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Detailed explanations of Kubernetes system concepts and abstractions.

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Assigning Pods to Nodes

You can constrain a pod to only be able to run on particular nodes or to prefer to run on particular nodes. There are several ways to do this, and they all use label selectors to make the selection. Generally such constraints are unnecessary, as the scheduler will automatically do a reasonable placement (e.g. spread your pods across nodes, not place the pod on a node with insufficient free resources, etc.) but there are some circumstances where you may want more control on a node where a pod lands, e.g. to ensure that a pod ends up on a machine with an SSD attached to it, or to co-locate pods from two different services that communicate a lot into the same availability zone.

You can find all the files for these examples in our docs repo here.

nodeSelector

nodeSelector is the simplest form of constraint. nodeSelector is a field of PodSpec. It specifies a map of key-value pairs. For the pod to be eligible to run on a node, the node must have each of the indicated key-value pairs as labels (it can have additional labels as well). The most common usage is one key-value pair.

Let’s walk through an example of how to use nodeSelector.

Step Zero: Prerequisites

This example assumes that you have a basic understanding of Kubernetes pods and that you have turned up a Kubernetes cluster.

Step One: Attach label to the node

Run kubectl get nodes to get the names of your cluster’s nodes. Pick out the one that you want to add a label to, and then run kubectl label nodes <node-name> <label-key>=<label-value> to add a label to the node you’ve chosen. 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.

You can verify that it worked by re-running kubectl get nodes --show-labels and checking that the node now has a label.

Step Two: Add a nodeSelector field to your pod configuration

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:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    env: test
spec:
  containers:
  - name: nginx
    image: nginx

Then add a nodeSelector like so:

pod.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    env: test
spec:
  containers:
  - name: nginx
    image: nginx
    imagePullPolicy: IfNotPresent
  nodeSelector:
    disktype: ssd

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.

Interlude: built-in node labels

In addition to labels you attach, nodes come pre-populated with a standard set of labels. As of Kubernetes v1.4 these labels are

Affinity and anti-affinity

nodeSelector provides a very simple way to constrain pods to nodes with particular labels. The affinity/anti-affinity feature, currently in beta, greatly expands the types of constraints you can express. The key enhancements are

  1. the language is more expressive (not just “AND of exact match”)
  2. you can indicate that the rule is “soft”/”preference” rather than a hard requirement, so if the scheduler can’t satisfy it, the pod will still be scheduled
  3. you can constrain against labels on other pods running on the node (or other topological domain), rather than against labels on the node itself, which allows rules about which pods can and cannot be co-located

The affinity feature consists of two types of affinity, “node affinity” and “inter-pod affinity/anti-affinity.” Node affinity is like the existing nodeSelector (but with the first two benefits listed above), while inter-pod affinity/anti-affinity constrains against pod labels rather than node labels, as described in the third item listed above, in addition to having the first and second properties listed above.

nodeSelector continues to work as usual, but will eventually be deprecated, as node affinity can express everything that nodeSelector can express.

Node affinity (beta feature)

Node affinity was introduced as alpha in Kubernetes 1.2. Node affinity is conceptually similar to nodeSelector – it allows you to constrain which nodes your pod is eligible to schedule on, based on labels on the node.

There are currently two types of node affinity, called requiredDuringSchedulingIgnoredDuringExecution and 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 to how nodeSelector works, if labels on a node change at runtime such that the affinity 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 requiredDuringSchedulingIgnoredDuringExecution except that it will evict pods from nodes that cease to satisfy the pods’ node affinity requirements.

Thus an example of requiredDuringSchedulingIgnoredDuringExecution would be “only run the pod on nodes with Intel CPUs” and an example preferredDuringSchedulingIgnoredDuringExecution would be “try to run this set of pods in availability zone XYZ, but if it’s not possible, then allow some to run elsewhere”.

Node affinity is specified as field nodeAffinity of field affinity in the PodSpec.

Here’s an example of a pod that uses node affinity:

pod-with-node-affinity.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: with-node-affinity
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: kubernetes.io/e2e-az-name
            operator: In
            values:
            - e2e-az1
            - e2e-az2
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 1
        preference:
          matchExpressions:
          - key: another-node-label-key
            operator: In
            values:
            - another-node-label-value
  containers:
  - name: with-node-affinity
    image: k8s.gcr.io/pause:2.0

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-az1 or e2e-az2. In addition, among nodes that meet that criteria, nodes with a label whose key is another-node-label-key and whose value is another-node-label-value should be preferred.

You can see the operator In being used in the example. The new node affinity syntax supports the following operators: In, NotIn, Exists, DoesNotExist, Gt, Lt. There is no explicit “node anti-affinity” concept, but NotIn and DoesNotExist give that behavior.

If you specify both nodeSelector and nodeAffinity, both must be satisfied for the pod to be scheduled onto a candidate node.

If you specify multiple nodeSelectorTerms associated with nodeAffinity types, then the pod can be scheduled onto a node if one of the nodeSelectorTerms is satisfied.

If you specify multiple matchExpressions associated with nodeSelectorTerms, then the pod can be scheduled onto a node only if all matchExpressions can be satisfied.

If you remove or change the label of the node where the pod is scheduled, the pod won’t be removed. In other words, the affinity selection works only at the time of scheduling the pod.

For more information on node affinity, see the design doc here.

Inter-pod affinity and anti-affinity (beta feature)

Inter-pod affinity and anti-affinity were introduced in Kubernetes 1.4. Inter-pod affinity and anti-affinity allow you to constrain which nodes your pod is eligible to be scheduled based on labels on pods that are already running on the node rather than based on labels on nodes. The rules are of the form “this pod should (or, in the case of anti-affinity, should not) run in an X if that X is already running one or more pods that meet rule Y.” Y is expressed as a LabelSelector with an associated list of namespaces (or “all” namespaces); unlike nodes, because pods are namespaced (and therefore the labels on pods are implicitly namespaced), a label selector over pod labels must specify which namespaces the selector should apply to. Conceptually X is a topology domain like node, rack, cloud provider zone, cloud provider region, etc. You express it using a topologyKey which is the key for the node label that the system uses to denote such a topology domain, e.g. see the label keys listed above in the section Interlude: built-in node labels.

Note: Inter-pod affinity and anti-affinity require substantial amount of processing which can slow down scheduling in large clusters significantly. We do not recommend using them in clusters larger than several hundred nodes.

As with node affinity, there are currently two types of pod affinity and anti-affinity, called requiredDuringSchedulingIgnoredDuringExecution and preferredDuringSchedulingIgnoredDuringExecution which denote “hard” vs. “soft” requirements. See the description in the node affinity section earlier. An example of requiredDuringSchedulingIgnoredDuringExecution affinity would be “co-locate the pods of service A and service B in the same zone, since they communicate a lot with each other” and an example preferredDuringSchedulingIgnoredDuringExecution anti-affinity would be “spread the pods from this service across zones” (a hard requirement wouldn’t make sense, since you probably have more pods than zones).

Inter-pod affinity is specified as field podAffinity of field affinity in the PodSpec. And inter-pod anti-affinity is specified as field podAntiAffinity of field affinity in the PodSpec.

An example of a pod that uses pod affinity:

pod-with-pod-affinity.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: with-pod-affinity
spec:
  affinity:
    podAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
          matchExpressions:
          - key: security
            operator: In
            values:
            - S1
        topologyKey: failure-domain.beta.kubernetes.io/zone
    podAntiAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 100
        podAffinityTerm:
          labelSelector:
            matchExpressions:
            - key: security
              operator: In
              values:
              - S2
          topologyKey: kubernetes.io/hostname
  containers:
  - name: with-pod-affinity
    image: k8s.gcr.io/pause:2.0

The affinity on this pod defines one pod affinity rule and one pod anti-affinity rule. In this example, the podAffinity is requiredDuringSchedulingIgnoredDuringExecution while the podAntiAffinity is preferredDuringSchedulingIgnoredDuringExecution. The pod affinity rule says that the pod can schedule onto a node only if that node is in the same zone as at least one already-running pod that has a label with key “security” and value “S1”. (More precisely, the pod is eligible to run on node N if node N has a label with key failure-domain.beta.kubernetes.io/zone and some value V such that there is at least one node in the cluster with key failure-domain.beta.kubernetes.io/zone and value V that is running a pod that has a label with key “security” and value “S1”.) The pod anti-affinity rule says that the pod prefers to not schedule onto a node if that node is already running a pod with label having key “security” and value “S2”. (If the topologyKey were failure-domain.beta.kubernetes.io/zone then it would mean that the pod cannot schedule onto a node if that node is in the same zone as a pod with label having key “security” and value “S2”.) See the design doc. for many more examples of pod affinity and anti-affinity, both the requiredDuringSchedulingIgnoredDuringExecution flavor and the preferredDuringSchedulingIgnoredDuringExecution flavor.

The legal operators for pod affinity and anti-affinity are In, NotIn, Exists, DoesNotExist.

In principle, the topologyKey can be any legal label-key. However, for performance and security reasons, there are some constraints on topologyKey:

  1. For affinity and for RequiredDuringScheduling pod anti-affinity, empty topologyKey is not allowed.
  2. For RequiredDuringScheduling pod anti-affinity, the admission controller LimitPodHardAntiAffinityTopology was introduced to limit topologyKey to kubernetes.io/hostname. If you want to make it available for custom topologies, you may modify the admission controller, or simply disable it.
  3. For PreferredDuringScheduling pod anti-affinity, empty topologyKey is interpreted as “all topologies” (“all topologies” here is now limited to the combination of kubernetes.io/hostname, failure-domain.beta.kubernetes.io/zone and failure-domain.beta.kubernetes.io/region).
  4. Except for the above cases, the topologyKey can be any legal label-key.

In addition to labelSelector and topologyKey, you can optionally specify a list namespaces of namespaces which the labelSelector should match against (this goes at the same level of the definition as labelSelector and topologyKey). If omitted, it defaults to the namespace of the pod where the affinity/anti-affinity definition appears. If defined but empty, it means “all namespaces.”

All matchExpressions associated with requiredDuringSchedulingIgnoredDuringExecution affinity and anti-affinity must be satisfied for the pod to schedule onto a node.

More Practical Use-cases

Interpod Affinity and AntiAffinity can be even more useful when they are used with higher level collections such as ReplicaSets, Statefulsets, Deployments, etc. One can easily configure that a set of workloads should be co-located in the same defined topology, eg., the same node.

Always co-located in the same node

In a three node cluster, a web application has in-memory cache such as redis. We want the web-servers to be co-located with the cache as much as possible. Here is the yaml snippet of a simple redis deployment with three replicas and selector label app=store. The deployment has PodAntiAffinity configured to ensure the scheduler does not co-locate replicas on a single node.

apiVersion: apps/v1beta1 # for versions before 1.6.0 use extensions/v1beta1
kind: Deployment
metadata:
  name: redis-cache
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: store
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: app
                operator: In
                values:
                - store
            topologyKey: "kubernetes.io/hostname"
      containers:
      - name: redis-server
        image: redis:3.2-alpine

The below yaml snippet of the webserver deployment has podAntiAffinity and podAffinity configured. This informs the scheduler that all its replicas are to be co-located with pods that have selector label app=store. This will also ensure that each web-server replica does not co-locate on a single node.

apiVersion: apps/v1beta1 # for versions before 1.6.0 use extensions/v1beta1
kind: Deployment
metadata:
  name: web-server
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: web-store
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: app
                operator: In
                values:
                - web-store
            topologyKey: "kubernetes.io/hostname"
        podAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: app
                operator: In
                values:
                - store
            topologyKey: "kubernetes.io/hostname"
      containers:
      - name: web-app
        image: nginx:1.12-alpine

If we create the above two deployments, our three node cluster should look like below.

node-1 node-2 node-3
webserver-1 webserver-2 webserver-3
cache-1 cache-2 cache-3

As you can see, all the 3 replicas of the web-server are automatically co-located with the cache as expected.

$ kubectl get pods -o wide
NAME                           READY     STATUS    RESTARTS   AGE       IP           NODE
redis-cache-1450370735-6dzlj   1/1       Running   0          8m        10.192.4.2   kube-node-3
redis-cache-1450370735-j2j96   1/1       Running   0          8m        10.192.2.2   kube-node-1
redis-cache-1450370735-z73mh   1/1       Running   0          8m        10.192.3.1   kube-node-2
web-server-1287567482-5d4dz    1/1       Running   0          7m        10.192.2.3   kube-node-1
web-server-1287567482-6f7v5    1/1       Running   0          7m        10.192.4.3   kube-node-3
web-server-1287567482-s330j    1/1       Running   0          7m        10.192.3.2   kube-node-2

Best practice is to configure these highly available stateful workloads such as redis with AntiAffinity rules for more guaranteed spreading.

Never co-located in the same node

Highly Available database statefulset has one master and three replicas, one may prefer none of the database instances to be co-located in the same node.

node-1 node-2 node-3 node-4
DB-MASTER DB-REPLICA-1 DB-REPLICA-2 DB-REPLICA-3

Here is an example of Zookeeper statefulset configured with anti-affinity for high availability.

For more information on inter-pod affinity/anti-affinity, see the design doc here.

You may want to check Taints as well, which allow a node to repel a set of pods.

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