k8s NodeAffinity

關於 NodeAffinity 的文件 官方文件散落各地, 因此整理一篇來幫助記憶

在講 NodeAffinity 時
通常會認為在說 pod 用的 nodeAffinity

在 k8s 中還存在一個 VolumeNodeAffinity
是給 persistent volume 使用

api field

先看看 api field 能大概了解差異

# VolumeNodeAffinity
.spec.nodeAffinity.required.nodeSelectorTerms.matchExpressions
.spec.nodeAffinity.required.nodeSelectorTerms.matchFields

# NodeAffinity
.spec.affinity.nodeAffinity.preferredDuringSchedulingIgnoredDuringExecution.preference.matchExpressions
.spec.affinity.nodeAffinity.preferredDuringSchedulingIgnoredDuringExecution.preference.matchFields

.spec.affinity.nodeAffinity.requiredDuringSchedulingIgnoredDuringExecution.nodeSelectorTerms.matchExpressions
.spec.affinity.nodeAffinity.requiredDuringSchedulingIgnoredDuringExecution.nodeSelectorTerms.matchFields 

由於 VolumeNodeAffinity 的 field 也是 nodeAffinity 因此要小心別搞錯
另外 pod 的 nodeAffinity 後的 field 名稱非常長, 但基本只要看前面 preferred/equired 就能知道意思
詳細說明

這邊可以看到兩者都是使用 matchExpressions, matchFields
下面來說明差異

matchExpressions/matchFields

matchFields: select object field
matchExpressions: select object label

範例

matchExpressions

- matchExpressions:
  - key: topology.kubernetes.io/zone
    operator: In
    values:
    - antarctica-east1

matchFields

- matchFields:
  - key: metadata.name
    operator: NotIn
    values:
    - worker-1

matchExpressions/matchFields operator

operator 支援以下設定

The following are all the logical operators that you can use in the operator field for nodeAffinity and podAffinity mentioned above.

Operator	Behavior
In	The label value is present in the supplied set of strings
NotIn	The label value is not contained in the supplied set of strings
Exists	A label with this key exists on the object
DoesNotExist	No label with this key exists on the object

The following operators can only be used with nodeAffinity.

Operator	Behavior
Gt	The field value will be parsed as an integer, and that integer is less than the integer that results from parsing the value of a label named by this selector
Lt	The field value will be parsed as an integer, and that integer is greater than the integer that results from parsing the value of a label named by this selector

Gt and Lt operators will not work with non-integer values. If the given value doesn't parse as an integer, the Pod will fail to get scheduled. Also, Gt and Lt are not available for podAffinity.

label-selectors

另外一個同樣在 select label 的 LabelSelector 語法又稍微不同, 大部分用在 deployment/service

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  ...

這裡也能使用 matchExpressions
不過大多使用 matchLabels(map of {key,value})