DaemonSets for per-node workloads in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of DaemonSet Pod Distribution

You have a DaemonSet running on a Kubernetes cluster with 3 nodes. After running kubectl get pods -o wide, how many pods will you see for this DaemonSet?

A 3 pods, one on each node

B 1 pod, scheduled on the master node only

C 0 pods, DaemonSets do not create pods automatically

D 3 pods, but all scheduled on the same node

💡 Hint

DaemonSets ensure one pod per node unless node selectors or taints prevent scheduling.

A DaemonSet creates exactly one pod on each node in the cluster (unless restricted by node selectors or taints). So with 3 nodes, you get 3 pods.

🧠 Conceptual

intermediate

Purpose of DaemonSets

What is the main purpose of using a DaemonSet in Kubernetes?

A To run a copy of a pod on every node for node-level tasks

B To manage multiple replicas of a pod for load balancing

C To schedule pods only on master nodes

D To create pods that run only once and then exit

💡 Hint

Think about tasks that need to run on all nodes, like monitoring or logging.

DaemonSets are designed to run one pod on each node to perform node-specific tasks such as monitoring, logging, or networking.

❓ Troubleshoot

advanced

DaemonSet Pods Not Running on New Nodes

You added two new nodes to your Kubernetes cluster, but the DaemonSet pods are not appearing on these new nodes. What is the most likely cause?

A Pods are scheduled only on nodes labeled as 'worker' and new nodes are labeled 'master'

B DaemonSets only schedule pods on nodes present at creation time

C The DaemonSet controller needs to be restarted manually

D The new nodes are tainted and the DaemonSet has no tolerations

💡 Hint

Check node taints and tolerations for DaemonSet pods.

If new nodes have taints that the DaemonSet pods do not tolerate, pods will not be scheduled on those nodes.

❓ Configuration

advanced

DaemonSet Node Selector Configuration

You want your DaemonSet pods to run only on nodes labeled disk=ssd. Which snippet correctly configures this in the DaemonSet spec?

A "tolerations": [{"key": "disk", "value": "ssd"}]

B "nodeSelector": {"disk": "ssd"}

C "nodeSelector": {"ssd": "disk"}

D "affinity": {"nodeAffinity": {"requiredDuringSchedulingIgnoredDuringExecution": {"nodeSelectorTerms": [{"matchExpressions": [{"key": "disk", "operator": "In", "values": ["ssd"]}]}]}}}

💡 Hint

Node selectors use key-value pairs to filter nodes.

The nodeSelector field uses key-value pairs to select nodes. Option B correctly uses the label key and value.

✅ Best Practice

expert

Best Practice for Updating DaemonSet Pods

You need to update the container image of a DaemonSet without downtime. Which strategy is best to achieve this?

A Delete the DaemonSet and recreate it with the new image

B Manually delete pods one by one to force recreation with the new image

C Use the DaemonSet rolling update strategy with maxUnavailable set to 1

D Update the DaemonSet spec and immediately restart the kubelet on all nodes

💡 Hint

DaemonSets support rolling updates to minimize downtime.

The rolling update strategy allows updating pods gradually, controlling how many pods can be unavailable during the update.

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