All-Things-Docker-and-Kubernetes

DaemonSets

A DaemonSet is a Kubernetes resource that is similar with Deployments as Daemonsets also creates a Pod. The difference is that the main purpose of a Daemonset is to ensure that all (or some) nodes run a copy of a Pod. This is used for long-running processes.

As nodes are added to the cluster, Pods are added to them. As nodes are removed from the cluster, those Pods are garbage collected. Deleting a DaemonSet will clean up the Pods it created.

Some typical uses of a DaemonSet are:

running a cluster storage daemon on every node
running a logs collection daemon on every node
running a node monitoring daemon on every node

To see the daemonsets running:

$ kubectl get ds

To get more information about the daemonset in a specific namespace, e.g. kube-system:

$ kubectl describe <daemonset> -n kube-system

To see the daemonset in a specific namespace, e.g. kube-system:

$ kubectl get ds -n kube-system

NAME        DESIRED     CURRENT     READY   UP-TO-DATE  AVAILABLE
aws-node    2           2           2       2           2
kube-proxy   2           2           2       2           2

In the example above, we see that there’s two DaemonSets running:

kube-proxy
- handles iptables configs of the nodes
- allows network connectivity to happen with the pods on the node.
```
  kubectl logs -n kube-system kube-proxy-XXXXX
```
aws-node
- handles network overlay underneath
- contains the CNI plugins necessary for EC2 nodes to be able to communicate with the Amazon VPC CNI.
```
  kubectl describe ds aws-node -n kube-system 
```

As an example, we can use one of the manifest from one of the labs in this repo. Run the eksops.yml file to launch a Kubernetes cluster.

time eksctl create cluster -f eksops.yml

Once the cluster is setup, check out the logs of one of the kube-proxy pods.

~$ kubectl get pods -n kube-system | grep proxy
kube-proxy-f8dl7                                                   1/1     Running   3 (6m58s ago)   140d
kube-proxy-fdmnb                                                   1/1     Running   0               5m23s
kube-proxy-nhzsd                                                   1/1     Running   0               5m17s 

# proxy_pod_1 is a variable with the name of the first kube-proxy pod
proxy_pod_1=$(kubectl get pods -n kube-system | grep proxy | cut -d" " -f1 | head -1)

~$ kubectl logs -n kube-system $proxy_pod_1

I1228 15:55:44.614375       1 node.go:163] Successfully retrieved node IP: 10.0.0.100
I1228 15:55:44.614936       1 server_others.go:138] "Detected node IP" address="10.0.0.100"
I1228 15:55:44.615089       1 server_others.go:578] "Unknown proxy mode, assuming iptables proxy" proxyMode=""
I1228 15:55:44.842402       1 server_others.go:206] "Using iptables Proxier"
I1228 15:55:44.842586       1 server_others.go:213] "kube-proxy running in dual-stack mode" ipFamily=IPv4
I1228 15:55:44.842673       1 server_others.go:214] "Creating dualStackProxier for iptables"
I1228 15:55:44.842814       1 server_others.go:501] "Detect-local-mode set to ClusterCIDR, but no IPv6 cluster CIDR defined, , defaulting to no-op detect-local for IPv6"
I1228 15:55:44.842884       1 proxier.go:259] "Setting route_localnet=1, use nodePortAddresses to filter loopback addresses for NodePorts to skip it https://issues.k8s.io/90259"
I1228 15:55:44.843065       1 proxier.go:259] "Setting route_localnet=1, use nodePortAddresses to filter loopback addresses for NodePorts to skip it https://issues.k8s.io/90259"
I1228 15:55:44.843261       1 server.go:661] "Version info" version="v1.24.3"
I1228 15:55:44.843277       1 server.go:663] "Golang settings" GOGC="" GOMAXPROCS="" GOTRACEBACK=""
I1228 15:55:44.844361       1 conntrack.go:100] "Set sysctl" entry="net/netfilter/nf_conntrack_max" value=131072
I1228 15:55:44.844397       1 conntrack.go:52] "Setting nf_conntrack_max" nf_conntrack_max=131072
I1228 15:55:44.844659       1 conntrack.go:83] "Setting conntrack hashsize" conntrack hashsize=32768
I1228 15:55:44.858856       1 conntrack.go:100] "Set sysctl" entry="net/netfilter/nf_conntrack_tcp_timeout_close_wait" value=3600
I1228 15:55:44.861250       1 config.go:317] "Starting service config controller"
I1228 15:55:44.862031       1 shared_informer.go:255] Waiting for caches to sync for service config
I1228 15:55:44.862067       1 config.go:444] "Starting node config controller"
I1228 15:55:44.862072       1 shared_informer.go:255] Waiting for caches to sync for node config
I1228 15:55:44.864557       1 config.go:226] "Starting endpoint slice config controller"
I1228 15:55:44.864567       1 shared_informer.go:255] Waiting for caches to sync for endpoint slice config
I1228 15:55:44.962638       1 shared_informer.go:262] Caches are synced for service config
I1228 15:55:44.962638       1 shared_informer.go:262] Caches are synced for node config
I1228 15:55:44.964885       1 shared_informer.go:262] Caches are synced for endpoint slice config 

The manifest for the DaemonSet is similar to a ReplicaSet.

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 

To learn more, check out DaemonSets in Kubernetes.

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