DaemonSets

Theory

A DaemonSet ensures that a copy of a specific Pod runs on every node (or a subset using node selectors or affinity rules). Use cases include:

• Running log collection agents (e.g., FluentD)
• Running monitoring agents (e.g., Prometheus node-exporter)
• Running networking plugins (e.g., kube-proxy, weave-net)

DaemonSets are commonly used when you need:

• Node-local functionality
• To ensure one Pod per node
• Full visibility or access to host-level resources

Scheduling Behavior

• Prior to Kubernetes v1.12, DaemonSet pods were scheduled manually by the DaemonSet controller.
• From Kubernetes v1.12 onwards, they use the default scheduler with NodeAffinity under the hood.

DaemonSets vs ReplicaSets vs Deployments

Unlike ReplicaSets and Deployments, which distribute Pods across nodes based on desired counts, DaemonSets schedule one pod per node, regardless of cluster size.

---

Use Cases

1. Monitoring & Logging

Deploy logging and monitoring agents on each node:

2. kube-proxy

The Kubernetes networking component kube-proxy runs as a DaemonSet:

3. Networking Plugins

CNI plugins like weave-net, flannel, etc., are deployed using DaemonSets:

---

Practical

1. Viewing DaemonSets

kubectl get daemonsets --all-namespaces
kubectl describe daemonset <name> -n <namespace>

2. Count all DaemonSets in the cluster:

kubectl get daemonsets --all-namespaces --no-headers | wc -l

3. Identify kube-proxy DaemonSet

kubectl get daemonset kube-proxy -n kube-system

• Namespace: kube-system
• Scheduled on 1 node (from DESIRED column)

4. Get Image of kube-flannel-ds DaemonSet

kubectl get daemonset kube-flannel-ds -n kube-flannel -o jsonpath="{.spec.template.spec.containers[*].image}"

---

Create a DaemonSet

YAML Definition:

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: elasticsearch
  namespace: kube-system
  labels:
    k8s-app: fluentd-logging
spec:
  selector:
    matchLabels:
      name: fluentd
  template:
    metadata:
      labels:
        name: fluentd
    spec:
      containers:
      - name: fluentd
        image: registry.k8s.io/fluentd-elasticsearch:1.20
        resources:
          limits:
            memory: 200Mi
          requests:
            cpu: 100m
            memory: 200Mi
        volumeMounts:
        - name: varlog
          mountPath: /var/log
        - name: varlibdockercontainers
          mountPath: /var/lib/docker/containers
          readOnly: true
      terminationGracePeriodSeconds: 30
      volumes:
      - name: varlog
        hostPath:
          path: /var/log
      - name: varlibdockercontainers
        hostPath:
          path: /var/lib/docker/containers

Apply:

kubectl apply -f fluentd-daemonset.yaml

Imperative Creation:

kubectl create daemonset elasticsearch \
  --image=registry.k8s.io/fluentd-elasticsearch:1.20 \
  --namespace=kube-system \
  --dry-run=client -o yaml | kubectl apply -f -

---

Inspect Existing DaemonSet

Example:

kubectl get daemonsets
kubectl describe daemonsets monitoring-daemon

---

Appendix

Example DaemonSet YAML

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

---

This section is part of: Scheduling -> DaemonSets