DaemonSets for per-node workloads in Kubernetes - Mini Project: Build & Apply

DaemonSets for per-node workloads

📖 Scenario: You are managing a Kubernetes cluster where you need to run a monitoring agent on every node. This agent collects system metrics and sends them to a central dashboard. To ensure the agent runs on all current and future nodes automatically, you will use a DaemonSet.

🎯 Goal: Create a Kubernetes DaemonSet manifest that deploys a monitoring agent container on every node in the cluster.

📋 What You'll Learn

Create a DaemonSet manifest named monitor-agent in the default namespace

Use the container image busybox with the command ['sh', '-c', 'while true; do echo Monitoring; sleep 30; done']

Set the container name to monitor-agent-container

Ensure the DaemonSet runs one pod per node

Add a label app: monitor-agent to the pod template metadata

💡 Why This Matters

🌍 Real World

DaemonSets are used to deploy system-level agents like monitoring, logging, or security tools on every node automatically.

💼 Career

Understanding DaemonSets is essential for Kubernetes administrators and DevOps engineers to manage node-level workloads efficiently.

1

Create the DaemonSet skeleton

Create a YAML manifest for a DaemonSet named monitor-agent in the default namespace. Include the apiVersion, kind, metadata with the name and namespace, and an empty spec section.

Kubernetes

# Your code here

Hint

Start with the basic structure of a DaemonSet manifest including apiVersion, kind, and metadata.

2

Add the pod template with labels and container

Inside the spec section, add a selector with matchLabels set to app: monitor-agent. Then add a template section with metadata labels app: monitor-agent and a spec that defines one container named monitor-agent-container using the image busybox.

Kubernetes

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: monitor-agent
  namespace: default
spec:
  # Add selector and template with labels and container here

Hint

Remember the selector.matchLabels must match the pod template labels exactly.

3

Add the container command to run the monitoring loop

In the container spec, add the command field with the value ['sh', '-c', 'while true; do echo Monitoring; sleep 30; done'] to keep the container running and printing 'Monitoring' every 30 seconds.

Kubernetes

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: monitor-agent
  namespace: default
spec:
  selector:
    matchLabels:
      app: monitor-agent
  template:
    metadata:
      labels:
        app: monitor-agent
    spec:
      containers:
      - name: monitor-agent-container
        image: busybox
        # Add the command to run the monitoring loop here

Hint

The command field overrides the default container command to run your monitoring loop.

4

Display the DaemonSet manifest

Print the complete DaemonSet YAML manifest to the console.

Kubernetes

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: monitor-agent
  namespace: default
spec:
  selector:
    matchLabels:
      app: monitor-agent
  template:
    metadata:
      labels:
        app: monitor-agent
    spec:
      containers:
      - name: monitor-agent-container
        image: busybox
        command: ['sh', '-c', 'while true; do echo Monitoring; sleep 30; done']

# Print the manifest below

Hint

Use a print statement to output the entire YAML manifest as a string.

Practice

1. What is the main purpose of a DaemonSet in Kubernetes?

easy

A. To expose services outside the cluster

B. To manage persistent storage volumes

C. To run one pod on every node in the cluster

D. To schedule batch jobs at specific times

Solution

1. Step 1: Understand DaemonSet functionality

   A DaemonSet ensures that a copy of a pod runs on all or some nodes in the cluster.

2. Step 2: Compare with other Kubernetes objects

   Other objects like Jobs or Services do not guarantee one pod per node; DaemonSet does.

3. Final Answer:

   To run one pod on every node in the cluster -> Option C

4. Quick Check:

   DaemonSet = one pod per node [OK]

Hint: DaemonSet always runs pods on all nodes [OK]

Common Mistakes:

• Confusing DaemonSet with Deployment
• Thinking DaemonSet manages storage
• Assuming DaemonSet schedules batch jobs

2. Which of the following is the correct way to specify a DaemonSet selector in the YAML manifest?

easy

A. selector:
labels:
app: my-daemon

B. selector:
matchLabels:
app: my-daemon

C. selector:
matchExpressions:
- key: app
operator: Equals
values: [my-daemon]

D. selector:
matchLabels:
app: my-daemon
matchExpressions:
- key: tier
operator: In
values: [backend]

Solution

1. Step 1: Recall DaemonSet selector syntax

   The selector must use matchLabels or matchExpressions under selector.

2. Step 2: Identify correct syntax

   selector: matchLabels: app: my-daemon uses matchLabels correctly with key-value pairs. selector: labels: app: my-daemon is invalid because it uses labels instead of matchLabels. selector: matchExpressions: - key: app operator: Equals values: [my-daemon] has wrong operator case and structure. selector: matchLabels: app: my-daemon matchExpressions: - key: tier operator: In values: [backend] mixes both but DaemonSet selector only supports one selector type.

3. Final Answer:

   selector: matchLabels: app: my-daemon -> Option B

4. Quick Check:

   DaemonSet selector uses matchLabels [OK]

Hint: Use matchLabels under selector for DaemonSet [OK]

Common Mistakes:

• Using 'labels' instead of 'matchLabels'
• Incorrect operator casing in matchExpressions
• Mixing matchLabels and matchExpressions in selector

3. Given this DaemonSet YAML snippet, what will happen when a new node joins the cluster?

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: log-agent
spec:
  selector:
    matchLabels:
      app: log-agent
  template:
    metadata:
      labels:
        app: log-agent
    spec:
      containers:
      - name: log-agent
        image: log-agent:latest

medium

A. A pod named log-agent will be created on the new node automatically

B. No pod will be created until manually triggered

C. The DaemonSet will fail due to missing node selector

D. Pods will be created only on nodes labeled 'log-agent'

Solution

1. Step 1: Understand DaemonSet behavior on new nodes

   DaemonSets automatically create pods on every node that matches the selector, including new nodes joining the cluster.

2. Step 2: Check node selector usage

   This DaemonSet has no node selector restricting nodes, so it applies to all nodes.

3. Final Answer:

   A pod named log-agent will be created on the new node automatically -> Option A

4. Quick Check:

   DaemonSet pods auto-create on new nodes [OK]

Hint: DaemonSet pods auto-launch on all nodes by default [OK]

Common Mistakes:

• Assuming manual pod creation on new nodes
• Thinking node selector is mandatory
• Believing pods only run on labeled nodes

4. You created a DaemonSet but no pods are running on any node. Which of these is the most likely cause?

medium

A. The DaemonSet is created in the wrong namespace

B. The DaemonSet spec is missing the container image

C. The cluster has no nodes available

D. The pod template labels do not match the DaemonSet selector

Solution

1. Step 1: Check DaemonSet selector and pod template labels

   DaemonSet requires the selector to match the pod template labels exactly; otherwise, pods won't be created.

2. Step 2: Evaluate other options

   Missing container image causes pod creation failure but usually shows pod errors. No nodes means no pods but is rare. Wrong namespace means DaemonSet exists but unrelated to nodes.

3. Final Answer:

   The pod template labels do not match the DaemonSet selector -> Option D

4. Quick Check:

   Selector and pod labels must match [OK]

Hint: Match pod labels with selector to run DaemonSet pods [OK]

Common Mistakes:

• Ignoring label mismatch between selector and template
• Assuming missing image stops pod creation silently
• Not verifying cluster node availability

5. You want to run a logging agent on all Linux nodes but exclude Windows nodes in your Kubernetes cluster. How should you configure the DaemonSet?

hard

A. Add a nodeSelector with kubernetes.io/os: linux in the pod spec

B. Add a tolerations for Windows taints in the pod spec

C. Set the DaemonSet selector to os: linux label

D. Use a Deployment instead of DaemonSet with node affinity

Solution

1. Step 1: Identify how to target Linux nodes only

   Kubernetes labels nodes with kubernetes.io/os set to their OS. Using nodeSelector with kubernetes.io/os: linux restricts pods to Linux nodes.

2. Step 2: Evaluate other options

   Tolerations are for taints, not OS filtering. Selector matches pod labels, not nodes. Deployment does not guarantee one pod per node.

3. Final Answer:

   Add a nodeSelector with kubernetes.io/os: linux in the pod spec -> Option A

4. Quick Check:

   Use nodeSelector to filter nodes by OS [OK]

Hint: Use nodeSelector with OS label to filter nodes [OK]

Common Mistakes:

• Using selector to filter nodes instead of nodeSelector
• Confusing tolerations with node filtering
• Using Deployment instead of DaemonSet for per-node pods