podAffinity and podAntiAffinity's matchLabels, are a little confusing

[Kenneth Massada]

Could someone help clarify difference between empty and null? https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/ says An empty label selector (that is, one with zero requirements) selects every object in the collection. A null label selector (which is only possible for optional selector fields) selects no objects consider this: affinity: podAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchLabels: app: nginx topologyKey: kubernetes.io/hostname podAntiAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchLabels: null topologyKey: kubernetes.io/hostname The current behavior is then schedule pod on all nodes that have a pod app:nginx BUT do not schedule pod on "every" of the nodes. or schedule it none nodes and node that has a pod app:nginx consider affinity: podAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchLabels: app: nginx topologyKey: kubernetes.io/hostname podAntiAffinity: requiredDuringSchedulingIgnoredDuringExecution: - topologyKey: kubernetes.io/hostname The current behavior is then schedule pod on all nodes that have a pod app:nginx BUT do not schedule it on "none" of the nodes. schedule it on all nodes and node that has a pod app:nginx {}/null act as every Empty act as all so according to the YAML construct "{}" is also null.

however, {} this looks more like empty? so is empty omitting?

I guess the use case here is

PodAffinity: app:nginx AND podAntiAffinity: NONE

Kenneth

[David Oppenheimer]

It's not clear to me if you are asking about YAML/API server behavior, or the semantics of label selectors, or the semantics of pod affinity/anti-affinity.

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[Kenneth Massada]

The semantics of label selectors, or the semantics of pod affinity/anti-affinity, they both are very related.

What does "empty" mean in  pod affinity/anti-affinity labelselectors, what does null/{} mean in  pod affinity/anti-affinity, are they the same? 

I can use this to colocate a pod with another. what if the plan was to make sure they were not colocated? 

apiVersion: v1

kind: Pod

metadata:

  name: nginx-pod3

spec:

  affinity:

    podAffinity:

      requiredDuringSchedulingIgnoredDuringExecution:

      - labelSelector:

          matchLabels:

            app: nginx

        topologyKey: kubernetes.io/hostname

    podAntiAffinity:

      requiredDuringSchedulingIgnoredDuringExecution:

      - topologyKey: kubernetes.io/hostname

  containers:

  - name: nginx

    image: nginx:1.7.9

    ports:

    - containerPort: 80

On Friday, April 28, 2017 at 3:44:34 PM UTC-7, David Oppenheimer wrote:

It's not clear to me if you are asking about YAML/API server behavior, or the semantics of label selectors, or the semantics of pod affinity/anti-affinity.

On Fri, Apr 28, 2017 at 3:26 PM, 'Kenneth Massada' via Kubernetes user discussion and Q&A <kubernet...@googlegroups.com> wrote:

Could someone help clarify difference between empty and null? https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/ says An empty label selector (that is, one with zero requirements) selects every object in the collection. A null label selector (which is only possible for optional selector fields) selects no objects consider this: affinity: podAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchLabels: app: nginx topologyKey: kubernetes.io/hostname podAntiAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchLabels: null topologyKey: kubernetes.io/hostname The current behavior is then schedule pod on all nodes that have a pod app:nginx BUT do not schedule pod on "every" of the nodes. or schedule it none nodes and node that has a pod app:nginx consider affinity: podAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchLabels: app: nginx topologyKey: kubernetes.io/hostname podAntiAffinity: requiredDuringSchedulingIgnoredDuringExecution: - topologyKey: kubernetes.io/hostname The current behavior is then schedule pod on all nodes that have a pod app:nginx BUT do not schedule it on "none" of the nodes. schedule it on all nodes and node that has a pod app:nginx {}/null act as every Empty act as all so according to the YAML construct "{}" is also null.

however, {} this looks more like empty? so is empty omitting?

I guess the use case here is

PodAffinity: app:nginx AND podAntiAffinity: NONE

Kenneth

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