Node Affinity

August 26, 2021 - Thomas Jungbauer ( Lastmod: 2024-05-08 ) - 2 min read

Node Affinity allows to place a pod to a specific group of nodes. For example, it is possible to run a pod only on nodes with a specific CPU or disktype. The disktype was used as an example for the nodeSelector and yes … Node Affinity is conceptually similar to nodeSelector but allows a more granular configuration.

Pod Placement Series

Please check out other ways of pod placements:

Expand me...

Using Node Affinity

Currently two types of affinity settings are known:

requiredDuringSchedulingIgnoreDuringExecuption (short required) - a hard requirement which must be met before a pod can be scheduled
preferredDuringSchedulingIgnoredDuringExecution (short preferred) - a soft requirement the scheduler tries to meet, but does not guarantee it

Both types can be specified. In such case the node must first meet the required rule and then attempt to meet the preferred rule.

Preparing node labels

Remember the prerequisites explained in the . Pod Placement - Introduction. We have 4 compute nodes and an example web application up and running.

Before we start with affinity rules we need to label all nodes. Let’s create 2 zones (east and west) for our compute nodes.

You can skip this, if these labels are still set.

Figure 1. Node Zones

oc label nodes compute-0 compute-1 topology.kubernetes.io/zone=east

oc label nodes compute-2 compute-3 topology.kubernetes.io/zone=west

Configure node affinity rule

Like pod affinity the node affinity is defined on the pod specification:

kind: DeploymentConfig
apiVersion: apps.openshift.io/v1
metadata:
  name: django-psql-example
  namespace: podtesting
[...]
spec:
[...]
  template:
[...]
    spec:
[...]
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
              - matchExpressions:
                  - key: topology.kubernetes.io/zone
                    operator: In
                    values:
                      - west

In this example the pods are started only on nodes of the zone "West". Since the value is an array, multiple zones can be defined letting the web application be executed on West and East for example. With this setup you can control on which node a specific application shall be executed. For example: you have a group of nodes which provide a GPU and your GPU application must be started only on this group of nodes.

Like with pod affinity you can combine required and preferred settings.

What happened to Node Anti-Affinity?

Unlike Pod Anti-Affinity, there is no concept to define a node Anti-Affinity. Instead you can use the NotIn and DoesNotExist operators to achieve this bahaviour.

Cleanup

As cleanup simply remove the affinity specification from the DeploymentConf. The node labels can stay as they are since they do not hurt.

Summary

This concludes the quick overview of the node affinity. Further information can be found at Node Affinity

Using ApplicationSet with Matrix Generator and define individual Namespaces – Apr 17, 2025
During my day-to-day business, I am discussing the following setup with many customers: Configure App-of-Apps. Here I try to explain how I use an ApplicationSet that watches over a folder in Git and automatically adds a new Argo CD Application whenever a new folder is found. This works great, but there is a catch: The ApplicationSet uses the same Namespace default for all Applications. This is not always desired, especially when you have different teams working on different Applications. Recently I was asked by the customer if this can be fixed and if it is possible to define different Namespaces for each Application. The answer is yes, and I would like to show you how to do this.
Introducing AdminNetworkPolicies – Nov 6, 2024
Classic Kubernetes/OpenShift offer a feature called NetworkPolicy that allows users to control the traffic to and from their assigned Namespace. NetworkPolicies are designed to give project owners or tenants the ability to protect their own namespace. Sometimes, however, I worked with customers where the cluster administrators or a dedicated (network) team need to enforce these policies. Since the NetworkPolicy API is namespace-scoped, it is not possible to enforce policies across namespaces. The only solution was to create custom (project) admin and edit roles, and remove the ability of creating, modifying or deleting NetworkPolicy objects. Technically, this is possible and easily done. But shifts the whole network security to cluster administrators. Luckily, this is where AdminNetworkPolicy (ANP) and BaselineAdminNetworkPolicy (BANP) comes into play.
Using Kustomize to post render a Helm Chart – Oct 13, 2024
Lately I came across several issues where a given Helm Chart must be modified after it has been rendered by Argo CD. Argo CD does a helm template to render a Chart. Sometimes, especially when you work with Subcharts or when a specific setting is not yet supported by the Chart, you need to modify it later … you need to post-render the Chart. In this very short article, I would like to demonstrate this on a real-live example I had to do. I would like to inject annotations to a Route objects, so that the certificate can be injected. This is done by the cert-utils operator. For the post-rendering the Argo CD repo pod will be extended with a sidecar container, that is watching for the repos and patches them if required.
Managing Certificates using GitOps approach – Jul 4, 2024
The article SSL Certificate Management for OpenShift on AWS explains how to use the Cert-Manager Operator to request and install a new SSL Certificate. This time, I would like to leverage the GitOps approach using the Helm Chart cert-manager I have prepared to deploy the Operator and order new Certificates. I will use an ACME Letsencrypt issuer with a DNS challenge. My domain is hosted at AWS Route 53. However, any other integration can be easily used.
Update Cluster Version using GitOps approach – Jun 7, 2024
During a GitOps journey at one point, the question arises, how to update a cluster? Nowadays it is very easy to update a cluster using CLI or WebUI, so why bother with GitOps in that case? The reason is simple: Using GitOps you can be sure that all clusters are updated to the correct, required version and the version of each cluster is also managed in Git. All you need is the channel you want to use and the desired cluster version. Optionally, you can define the exact image SHA. This might be required when you are operating in a restricted environment.