What is a DaemonSet ?

A DaemonSet is a resource that 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.
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 (ex: fluent, filebeat)
– running a node monitoring daemon on every node

Simple case : one DaemonSet, covering all nodes
More complex cases : use multiple DaemonSets for a single type of daemon, but with different flags and/or different memory and cpu requests for different hardware types.

Creating a daemonset

Required Fields

As with all other Kubernetes config, it requires : apiVersion, kind, and metadata fields.
It also requires a .spec.template and a .spec.selector section.

The .spec.template is a pod template. It has exactly the same schema as a Pod, except it is nested and does not have an apiVersion or kind.
In addition to required fields for a Pod, a Pod template in a DaemonSet has to specify appropriate labels (see pod selector below).
A Pod Template in a DaemonSet must have a RestartPolicy equal to Always, or be unspecified, which defaults to Always

Pod Selector

.spec.selector  must match the .spec.template.metadata.labels.
Config with these not matching will be rejected by the API

The .spec.selector consists of two fields and at least one of them has to be valued :
– matchLabels – works the same as the .spec.selector of a ReplicationController.
– matchExpressions – allows to build more sophisticated selectors by specifying key, list of values and an operator that relates the key and values.
When the two are specified the result is ANDed.

Examples

Here is a fluentd daemonset example:

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd-elasticsearch
  namespace: kube-system
  labels:
    k8s-app: fluentd-logging
spec:
  selector:
    matchLabels:
      name: fluentd-elasticsearch
  template:
    metadata:
      labels:
        name: fluentd-elasticsearch
    spec:
      containers:
      - name: fluentd-elasticsearch
        image: quay.io/fluentd_elasticsearch/fluentd:v2.5.2
        resources:
          limits:
            memory: 200Mi
          requests:
            cpu: 100m
            memory: 200Mi
        volumeMounts:
        - name: varlog
          mountPath: /var/log
        - name: varlibdockercontainers
          mountPath: /var/lib/docker/containers
          readOnly: true
      terminationGracePeriodSeconds: 30
      volumes:
      - name: varlog
        hostPath:
          path: /var/log
      - name: varlibdockercontainers
        hostPath:
          path: /var/lib/docker/containers