How to access StatefulSet with other PODs - elasticsearch

Question

I have a running Elasticsearch STS with a headless Service assigned to it:
svc.yaml:
kind: Service
apiVersion: v1
metadata:
name: elasticsearch
namespace: elasticsearch-namespace
labels:
app: elasticsearch
spec:
selector:
app: elasticsearch
clusterIP: None
ports:
- port: 9200
name: rest
- port: 9300
name: inter-node
stateful.yaml:
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: es-cluster
namespace: elasticsearch-namespace
spec:
serviceName: elasticsearch
replicas: 3
selector:
matchLabels:
app: elasticsearch
template:
metadata:
labels:
app: elasticsearch
spec:
containers:
- name: elasticsearch
image: docker.elastic.co/elasticsearch/elasticsearch:7.2.0
resources:
limits:
cpu: 1000m
requests:
cpu: 100m
ports:
- containerPort: 9200
name: rest
protocol: TCP
- containerPort: 9300
name: inter-node
protocol: TCP
volumeMounts:
- name: elasticsearch-persistent-storage
mountPath: /usr/share/elasticsearch/data
env:
- name: cluster.name
value: k8s-logs
- name: node.name
valueFrom:
fieldRef:
fieldPath: metadata.name
- name: discovery.seed_hosts
value: "es-cluster-0.elasticsearch,es-cluster-1.elasticsearch,es-cluster-2.elasticsearch"
- name: cluster.initial_master_nodes
value: "es-cluster-0,es-cluster-1,es-cluster-2"
- name: ES_JAVA_OPTS
value: "-Xms512m -Xmx512m"
initContainers:
- name: fix-permissions
image: busybox
command: ["sh", "-c", "chown -R 1000:1000 /usr/share/elasticsearch/data"]
securityContext:
privileged: true
volumeMounts:
- name: elasticsearch-persistent-storage
mountPath: /usr/share/elasticsearch/data
- name: increase-vm-max-map
image: busybox
command: ["sysctl", "-w", "vm.max_map_count=262144"]
securityContext:
privileged: true
- name: increase-fd-ulimit
image: busybox
command: ["sh", "-c", "ulimit -n 65536"]
securityContext:
privileged: true
volumeClaimTemplates:
- metadata:
name: elasticsearch-persistent-storage
labels:
app: elasticsearch
spec:
accessModes: [ "ReadWriteOnce" ]
storageClassName: elasticsearch-storageclass
resources:
requests:
storage: 20Gi
The question is: how to access this STS with PODs of Deployment Kind? Let's say, using this Redis POD:
apiVersion: apps/v1
kind: Deployment
metadata:
name: redis-ws-app
labels:
app: redis-ws-app
spec:
replicas: 1
selector:
matchLabels:
app: redis-ws-app
template:
metadata:
labels:
app: redis-ws-app
spec:
containers:
- name: redis-ws-app
image: redis:latest
command: [ "redis-server"]
ports:
- containerPort: 6379
I have been trying to create another service, that would enable me to access it from the outside, but without any luck:
kind: Service
apiVersion: v1
metadata:
name: elasticsearch-tcp
namespace: elasticsearch-namespace
labels:
app: elasticsearch
spec:
selector:
app: elasticsearch
ports:
- protocol: TCP
port: 9200
targetPort: 9200

Answer 1

You would reach it directly hitting the headless service. As an example, this StatefulSet and this Service.
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: web
spec:
selector:
matchLabels:
app: nginx
serviceName: "nginx"
replicas: 4
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
ports:
- containerPort: 80
name: web
---
kind: Service
apiVersion: v1
metadata:
name: nginx-headless
spec:
selector:
app: nginx
clusterIP: None
ports:
- port: 80
name: http
I could reach the pods of the statefulset, through the headless service from any pod within the cluster:
/ # curl -I nginx-headless
HTTP/1.1 200 OK
Server: nginx/1.19.0
Date: Tue, 09 Jun 2020 12:36:47 GMT
Content-Type: text/html
Content-Length: 612
Last-Modified: Tue, 26 May 2020 15:00:20 GMT
Connection: keep-alive
ETag: "5ecd2f04-264"
Accept-Ranges: bytes
The singularity of the headless service, is that it doesn't create iptable rules for that service. So, when you query that service, it goes to kube-dns (or CoreDNS), and it returns the backends, rather then the IP address is the service itself. So, if you do nslookup, for example, it will return all the backends (pods) of that service:
/ # nslookup nginx-headless
Name: nginx-headless
Address 1: 10.56.1.44
Address 2: 10.56.1.45
Address 3: 10.56.1.46
Address 4: 10.56.1.47
And it won't have any iptable rules assigned to it:
$ sudo iptables-save | grep -i nginx-headless
$
Unlike a normal service, that would return the IP address of the service itself:
/ # nslookup nginx
Name: nginx
Address 1: 10.60.15.30 nginx.default.svc.cluster.local
And it will have iptable rules assigned to it:
$ sudo iptables-save | grep -i nginx
-A KUBE-SERVICES ! -s 10.56.0.0/14 -d 10.60.15.30/32 -p tcp -m comment --comment "default/nginx: cluster IP" -m tcp --dport 80 -j KUBE-MARK-MASQ
-A KUBE-SERVICES -d 10.60.15.30/32 -p tcp -m comment --comment "default/nginx: cluster IP" -m tcp --dport 80 -j KUBE-SVC-4N57TFCL4MD7ZTDA

Answer 2

User #suren was right about the headless service. In my case, I was just using a wrong reference.
The Kube-DNS naming convention is
service.namespace.svc.cluster-domain.tld
and the default cluster domain is cluster.local
In my case, the in order to reach the pod, one has to use:
curl -I elasticsearch.elasticsearch-namespace