1. Overview

Stateless applications focus more on groups, and any member can be replaced. Stateless applications focus on individuals. The nginx and myapp managed by deployment controller belong to stateless applications, such as mysql, redis, zookeeper and so on. They also have master-slave and sequential.

The stateful set controller can manage stateful applications, but it is also very troublesome to implement. It needs to script the operation and maintenance management process and inject it into stateful set to use. Although some people have done stateful script well on the Internet, it is recommended that we not easily redis, mysql and other stateful applications. Use migration to k8s.

In k8s, statefulset management has the following special effects:

(a) Each Pod is stable and has a unique network identifier;
b) Stable and durable storage devices;
c) Require orderly and smooth deployment and expansion;
d) Require orderly and smooth termination and deletion;
E. Orderly rolling updates should first update the slave node and then update the master node.

statefulset consists of three components:

Headless service (headless service, i.e. no name);
B) statefulset controller;
Volume ClaimTemplate

2. Create StatefulSet Controller

kubectl explain sts

cat stateful-demo.yaml 
apiVersion: v1
kind: Service
metadata:
  name: myapp-svc
  labels:
    app: myapp-svc
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: myapp-pod
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: myapp
spec:
  serviceName: myapp-svc
  replicas: 2
  selector:
    matchLabels:
      app: myapp-pod
  template:
    metadata:
      labels:
        app: myapp-pod
    spec:
      containers:
      - name: myapp
        image: ikubernetes/myapp:v1
        ports:
        - containerPort: 80
          name: web
        volumeMounts:
        - name: myappdata
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
  - metadata:
      name: myappdata
    spec:
      accessModes: ["ReadWriteOnce"]
      #storageClassName: "gluster-dynamic"
      resources:
        requests:
          storage: 5Gi

Volume ClaimTemplates: Store volume application template, define volume for each pod; automatically create pvc for the namespace where the pod resides.

kubectl apply -f stateful-demo.yaml
kubectl get pods
NAME                             READY     STATUS             RESTARTS   AGE
myapp-0                          1/1       Running            0          4m
myapp-1                          1/1       Running            0          4m

# pod and service will be deleted, but pvc will not be deleted, so it can be restored.
kubectl delete -f stateful-demo.yaml
 
# Format for parsing pod: pod name. SVC name. namespace name. svc.cluster.local
kubectl exec -it myapp-0 -- /bin/sh
/ # nslookup myapp-0.myapp-svc.default.svc.cluster.local
nslookup: can't resolve '(null)': Name does not resolve
Name:      myapp-0.myapp-svc.default.svc.cluster.local
Address 1: 10.244.1.110 myapp-0.myapp-svc.default.svc.cluster.local

# Five pod s
kubectl scale sts myapp --replicas=5
# patch patching can also be used to expand and shrink
kubectl patch sts myapp -p '{"spec":{"replicas":2}}'
# Update strategy
kubectl explain sts.spec.updateStrategy.rollingUpdate
//Suppose there are four pods (pod0, pod1, pod2, pod3). If the partition is set to 5, then it means that the four pods are not updated if the partition is greater than or equal to 5.
//If partition is 4, it means that pod updates greater than or equal to 4, that is, POD 3 updates, and no other pod updates.
//If partiton is 3, then the pod updates greater than or equal to 3 are POD 2 and POD 3 updates, and no other pod updates.
kubectl patch sts myapp -p '{"spec":{"updateStrategy":{"rollingUpdate":{"partition":4}}}}'
kubectl describe sts myapp
Update Strategy:    RollingUpdate
Partition:        4
# Upgrade myapp to v2 version
kubectl set image sts/myapp myapp=ikubernetes/myapp:v2
kubectl get pods myapp-4 -o yaml
 containerStatuses:
    image: ikubernetes/myapp:v2

You can refer to the stateful applications done by others on github: GitHub k8s stateful Set redis | MySQL

Reference Blog: http://blog.itpub.net/28916011/viewspace-2215046/