How to set hostname for kubernetes pod in statefulset - kubernetes

Question

I am using statefulset and I spin up multiple pods but they are not replica of each other. I want to set the hostname of the pods and passing these hostname as a env variable to all the pods so that they can communicate with each other.
I tried to use hostname under pod spec but hostname is never to set to specified hostname. However, it is set to hostname as podname-0.
# Source: testrep/templates/statefulset.yaml
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: orbiting-butterfly-testrep
labels:
app.kubernetes.io/name: testrep
helm.sh/chart: testrep-0.1.0
app.kubernetes.io/instance: orbiting-butterfly
app.kubernetes.io/managed-by: Tiller
spec:
replicas: 1
selector:
matchLabels:
app.kubernetes.io/name: testrep
app.kubernetes.io/instance: orbiting-butterfly
strategy:
type: Recreate
template:
metadata:
labels:
app.kubernetes.io/name: testrep
app.kubernetes.io/instance: orbiting-butterfly
spec:
nodeSelector:
testol: ad3
hostname: test1
containers:
- name: testrep
image: "test/database:v1"
imagePullPolicy: IfNotPresent
env:
- name: DB_HOSTS
value: test1,test2,test3

Answer 1

As per documentation:
StatefulSet is the workload API object used to manage stateful applications.
Manages the deployment and scaling of a set of Pods, and provides guarantees about the ordering and uniqueness of these Pods.
StatefulSets are valuable for applications that require one or more of the following:
Stable, unique network identifiers.
Stable, persistent storage.
Ordered, graceful deployment and scaling.
Ordered, automated rolling updates.
Statefulset Limitations:
StatefulSets currently require a Headless Service to be responsible for the network identity of the Pods. You are responsible for creating this Service.
Pod Identity
StatefulSet Pods have a unique identity that is comprised of an ordinal, a stable network identity, and stable storage. The identity sticks to the Pod, regardless of which node it’s (re)scheduled on.
For a StatefulSet with N replicas, each Pod in the StatefulSet will be assigned an integer ordinal, from 0 up through N-1, that is unique over the Set.
Stable Network ID
Each Pod in a StatefulSet derives its hostname from the name of the StatefulSet and the ordinal of the Pod. The pattern for the constructed hostname is $(statefulset name)-$(ordinal). The example above will create three Pods named web-0,web-1,web-2. A StatefulSet can use a Headless Service to control the domain of its Pods. The domain managed by this Service takes the form: $(service name).$(namespace).svc.cluster.local, where “cluster.local” is the cluster domain. As each Pod is created, it gets a matching DNS subdomain, taking the form: $(podname).$(governing service domain), where the governing service is defined by the serviceName field on the StatefulSet.
Note:
You are responsible for creating the Headless Service responsible for the network identity of the pods.
So as described by vjdhama Please create your Statefulset with Headless Service.
You can find this example in the docs:
apiVersion: v1
kind: Service
metadata:
name: nginx
labels:
app: nginx
spec:
ports:
- port: 80
name: web
clusterIP: None
selector:
app: nginx
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: web
spec:
selector:
matchLabels:
app: nginx # has to match .spec.template.metadata.labels
serviceName: "nginx" # has to match headless Service metadata.name
replicas: 3 # by default is 1
template:
metadata:
labels:
app: nginx # has to match .spec.selector.matchLabels
spec:
terminationGracePeriodSeconds: 10
containers:
- name: nginx
image: k8s.gcr.io/nginx-slim:0.8
ports:
- containerPort: 80
In this scenario Pod DNS and Pod Hostnames should be respectively:
Pod DNS
web-{0..N-1}.nginx.default.svc.cluster.local
Pod Hostname
web-{0..N-1}
NAME READY STATUS RESTARTS AGE IP
pod/web-0 1/1 Running 0 5m 192.168.148.78
pod/web-1 1/1 Running 0 4m53s 192.168.148.79
pod/web-2 1/1 Running 0 4m51s 192.168.148.80
From the Pod perspective:
root#web-2:# nslookup nginx
Server: 10.96.0.10
Address: 10.96.0.10#53
Name: nginx.default.svc.cluster.local
Address: 192.168.148.80
Name: nginx.default.svc.cluster.local
Address: 192.168.148.78
Name: nginx.default.svc.cluster.local
Address: 192.168.148.79
So you can call each of the respective pods using the Pod DNS, like:
web-0.nginx.default.svc.cluster.local
Update:
Exposing single pod from StatefulSet.
Pod Name Label
When the StatefulSet controller creates a Pod, it adds a label, statefulset.kubernetes.io/pod-name, that is set to the name of the Pod. This label allows you to attach a Service to a specific Pod in the StatefulSet.
https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/#pod-name-label
You can find here tricky way.
Using described above advantages of Statefulset:
The pattern for the constructed hostname is $(statefulset name)-$(ordinal). The example above will create three Pods named web-0,web-1,web-2.
So as an example:
apiVersion: v1
kind: Service
metadata:
name: app-0
spec:
type: LoadBalancer
selector:
statefulset.kubernetes.io/pod-name: web-0
ports:
- protocol: TCP
port: 80
targetPort: 80
Will do this for you.
Hope this help.

Answer 2

Kubernetes has service discovery baked in, so you don't have to do that. You can setup a headless service for your StatefulSet to let other applications talk to it.
The pods that are created with StatefulSets are ordered and setup sequentially. Hence the postfix integer value. You can read more on it here.