Labels and Selectors

(this post is part of the material I cover in my devops course)

Motivation

-Service deployments and batch processing pipelines are often multi-dimensional entities.

• For example, you may have multiple micro-services per tier.

Labels

• Labels are key/value pairs that are attached to objects, such as pods or deployments
• Labels can be used to organize and to select subsets of objects.
  They are said to be "identifying", so you can base queries on them.
• Non-identifying information should be recorded using annotations.
• Labels can be attached to objects at creation time or later and can be changed
• Each object can have a set of key/value labels defined
• Each Key must be unique for a given object.
• More info here
• exanple:

1"metadata": {
2  "labels": {
3    "key1" : "value1",
4    "key2" : "value2"
5  }
6}

Label syntax

• Labels are key/value pairs.
• Valid label keys have two segments:
  • an optional prefix
  • a name separated by a slash (/) Example:

1app.example.com/name=my-app

• Read more about label syntax here

Lable Selectors

• Labels do not provide uniqueness.
• Via a label selector, the client/user can identify a set of objects.
• The label selector is the core grouping primitive in Kubernetes.
• The API currently supports two types of selectors:
  • equality-based
  • set-based
• equality based example:

 1List only pods from app b:
 2    kubectl get pods --selector=app.k8s.io/name==appB
 3also with one equation mark:
 4    kubectl get pods --selector=app.k8s.io/name=appB
 5List only pods NOT from app b:
 6    kubectl get pods --selector=app.k8s.io/name!=appB
 7The , (comma operator) acts as a logical AND operation.
 8There is NO logical OR opration.
 9So:
10    kubectl get pods --selector=app.k8s.io/name==appB,app.k8s.io/release==3

• set based example:

1Select all pods that are in the (appB, appC) set:
2    kubectl get pods --selector="app.k8s.io/name in (appB, appC)"
3Select all pods that are NOT in the (appB, appC) set:
4    kubectl get pods --selector="app.k8s.io/name notin (appB, appC)"
5Select all of these pods that HAVE the single "nice" label:
6    kubectl get pods --selector=nice
7Select all of these pods that DO NOT have the label "nice:
8    kubectl get pods --selector='!nice'
9    (notice we have used a bash-single-quotation mark)