DaemonSet

DaemonSet is an object (daemon) that runs a Pod on each node of the cluster and guarantees that there is only one Pod. This is very suitable for some system-level applications, such as log collection, resource monitoring, etc., such applications require Each node runs and does not require too many instances. A good example is kube-proxy of Kubernetes.

DaemonSet is related to the node. If the node is abnormal, it will not be recreated on other nodes.

Figure 1 DaemonSet

The following is an example of a DaemonSet.

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: nginx-daemonset
  labels:
    app: nginx-daemonset
spec:
  selector:
    matchLabels:
      app: nginx-daemonset
  template:
    metadata:
      labels:
        app: nginx-daemonset
    spec:
      nodeSelector:                 # 节点选择，当节点拥有daemon=need时才在节点上创建Pod
        daemon: need
      containers:
      - name: nginx-daemonset
        image: nginx:alpine
        resources:
          limits:
            cpu: 250m
            memory: 512Mi
          requests:
            cpu: 250m
            memory: 512Mi
      imagePullSecrets:
      - name: default-secret

It can be seen here that there is no replicas parameter in Deployment or StatefulSet, because each node is fixed one.

There is a nodeSelector in the Pod template, which specifies that Pods are created only on nodes with "daemon=need". As shown in the figure below, DaemonSet only creates Pods on nodes with specified labels. If you need to create a Pod on each node, you can delete the label.

Figure 2 DaemonSet creates Pod on the node with the specified label

Create DaemonSet:

$ kubectl create -f daemonset.yaml
daemonset.apps/nginx-daemonset created

The query found that nginx-daemonset has no Pod created.

$ kubectl get ds
NAME              DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE   NODE SELECTOR   AGE
nginx-daemonset   0         0         0       0            0           daemon=need     16s
​
$ kubectl get pods
No resources found in default namespace.

This is because there is no daemon=need label on the node. Use the following command to query the label of the node.

$ kubectl get node --show-labels
NAME            STATUS   ROLES    AGE   VERSION                            LABELS
192.168.0.212   Ready    <none>   83m   v1.15.6-r1-20.3.0.2.B001-15.30.2   beta.kubernetes.io/arch=amd64 ...
192.168.0.94    Ready    <none>   83m   v1.15.6-r1-20.3.0.2.B001-15.30.2   beta.kubernetes.io/arch=amd64 ...
192.168.0.97    Ready    <none>   83m   v1.15.6-r1-20.3.0.2.B001-15.30.2   beta.kubernetes.io/arch=amd64 ...

Tag the node 192.168.0.212, and then check again, and found that a Pod has been created, and the Pod is on the node 192.168.0.212.

$ kubectl label node 192.168.0.212 daemon=need
node/192.168.0.212 labeled

$ kubectl get ds
NAME              DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE   NODE SELECTOR   AGE
nginx-daemonset   1         1         0       1            0           daemon=need     116s

$ kubectl get pod -owide
NAME                    READY   STATUS    RESTARTS   AGE   IP           NODE            
nginx-daemonset-g9b7j   1/1     Running   0          18s   172.16.3.0   192.168.0.212

Then label the node 192.168.0.94 and find that another Pod has been created:

$ kubectl label node 192.168.0.94 daemon=need
node/192.168.0.94 labeled

$ kubectl get ds
NAME              DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE   NODE SELECTOR   AGE
nginx-daemonset   2         2         1       2            1           daemon=need     2m29s

$ kubectl get pod -owide
NAME                    READY   STATUS              RESTARTS   AGE   IP           NODE            
nginx-daemonset-6jjxz   0/1     ContainerCreating   0          8s    <none>       192.168.0.94    
nginx-daemonset-g9b7j   1/1     Running             0          42s   172.16.3.0   192.168.0.212 

If you modify the label of the 192.168.0.94 node, you can find that DaemonSet will delete the Pod on this node.

$ kubectl label node 192.168.0.94 daemon=no --overwrite
node/192.168.0.94 labeled

$ kubectl get ds
NAME              DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE   NODE SELECTOR   AGE
nginx-daemonset   1         1         1       1            1           daemon=need     4m5s

$ kubectl get pod -owide
NAME                    READY   STATUS    RESTARTS   AGE     IP           NODE            
nginx-daemonset-g9b7j   1/1     Running   0          2m23s   172.16.3.0   192.168.0.212