Daemon Sets

Daemon Sets

In this tutorial we will discuss about Daemon Sets in Kubernetes. A Daemon Set makes sure that all of the Nodes in the Kubernetes Cluster run a copy of a POD.

Every node will have a copy of the pod. When nodes are added to the cluster, PODs are added to them and when nodes are removed from the cluster, the PODs are removed. If we delete a DaemonSet it will clean up the PODs it created.

So far we have deployed various PODs on different nodes in our cluster. With the help of replica sets and deployments we made sure multiple copies of our applications are made available across various different worker nodes.

Daemon sets are like replica sets, as in it helps you deploy multiple instances of POD. But it runs one copy of your POD on each node in your cluster.

Whenever a new node is added to the cluster a replica of the POD is automatically added to that node.

And when the node is removed from the cluster then the POD is automatically removed.

The daemon sets ensure that one copy of the POD always present in all nodes in the cluster.

Use cases of daemon sets

For example you would like to deploy a monitoring agent or log collector on each nodes in the cluster. So you can monitor your cluster better. Daemon set is perfect for that as it can deploy your monitoring agent in the form of a POD in all the nodes in the cluster.

Then, you don’t have to worry about adding/removing monitoring agents from these nodes when there are changes in your cluster. As the daemon set will take care of that for you.

Kube proxy is one good use case of daemon sets. The kube proxy component can be deployed as a daemon set in the cluster.

DaemonSet definition

Creating DaemonSet is similar to the ReplicaSet creation process. It has nested POD specification under the template section and selectors to link the daemon set to the PODs

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

Once ready, create the daemon set using the following command.

$ kubectl create -f daemon-set-definition.yaml

Now, to view the created daemon set run the following command.

$ kubectl get daemonsets

And of course to view more details run the following command.

$ kubectl describe daemonsets monitoring-daemon

How does it work?

So, how does a daemon set work? How does it schedule PODs on each node? and how does it ensure that every node has a POD?

Now, If you were asked to schedule a POD on each node in the cluster, how would you do it?

Firstly, On each POD set the nodeName property in its specification before it is created, they automatically land on the respective nodes.

So, that’s how it is used to be until Kubernetes version v1.12. From v1.12 on wards the daemon set used the default scheduler and node affinity rules that we discussed in