If you are not familiar with the oc command, refer to OpenShift - Getting Started with the oc command.

There are a few different ways to run a pod on a specific node.

• Using nodeSelector (this article)
  • ​Nodes are labeled with a key=value (e.g. region=east)
  • If namespace node-selector is an exact match of a nodes key=value label, then the pod can be scheduled run on the node
  • If a pods "nodeSelector" is an exact match of a nodes key=value label, then the pod can be scheduled run on the node
• Using nodeAffinity
  • Nodes are labeled with a key=value (e.g. region=east)
  • If a pods "nodeAffinity" regular expression matches a nodes key=value label, the pod can be scheduled to run on the node
• Using podAffinity
  • Pods are labeled with a key=value (e.g. region=east)
  • If a pods "podAffinity" regular expression matches another pods key=value label, the pod can be scheduled to run on the same node that the other pods is running on
• Using Taint and Toleration
  • Nodes are labeled with a key=value:taint (e.g. region=east:NoSchedule)
  • If a pods tolerations uses "Exists" and the pods tolerations key matches the nodes toleration key, the pod is allowed to run on the node
  • If a pods tolerations uses "Equal" and the pods tolerations key and value is an exact match of the nodes tolerations key and value, the pod is allowed to run on the node

The oc new-app command is used to deploy an application. There are various ways to deploy an app.

• From GitHub (https://github.com)
• From Docker Hub (https://hub.docker.com)
• From an image
• From a build
• From a JSON or YAML file (templates)

The scheduler is responsible for determining which worker node a resource should get created on. For example, when deploying a new application to OpenShift, the scheduler determines which worker node the pod should be created on, typically the worker node with the most available memory and CPU. Check out my article on the default scheduler.

The oc get nodes command will return the list of nodes. Notice there are two worker nodes.

~]# oc get nodes
NAME                  STATUS    ROLES     AGE       VERSION
my-node-infra-4k6z9   Ready     infra     273d      v1.11.0+d4cacc0
my-node-master-0      Ready     master    273d      v1.11.0+d4cacc0
my-node-worker-5n4fj  Ready     worker    273d      v1.11.0+d4cacc0
my-node-worker-v8r9r  Ready     worker    273d      v1.11.0+d4cacc0

Let's say you want to be able to specify the worker node that an application / container / deployment / pod will run. To accomplish this, you will need to label the worker nodes. The oc label node command can be used to apply one or more labels to a worker node. The worker node doesn't have to be labeled "region". You can pick any key value pair you would like.

~]$ oc label node my-node-worker-5n4fj region=east
node/my-node-worker-5n4fj labeled
~]$ oc label node my-node-worker-v8r9r region=west
node/my-node-worker-v8r9r labeled

The oc describe node command can be used to see the labels that have been applied to the worker nodes.

~]$ oc describe node my-node-worker-5n4fj
Name:               my-node-worker-5n4fj
Roles:              infra,worker
Labels:             region=east
~]$ oc describe node my-node-worker-v8r9r
Name:               my-node-worker-v8r9r
Roles:              infra,worker
Labels:             region=west

namespace node selector

namespaces can be configured with a node selector so that all pods in the name space are scheduled to run on a nodes that contain a certain label. The oc patch command can be used to configure a namespace with the node-selector annotation so that all pods in the namespace are scheduled to run on a node that have a matching label. In this example, namespace my-project is configured with the node-selector region=east annotation so that all pods in my-project are scheduled to run on a node that has label region: east.

oc patch namespace my-project --patch '{"metadata":{"annotations":{"scheduler.alpha.kubernetes.io/node-selector": "region=east"}}}'

The oc get namespace command should now show that the namespaces has the annotation.

~]$ oc get namespace my-project --output yaml
metadata:
  annotations:
    scheduler.alpha.kubernetes.io/node-selector: region=east

deployment node selector

deployments can be configured with a node selector so that pods managed by the deployment are scheduled to run on a nodes that contain a certain label. For example, let's say there is a node labeled region=east.

~]$ oc describe node worker-c4v5h
Name:               worker-c4v5h
Roles:              infra,worker
Labels:             region=east

Or in the OpenShift console, at Compute > Nodes you can view a nodes labels.

If the deployment YAML has nodeSelector region: east all pods managed by the deployment will be scheduled to run on a node labeled region: east.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  template:
    spec:
      nodeSelector:
        region: east

Or the oc patch command can be used.

~]$ oc patch deployment my-deployment --patch '{"spec":{"template":{"spec":{"nodeSelector":{"region":"east"}}}}}'
deployment.apps/my-deployment patched

Or in the OpenShift console, at Workloads > Deployments > your deployment > YAML.

Likewise, in the OpenShift console, at Workloads > Deployments > your deployment > Details, you can see if the deployment is configured with a node selector.