This example shows how to assign a pod to a specific node or to one of a set of nodes using node labels and the nodeSelector field in a pod specification. Generally this is unnecessary, as the scheduler will take care of things for you, but you may want to do so in certain circumstances like to ensure that your pod ends up on a machine with an SSD attached to it.
You can find all the files for this example in our docs repo here.
This example assumes that you have a basic understanding of Kubernetes pods and that you have turned up a Kubernetes cluster.
kubectl get nodes to get the names of your cluster’s nodes. Pick out the one that you want to add a label to.
Then, to add a label to the node you’ve chosen, run
kubectl label nodes <node-name> <label-key>=<label-value>. For example, if my node name is ‘kubernetes-foo-node-1.c.a-robinson.internal’ and my desired label is ‘disktype=ssd’, then I can run
kubectl label nodes kubernetes-foo-node-1.c.a-robinson.internal disktype=ssd.
If this fails with an “invalid command” error, you’re likely using an older version of kubectl that doesn’t have the
label command. In that case, see the previous version of this guide for instructions on how to manually set labels on a node.
Also, note that label keys must be in the form of DNS labels (as described in the identifiers doc), meaning that they are not allowed to contain any upper-case letters.
You can verify that it worked by re-running
kubectl get nodes and checking that the node now has a label.
Take whatever pod config file you want to run, and add a nodeSelector section to it, like this. For example, if this is my pod config:
- name: nginx
Then add a nodeSelector like so:
When you then run
kubectl create -f pod.yaml, the pod will get scheduled on the node that you attached the label to! You can verify that it worked by running
kubectl get pods -o wide and looking at the “NODE” that the pod was assigned to.
During the first half of 2016 we are rolling out a new mechanism, called affinity for controlling which nodes your pods wil be scheduled onto.
nodeSelector, affinity is based on labels. But it allows you to write much more expressive rules.
nodeSelector wil continue to work during the transition, but will eventually be deprecated.
Kubernetes v1.2 offers an alpha version of the first piece of the affinity mechanism, called node affinity.
There are currently two types of node affinity, called
preferredDuringSchedulingIgnoredDuringExecution. You can think of them as “hard” and “soft” respectively,
in the sense that the former specifies rules that must be met for a pod to schedule onto a node (just like
nodeSelector but using a more expressive syntax), while the latter specifies preferences that the scheduler
will try to enforce but will not guarantee. The “IgnoredDuringExecution” part of the names means that, similar
nodeSelector works, if labels on a node change at runtime such that the rules on a pod are no longer
met, the pod will still continue to run on the node. In the future we plan to offer
requiredDuringSchedulingRequiredDuringExecution which will be just like
except that it will evict pods from nodes that cease to satisfy the pods’ node affinity requirements.
Node affinity is currently expressed using an annotation on Pod. In v1.3 it will use a field, and we will also introduce the second piece of the affinity mechanism, called pod affinity, which allows you to control whether a pod schedules onto a particular node based on which other pods are running on the node, rather than the labels on the node.
Here’s an example of a pod that uses node affinity:
This node affinity rule says the pod can only be placed on a node with a label whose key is
kubernetes.io/e2e-az-name and whose value is either
e2e-az2. In addition,
among nodes that meet that criteria, nodes with a label whose key is
another-annotation-key and whose
another-annotation-value should be preferred.
You can see the operator
In being used in the example. The new node affinity syntax supports the following operators:
If you specify both
nodeAffinity, both must be satisfied for the pod
to be scheduled onto a candidate node.
In addition to labels you attach yourself, nodes come pre-populated with a standard set of labels. As of Kubernetes v1.2 these labels are
While this example only covered one node, you can attach labels to as many nodes as you want. Then when you schedule a pod with a nodeSelector, it can be scheduled on any of the nodes that satisfy that nodeSelector. Be careful that it will match at least one node, however, because if it doesn’t the pod won’t be scheduled at all.