A DaemonSet 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:
ceph, on each node.
collectd, Datadog agent, New Relic agent, or Ganglia
In a simple case, one DaemonSet, covering all nodes, would be used for each type of daemon. A more complex setup might use multiple DaemonSets for a single type of daemon, but with different flags and/or different memory and cpu requests for different hardware types.
You can describe a DaemonSet in a YAML file. For example, the
daemonset.yaml file below describes a DaemonSet that runs the fluentd-elasticsearch Docker image:
kubectl create -f https://k8s.io/examples/controllers/daemonset.yaml
As with all other Kubernetes config, a DaemonSet needs
metadata fields. For
general information about working with config files, see deploying applications,
configuring containers, and object management using kubectl documents.
A DaemonSet also needs a
.spec.template is one of the required fields in
In addition to required fields for a Pod, a Pod template in a DaemonSet has to specify appropriate labels (see pod selector).
A Pod Template in a DaemonSet must have a
Always, or be unspecified, which defaults to
.spec.selector field is a pod selector. It works the same as the
As of Kubernetes 1.8, you must specify a pod selector that matches the labels of the
.spec.template. The pod selector will no longer be defaulted when left empty. Selector
defaulting was not compatible with
kubectl apply. Also, once a DaemonSet is created,
.spec.selector can not be mutated. Mutating the pod selector can lead to the
unintentional orphaning of Pods, and it was found to be confusing to users.
.spec.selector is an object consisting of two fields:
matchLabels- works the same as the
.spec.selectorof 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.
.spec.selector is specified, it must match the
.spec.template.metadata.labels. Config with these not matching will be rejected by the API.
Also you should not normally create any Pods whose labels match this selector, either directly, via another DaemonSet, or via other controller such as ReplicaSet. Otherwise, the DaemonSet controller will think that those Pods were created by it. Kubernetes will not stop you from doing this. One case where you might want to do this is manually create a Pod with a different value on a node for testing.
If you specify a
.spec.template.spec.nodeSelector, then the DaemonSet controller will
create Pods on nodes which match that node
selector. Likewise if you specify a
then DaemonSet controller will create Pods on nodes which match that node affinity.
If you do not specify either, then the DaemonSet controller will create Pods on all nodes.
Normally, the machine that a Pod runs on is selected by the Kubernetes scheduler. However, Pods
created by the DaemonSet controller have the machine already selected (
.spec.nodeName is specified
when the Pod is created, so it is ignored by the scheduler). Therefore:
unschedulablefield of a node is not respected by the DaemonSet controller.
A DaemonSet ensures that all eligible nodes run a copy of a Pod. Normally, the node that a Pod runs on is selected by the Kubernetes scheduler. However, DaemonSet pods are created and scheduled by the DaemonSet controller instead. That introduces the following issues:
Pendingstate, but DaemonSet pods are not created in
Pendingstate. This is confusing to the user.
ScheduleDaemonSetPods allows you to schedule DaemonSets using the default
scheduler instead of the DaemonSet controller, by adding the
to the DaemonSet pods, instead of the
.spec.nodeName term. The default
scheduler is then used to bind the pod to the target host. If node affinity of
the DaemonSet pod already exists, it is replaced. The DaemonSet controller only
performs these operations when creating or modifying DaemonSet pods, and no
changes are made to the
spec.template of the DaemonSet.
nodeAffinity: requiredDuringSchedulingIgnoredDuringExecution: nodeSelectorTerms: - matchFields: - key: metadata.name operator: In values: - target-host-name
node.kubernetes.io/unschedulable:NoSchedule toleration is added
automatically to DaemonSet Pods. The DaemonSet controller ignores
unschedulable Nodes when scheduling DaemonSet Pods. You must enable
TaintNodesByCondition to ensure that the default scheduler behaves the same
way and schedules DaemonSet pods on
When this feature and
TaintNodesByCondition are enabled together, if DaemonSet
uses the host network, you must also add the
Although Daemon Pods respect taints and tolerations, the following tolerations are added to DaemonSet Pods automatically according to the related features.
|Toleration Key||Effect||Alpha Features||Version||Description|
Some possible patterns for communicating with Pods in a DaemonSet are:
hostPort, so that the pods are reachable via the node IPs. Clients know the list of node IPs somehow, and know the port by convention.
endpointsresource or retrieve multiple A records from DNS.
If node labels are changed, the DaemonSet will promptly add Pods to newly matching nodes and delete Pods from newly not-matching nodes.
You can modify the Pods that a DaemonSet creates. However, Pods do not allow all fields to be updated. Also, the DaemonSet controller will use the original template the next time a node (even with the same name) is created.
You can delete a DaemonSet. If you specify
kubectl, then the Pods
will be left on the nodes. You can then create a new DaemonSet with a different template.
The new DaemonSet with the different template will recognize all the existing Pods as having
matching labels. It will not modify or delete them despite a mismatch in the Pod template.
You will need to force new Pod creation by deleting the Pod or deleting the node.
In Kubernetes version 1.6 and later, you can perform a rolling update on a DaemonSet.
It is certainly possible to run daemon processes by directly starting them on a node (e.g. using
systemd). This is perfectly fine. However, there are several advantages to
running such processes via a DaemonSet:
kubectl) for daemons and applications.
It is possible to create Pods directly which specify a particular node to run on. However, a DaemonSet replaces Pods that are deleted or terminated for any reason, such as in the case of node failure or disruptive node maintenance, such as a kernel upgrade. For this reason, you should use a DaemonSet rather than creating individual Pods.
It is possible to create Pods by writing a file to a certain directory watched by Kubelet. These are called static pods. Unlike DaemonSet, static Pods cannot be managed with kubectl or other Kubernetes API clients. Static Pods do not depend on the apiserver, making them useful in cluster bootstrapping cases. Also, static Pods may be deprecated in the future.
DaemonSets are similar to Deployments in that they both create Pods, and those Pods have processes which are not expected to terminate (e.g. web servers, storage servers).
Use a Deployment for stateless services, like frontends, where scaling up and down the number of replicas and rolling out updates are more important than controlling exactly which host the Pod runs on. Use a DaemonSet when it is important that a copy of a Pod always run on all or certain hosts, and when it needs to start before other Pods.