Stable identity

Learn the stable name and ordinal of Pods.

Stable identity

A StatefulSet pod's identity consists of three parts and doesn't change even when the pod is recreated: the ordinal (db-0), the network name (db-0.db) and the storage (the data-db-0 PVC). Applications use this identity to determine roles; e.g. "the one with ordinal 0 is the primary":

# start-up script
SIRA=${HOSTNAME##*-}       # db-2 -> 2
if [ "$SIRA" = "0" ]; then
  echo "starting as primary"
else
  echo "replicating from db-0.db"
fi

Verifying the identity

A pod's host name is the same as the pod name. You can query the DNS names with a temporary pod from inside the cluster:

$ kubectl exec db-1 -- hostname
db-1
$ kubectl run -it --rm dns --image=busybox:1.36 --restart=Never -- nslookup db-0.db
Name:      db-0.db.default.svc.cluster.local
Address:   10.244.1.12

Exercise Query the identity

Write two commands: run hostname in db-1; in an interactive pod dns with busybox:1.36 that is deleted on exit and doesn't restart, run nslookup db-0.db.

Show example solution

kubectl exec db-1 -- hostname
kubectl run -it --rm dns --image=busybox:1.36 --restart=Never -- nslookup db-0.db

Test yourself

If the db-2 pod moves to another node, which changes?
- The pod IP address
- The pod name
- The DNS name (db-2.db)
- The attached PVC
Show answer

The pod IP address — The IP changes, but the DNS name is updated to point at the new IP.

Print the pod's host name.

kubectl exec db-1 -- ____

Show answer

kubectl exec db-1 -- hostname

Match.

ordinal

db-0, db-1
network name

db-0.db
storage

data-db-0
pod IP

Can change

Next lesson: VolumeClaimTemplate

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