DaemonSet

In Kubernetes, a DaemonSet ensures that all (or some) nodes run a copy of a Pod.

As nodes are added to the cluster, pods are added to them. As nodes are removed from the cluster, those pods are garbage collected.

DaemonSets are typically used for deploying system-level agents and tools, such as log collectors, monitoring agents, or other utilities that should run on every node.

Official docs: https://kubernetes.io/docs/concepts/workloads/controllers/daemonset/

Hands-On: Working with DaemonSets

We will create and examine a DaemonSet to understand it's behavior.

1. Create a Namespace for the Examples

First, we'll create a namespace for these examples and set it as the default.

# task 01-create-namespace
# - Create a namespace for these examples and set as default.
kubectl apply -f Namespace.yaml
kubens 04--daemonset

2. Apply the DaemonSet Configuration

The pod we will use in our example DaemonSet runs the fluentd, a tool used to collect logs from a variety of sources and ship them to a log aggregator. As written, this DaemonSet is not configure to actually ship them anywhere, but I wanted to at least use an application that made sense in this use case.

# DaemonSet.fluentd-minimal.yaml
apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd-minimal
  namespace: 04--daemonset
spec:
  selector:
    matchLabels:
      app: fluentd
  template:
    metadata:
      labels:
        app: fluentd
    spec:
      containers:
        - name: fluentd
          image: fluentd:v1.16-1

The specification for a DaemonSet is quite similar to that of a Deployment, but instead of specifying a number of replicas, it will run a single replica on all nodes (or those specified).

# task 03-apply-minimal-daemonset
# - Apply a minimal fluentd daemonset configuration.
kubectl apply -f DaemonSet.fluentd-minimal.yaml

We can then run a kubectl get pods to see the pods and which nodes they are running on:

❯ kubectl get nodes
NAME                 STATUS   ROLES           AGE   VERSION
kind-control-plane   Ready    control-plane   21d   v1.30.0
kind-worker          Ready    <none>          21d   v1.30.0
kind-worker2         Ready    <none>          21d   v1.30.0

❯ kubectl get pods -o wide
NAME                    READY   STATUS    RESTARTS   AGE   IP            NODE           NOMINATED NODE   READINESS GATES
fluentd-minimal-ssjwj   1/1     Running   0          10s   10.244.1.66   kind-worker2   <none>           <none>
fluentd-minimal-xxp8w   1/1     Running   0          10s   10.244.2.16   kind-worker    <none>           <none>

💡 Note: By default a DaemonSet will run on all worker nodes but NOT control plane nodes. To also run on the control plane you must add a "toleration" allowing it.

3. Delete the Namespace to Clean Up

Finally, clean up by deleting the namespace, which will also delete all resources within it.

# task 04-delete-namespace
# - Delete the namespace to clean up.
kubectl delete -f Namespace.yaml