Why DaemonSets in Kubernetes?

Sometimes, you may need to run a process on all the nodes of the cluster. Think of log-collecting services like Prometheus Log Exporter or storage daemons like glusterd. Such services need to be started on each node as soon as the node joins the cluster. You may think: we can use a cron job that runs on machine boot/reboot. Perhaps use the /etc/init.local file to ensure that a specific process or command gets executed as soon as the server gets started. While certainly a valid solution, using the node itself to control the daemons that run on it (especially within a Kubernetes cluster) suffers some drawbacks:

• We need the process to remain running on the node as long as it is part of the cluster. It should be terminated when the node is evicted.
• The process may need a particular runtime environment that may or may not be available on the node (for example, a specific JDK version, a required kernel library, a specific Linux distro...etc.). So, the process should run inside a container. Kubernetes uses Pods to run containers. This daemon should be aware that it is running within Kubernetes. Hence, it has access to other pods in the cluster and is part of the network.

Because of those drawbacks, Kubernetes offers Daemonsets. A Daemonset is another controller that manages pods like Deployments, ReplicaSets, and StatefulSets. It was created for one particular purpose: ensuring that the pods it manages to run on all the cluster nodes. As soon as a node joins the cluster, the DaemonSet ensures that it has the necessary pods running on it. When the node leaves the cluster, those pods are garbage collected.

Your First DeamonSet Deployment

Let’s deploy fluentd to collect node data using DaemonSet. fluentd is an open-source application used for collecting and normalizing data. Such data is often logs like web servers or the system log. fluentd is deployed as a background service. If we need it to run on every node of a Kubernetes cluster, we can create a YAML file as follows:

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd
  namespace: kube-system
  labels:
	tier: management
	app: fluentd
spec:
  selector:
	matchLabels:
  	name: fluentd
  template:
	metadata:
  	labels:
    	name: fluentd
	spec:
  	containers:
    	- resources:
        	limits:
          	memory: 200Mi
        	requests:
          	cpu: 100m
          	memory: 200Mi
      	securityContext:
        	privileged: true
      	image: fluent/fluentd
      	name: fluentd-elasticsearch
      	volumeMounts:
        	- name: varlog
          	mountPath: /var
        	- name: varlibdockercontainers
          	mountPath: /var/lib/docker/containers
          	readOnly: true
  	volumes:
    	- name: varlog
      	hostPath:
        	path: /var
    	- name: varlibdockercontainers
      	hostPath:
        	path: /var/lib/docker/containers

Let’s see what this definition do for us:

• Like all other Kubernetes resources, it defines an apiVersion, kind, and metadata.
• The spec part contains the pod selector. In our example, we are matching pods that have the app=fluentd label.
• The template part defines how the pod is built. It establishes some resource limits like CPU and memory to ensure that the pod will stay within utilization limits.
• The securityContext can grant privileged (root) access for the pod, and the containers' hosts. This is common in DaemonSets; as the pod may need to access restricted resources on the node.
• The pod template also contains the image that the container is using.
• Finally, the pod template includes the volume that this pod has access to the /var filesystem and /var/lib/docker/containers to enable the container to access the host instance.
• The volumes part contains the necessary instructions on how the volumes will be accessed. The type of this volume is hostPath.

Apply the above configuration using

kubectl apply -f daemonset.yaml

Let’s see what the DaemonSet created:

kubectl -n kube-system get pods
NAME                                	READY   STATUS	RESTARTS   AGE
coredns-696c56cf6f-gjcs9            	1/1 	Running   0      	6h46m
coredns-696c56cf6f-twj4l            	1/1 	Running   0      	6h50m
coredns-autoscaler-bc55cb685-xpzk6  	1/1 	Running   0      	6h50m
fluentd-f657w                       	1/1 	Running   0      	5h26m
fluentd-wsvdf                       	1/1 	Running   0      	5h26m
heapster-5678f88989-fvdj7           	2/2 	Running   0      	6h46m
kube-proxy-xdggl                    	1/1 	Running   0      	6h47m
kube-proxy-z899d                    	1/1 	Running   0      	6h47m
kubernetes-dashboard-7b749f655b-lbgnp   1/1 	Running   1      	6h50m
metrics-server-5b7d5c6f8d-kss58     	1/1 	Running   1      	6h50m
omsagent-fk4qg                      	1/1 	Running   1      	6h47m
omsagent-lgqkz                      	1/1 	Running   0      	6h47m
omsagent-rs-7b459857cd-g9gsx        	1/1 	Running   1      	6h50m
tunnelfront-5d4d658788-28tdc        	1/1 	Running   0      	6h50

Since we opted to deploy this DaemonSet on the kube-system namespace (rather than the default), we must pass in the namespace name when issuing any kubectl command to it by using -n option.

Additionally, we have some other pods running in the kube-system for a variety of tasks, including coredns for name resolution, kube-proxy, and others. The pods that we are interested in are fluentd-f657w and fluentd-wsvdf.

Restrict DaemonSets To Run On Specific Nodes

By default, a DaemonSet schedules its pods on all the cluster nodes. But sometimes you may need to run specific processes on specific nodes. For example, nodes that host database pods need different monitoring or logging rules. DaemonSets allow you to select which nodes you want to run the pods on. You can do this by using nodeSelector. With nodeSelector, you can select nodes by their labels the same way you do with pods. However, Kubernetes also allows you to select nodes based on some already-defined node properties. For example, kubernetes.io/hostname matches the node name. So, our example cluster has two nodes. We can modify the DaemonSet definition to run only on the first node. Lets’ first get the node names:

kubectl get nodes
NAME                   	STATUS   ROLES   AGE	VERSION
aks-agentpool-30423418-0   Ready	agent   7h2m   v1.12.8
aks-agentpool-30423418-1   Ready	agent   7h2m   v1.12.8

Our DaemonSet definition now should look like this:

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd
  namespace: kube-system
  labels:
	tier: management
	app: fluentd
spec:
  selector:
	matchLabels:
  	name: fluentd
  template:
	metadata:
  	labels:
    	name: fluentd
	spec:
  	containers:
    	- resources:
        	limits:
          	memory: 200Mi
        	requests:
          	cpu: 100m
          	memory: 200Mi
      	securityContext:
        	privileged: true
      	image: fluent/fluentd
      	name: fluentd-elasticsearch
      	volumeMounts:
        	- name: varlog
          	mountPath: /var
        	- name: varlibdockercontainers
          	mountPath: /var/lib/docker/containers
          	readOnly: true
  	nodeSelector:
    	  kubernetes.io/hostname: aks-agentpool-30423418-0
  	volumes:
    	- name: varlog
      	hostPath:
        	path: /var
    	- name: varlibdockercontainers
      	hostPath:
        	path: /var/lib/docker/containers

The added part here is:

nodeSelector:
    	  kubernetes.io/hostname: aks-agentpool-30423418-0

Run the new definition using kubectl:

kubectl apply -f daemonset.yaml

If we check the pods now:

kubectl -n kube-system get pods
NAME                                	READY   STATUS	RESTARTS   AGE
coredns-696c56cf6f-gjcs9            	1/1 	Running   0      	6h55m
coredns-696c56cf6f-twj4l            	1/1 	Running   0      	6h59m
coredns-autoscaler-bc55cb685-xpzk6  	1/1 	Running   0      	6h59m
fluentd-wg7rf                       	1/1 	Running   0      	3s
heapster-5678f88989-fvdj7           	2/2 	Running   0      	6h55m
kube-proxy-xdggl                    	1/1 	Running   0      	6h56m
kube-proxy-z899d                    	1/1 	Running   0      	6h56m
kubernetes-dashboard-7b749f655b-lbgnp   1/1 	Running   1      	6h59m
metrics-server-5b7d5c6f8d-kss58     	1/1 	Running   1      	6h59m
omsagent-fk4qg                      	1/1 	Running   1      	6h56m
omsagent-lgqkz                      	1/1 	Running   0      	6h56m
omsagent-rs-7b459857cd-g9gsx        	1/1 	Running   1      	6h59m
tunnelfront-5d4d658788-28tdc        	1/1 	Running   0      	6h59m

Notice that now we have only one pod running because only one node is matched.

How To Reach a DaemonSet Pod

There are several design patterns DaemonSet-pods communication in the cluster:

• The Push pattern: pods do not receive traffic. Instead, they push data to other services like ElasticSearch, for example.
• NodeIP and known port pattern: in this design, pods use the hostPort to acquire the node’s IP address. Clients can use the node IP and the known port (for example, port 80 if the DaemonSet has a web server) to connect to the pod.
• DNS pattern: create a Headless Service that selects the DaemonSet pods. Use Endpoints to discover DaemonSet pods.
• Service pattern: create a traditional service that selects the DaemonSet pods. Use NodePort to expose the pods using a random port. The drawback of this approach is that there is no way to choose a specific pod.

TL;DR

• DaemonSets are used in Kubernetes when you need to run one or more pods on all (or a subset of) the nodes in a cluster.
• The typical use case for a DaemonSet is logging and monitoring for the hosts. For example, a node needs a service (daemon) that collects health or log data and pushes them to a central system or database (like ELK stack).
• DaemonSets can be deployed to specific nodes either by the nodes’ user-defined labels or using values provided by Kubernetes like the node hostname.