container.training/slides/k8s/labels-annotations.md

Labels and annotations

• Most Kubernetes resources can have labels and annotations

• Both labels and annotations are arbitrary strings

  (with some limitations that we'll explain in a minute)

• Both labels and annotations can be added, removed, changed, dynamically

• This can be done with:

  • the kubectl edit command

  • the kubectl label and kubectl annotate

  • ... many other ways! (kubectl apply -f, kubectl patch, ...)

---

Viewing labels and annotations

• Let's see what we get when we create a Deployment

.lab[

• Create a Deployment:

  kubectl create deployment clock --image=jpetazzo/clock
  

• Look at its annotations and labels:

  kubectl describe deployment clock
  

]

So, what do we get?

---

Labels and annotations for our Deployment

• We see one label:

  Labels: app=clock
  

• This is added by kubectl create deployment

• And one annotation:

  Annotations: deployment.kubernetes.io/revision: 1
  

• This is to keep track of successive versions when doing rolling updates

---

And for the related Pod?

• Let's look up the Pod that was created and check it too

.lab[

• Find the name of the Pod:

  kubectl get pods
  

• Display its information:

  kubectl describe pod clock-xxxxxxxxxx-yyyyy
  

]

So, what do we get?

---

Labels and annotations for our Pod

• We see two labels:

    Labels: app=clock
            pod-template-hash=xxxxxxxxxx
  

• app=clock comes from kubectl create deployment too

• pod-template-hash was assigned by the Replica Set

  (when we will do rolling updates, each set of Pods will have a different hash)

• There are no annotations:

  Annotations: <none>
  

---

Selectors

• A selector is an expression matching labels

• It will restrict a command to the objects matching at least all these labels

.lab[

• List all the pods with at least app=clock:

  kubectl get pods --selector=app=clock
  

• List all the pods with a label app, regardless of its value:

  kubectl get pods --selector=app
  

]

---

Settings labels and annotations

• The easiest method is to use kubectl label and kubectl annotate

.lab[

• Set a label on the clock Deployment:

  kubectl label deployment clock color=blue
  

• Check it out:

  kubectl describe deployment clock
  

]

---

Other ways to view labels

• kubectl get gives us a couple of useful flags to check labels

• kubectl get --show-labels shows all labels

• kubectl get -L xyz shows the value of label xyz

.lab[

• List all the labels that we have on pods:

  kubectl get pods --show-labels
  

• List the value of label app on these pods:

  kubectl get pods -L app
  

]

---

class: extra-details

More on selectors

• If a selector has multiple labels, it means "match at least these labels"

  Example: --selector=app=frontend,release=prod

• --selector can be abbreviated as -l (for labels)

  We can also use negative selectors

  Example: --selector=app!=clock

• Selectors can be used with most kubectl commands

  Examples: kubectl delete, kubectl label, ...

---

Other ways to view labels

• We can use the --show-labels flag with kubectl get

.lab[

• Show labels for a bunch of objects:

  kubectl get --show-labels po,rs,deploy,svc,no
  

]

---

Differences between labels and annotations

• The key for both labels and annotations:

  • must start and end with a letter or digit

  • can also have . - _ (but not in first or last position)

  • can be up to 63 characters, or 253 + / + 63

• Label values are up to 63 characters, with the same restrictions

• Annotations values can have arbitrary characters (yes, even binary)

• Maximum length isn't defined

  (dozens of kilobytes is fine, hundreds maybe not so much)

???

:EN:- Labels and annotations :FR:- Labels et annotations