailable meaning

   minAvailable: 3 means at least 3 pods must stay running during disruptions.

2. Step 2: Calculate allowed disruptions

   With 5 pods total, maximum disruptions allowed = 5 - 3 = 2 pods.

3. Final Answer:

   2 pods -> Option B

4. Quick Check:

   5 total - 3 minAvailable = 2 allowed disruptions [OK]

Hint: Allowed disruptions = total pods - minAvailable [OK]

Common Mistakes:

• Confusing minAvailable with maxUnavailable
• Assuming all pods can be disrupted
• Ignoring total pod count

4. You applied this Pod Disruption Budget:

apiVersion: policy/v1
kind: PodDisruptionBudget
metadata:
  name: pdb-db
spec:
  maxUnavailable: 2
  selector:
    matchLabels:
      app: database

But Kubernetes reports an error: "spec.maxUnavailable: Invalid value: "2": must be less than the number of pods selected by the label selector". What is the likely cause?

medium

A. There are fewer than 3 pods with label app=database

B. maxUnavailable must be a percentage, not a number

C. The selector label is incorrect syntax

D. PodDisruptionBudget requires minAvailable, not maxUnavailable

Solution

1. Step 1: Understand maxUnavailable validation

   maxUnavailable must be less than the total number of pods matched by the selector.

2. Step 2: Analyze error message meaning

   The error says "must be less than the number of pods" meaning the number of pods with label app=database is less than or equal to 2.

3. Final Answer:

   There are fewer than 3 pods with label app=database -> Option A

4. Quick Check:

   maxUnavailable < total pods = error if not [OK]

Hint: maxUnavailable must be less than pod count [OK]

Common Mistakes:

• Using maxUnavailable as percentage incorrectly
• Miswriting selector labels
• Thinking minAvailable is mandatory

5. You have a deployment with 10 pods labeled app=frontend. You want to ensure that during voluntary disruptions, at least 80% of pods remain available. Which Pod Disruption Budget spec is correct?

hard

A. spec:
minAvailable: 80
selector:
matchLabels:
app: frontend

B. spec:
maxUnavailable: 8
selector:
matchLabels:
app: frontend

C. spec:
maxUnavailable: 20%
selector:
matchLabels:
app: frontend

D. spec:
minAvailable: 20%
selector:
matchLabels:
app: frontend

Solution

1. Step 1: Understand percentage usage in PDB

   maxUnavailable and minAvailable can accept percentages. To keep 80% available, maxUnavailable should be 20%.

2. Step 2: Evaluate options for correctness

   spec: minAvailable: 80 selector: matchLabels: app: frontend: minAvailable: 80 - absolute value exceeds total pods (10) - invalid.
   spec: maxUnavailable: 8 selector: matchLabels: app: frontend: maxUnavailable: 8 - allows 8 pods unavailable (only 20% available) - insufficient.
   spec: maxUnavailable: 20% selector: matchLabels: app: frontend: maxUnavailable: 20% - allows 20% (~2 pods) unavailable - ensures 80% available - correct.
   spec: minAvailable: 20% selector: matchLabels: app: frontend: minAvailable: 20% - ensures only at least 20% available - insufficient.

3. Final Answer:

   spec: maxUnavailable: 20% selector: matchLabels: app: frontend -> Option C

4. Quick Check:

   20% maxUnavailable = 80% pods available [OK]

Hint: Use maxUnavailable: 20% to keep 80% pods up [OK]

Common Mistakes:

• Using minAvailable with percentage incorrectly
• Confusing absolute numbers with percentages
• Not matching selector labels properly