What is a DaemonSet in OpenShift?

A DaemonSet is a Kubernetes workload that ensures exactly one copy of a Pod runs on every selected node in the cluster.

Unlike a Deployment, where you specify the number of replicas, a DaemonSet automatically creates and removes Pods as nodes are added or removed.

Think of a DaemonSet as “one Pod per node.”

Why DaemonSets are Important in OpenShift

Many OpenShift platform services must run on every node because they collect metrics, logs, or provide node-level networking.

For example:

                OpenShift Cluster

      ┌──────────────┬──────────────┬──────────────┐
      │              │              │
      ▼              ▼              ▼
   Worker-1       Worker-2       Worker-3
      │              │              │
      ▼              ▼              ▼
 Node Exporter   Node Exporter   Node Exporter

Each node has its own Node Exporter Pod.

If a new node joins:

Worker-4
↓
DaemonSet automatically creates
↓
Node Exporter Pod

No manual action is required.

DaemonSet vs Deployment

Example:

Deployment

Cluster
10 Nodes
↓
Deployment replicas = 3
↓
Only 3 Pods

DaemonSet

Cluster
10 Nodes
↓
DaemonSet
↓
10 Pods
(1 on each node)

How DaemonSet Works

New Node Added
        │
        ▼
DaemonSet Controller detects node
        │
        ▼
Creates Pod
        │
        ▼
Pod scheduled on new node

If a node is removed:

Node Deleted
↓
DaemonSet Pod Deleted

Real OpenShift DaemonSets

Run:

oc get daemonset -A

Typical output:

NAMESPACE                     NAME
openshift-monitoring          node-exporter
openshift-logging             collector
openshift-ovn-kubernetes      ovnkube-node
openshift-multus              multus
openshift-machine-config      machine-config-daemon

These are all critical platform components.

1. Node Exporter

Every Node
↓
Node Exporter
↓
CPU
Memory
Disk
Filesystem
Network

Used by Prometheus for infrastructure monitoring.

2. Machine Config Daemon

MachineConfig
↓
Machine Config Operator
↓
Machine Config Daemon
↓
Every Node

The Machine Config Daemon applies operating system changes on each RHCOS node.

3. OVN-Kubernetes

Every node participates in cluster networking.

Node
↓
OVN Pod
↓
Programs Open vSwitch
↓
Pod Networking

Without it, Pods cannot communicate.

4. Vector Collector

OpenShift Logging deploys Vector as a DaemonSet.

Every Node
↓
Vector
↓
Collect container logs
↓
Loki
Splunk
Elasticsearch

Each node collects its own logs locally and forwards them.

DaemonSet Architecture

                  DaemonSet Controller
                          │
        ┌─────────────────┼─────────────────┐
        │                 │                 │
        ▼                 ▼                 ▼
     Worker-1         Worker-2         Worker-3
        │                 │                 │
        ▼                 ▼                 ▼
    One Pod          One Pod          One Pod

Scheduling

The DaemonSet controller watches nodes.

Whenever a new node appears:

Node Ready
↓
Matches selector
↓
Create Pod
↓
Schedule Pod

Node Selectors

DaemonSets can target only specific nodes.

Example:

spec:
  template:
    spec:
      nodeSelector:
        node-role.kubernetes.io/worker: ""

Only worker nodes receive the Pod.

Taints and Tolerations

Control-plane nodes are often tainted.

Example:

master-0
↓
NoSchedule

To run there, the DaemonSet must tolerate the taint.

Example:

tolerations:
- operator: Exists

Without the toleration:

Master Node
×
No Pod

Example DaemonSet

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: node-exporter
spec:
  selector:
    matchLabels:
      app: node-exporter
  template:
    metadata:
      labels:
        app: node-exporter
    spec:
      containers:
      - name: node-exporter
        image: prom/node-exporter

You don’t specify replicas.

The controller calculates the number automatically.

DaemonSet Lifecycle

Cluster
3 Nodes
↓
DaemonSet
↓
3 Pods

Add two nodes:

Cluster
5 Nodes
↓
DaemonSet
↓
5 Pods

Remove one node:

Cluster
4 Nodes
↓
DaemonSet
↓
4 Pods

Common Use Cases

Monitoring

Node Exporter
↓
Metrics

Logging

Vector
↓
Collect logs

Networking

OVN
↓
Configure networking

Storage

CSI Node Plugin

Every Node
↓
Attach Volumes

Security

Examples:

• Falco
• Security agents
• Endpoint protection
• File integrity monitoring

DaemonSet vs StatefulSet

DaemonSet vs Deployment

Useful Commands

List DaemonSets:

oc get daemonset -A

Describe:

oc describe daemonset node-exporter -n openshift-monitoring

Check Pods:

oc get pods -o wide -n openshift-monitoring

Watch rollout:

oc rollout status daemonset/node-exporter -n openshift-monitoring

Edit:

oc edit daemonset node-exporter -n openshift-monitoring

Note: Do not edit OpenShift-managed DaemonSets (such as node-exporter, machine-config-daemon, or ovnkube-node) directly. They are managed by Operators, and your changes will typically be overwritten. Use the supported Operator configuration instead.

Troubleshooting DaemonSets

Missing Pod on a Node

Check:

oc get daemonset -A

Then:

oc describe daemonset <name>

Verify:

• Node Selector
• Taints
• Tolerations
• Resource availability
• Events

Pod Pending

Check:

oc describe pod <pod>

Common causes:

• No CPU
• No memory
• Taint not tolerated
• Missing image
• Failed image pull

New Node Has No Pod

Verify:

oc get nodes

Then:

oc get daemonset

Look for:

Desired Number Scheduled
Current Number Scheduled

If they differ:

Desired = 10
Current = 9

Investigate why one node isn’t running the DaemonSet Pod.

Real Production Example

A new worker node joins the cluster.

Within seconds:

New Worker
↓
OVN Pod
↓
Machine Config Daemon
↓
Node Exporter
↓
Vector
↓
CSI Driver

The node becomes fully integrated into the cluster without manual intervention.

Interview Answer (2 Minutes)

A DaemonSet is a Kubernetes workload that ensures one Pod runs on every selected node in the cluster. Unlike a Deployment, where you specify a replica count, a DaemonSet automatically creates a Pod on each matching node and removes it when the node leaves the cluster. In OpenShift, DaemonSets are used for node-level services such as the Machine Config Daemon, OVN-Kubernetes, Node Exporter, Vector log collectors, and CSI storage plugins because these components must run on every node. As the cluster scales, the DaemonSet scales automatically with it. When troubleshooting, I check the DaemonSet status, node selectors, taints and tolerations, events, and whether the number of scheduled Pods matches the number of eligible nodes. One important operational point is that OpenShift-managed DaemonSets are controlled by Operators, so they should be configured through the Operator rather than edited directly.