331 lines
9.1 KiB
Markdown
331 lines
9.1 KiB
Markdown
# StatefulSet
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StatefulSet是为了解决有状态服务的问题（对应Deployments和ReplicaSets是为无状态服务而设计），其应用场景包括：
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- 稳定的持久化存储，即Pod重新调度后还是能访问到相同的持久化数据，基于PVC来实现
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- 稳定的网络标志，即Pod重新调度后其PodName和HostName不变，基于Headless Service（即没有Cluster IP的Service）来实现
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- 有序部署，有序扩展，即Pod是有顺序的，在部署或者扩展的时候要依据定义的顺序依次依次进行（即从0到N-1，在下一个Pod运行之前所有之前的Pod必须都是Running和Ready状态），基于init containers来实现
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- 有序收缩，有序删除（即从N-1到0）
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从上面的应用场景可以发现，StatefulSet由以下几个部分组成：
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- 用于定义网络标志（DNS domain）的Headless Service
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- 用于创建PersistentVolumes的volumeClaimTemplates
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- 定义具体应用的StatefulSet
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StatefulSet中每个Pod的DNS格式为`statefulSetName-{0..N-1}.serviceName.namespace.svc.cluster.local`，其中
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- `serviceName`为Headless Service的名字
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- `0..N-1`为Pod所在的序号，从0开始到N-1
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- `statefulSetName`为StatefulSet的名字
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- `namespace`为服务所在的namespace，Headless Servic和StatefulSet必须在相同的namespace
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- `.cluster.local`为Cluster Domain，
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## 简单示例
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以一个简单的nginx服务[web.yaml](../manifests/test/web.yaml)为例：
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```yaml
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---
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apiVersion: v1
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kind: Service
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metadata:
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name: nginx
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labels:
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app: nginx
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spec:
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ports:
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- port: 80
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name: web
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clusterIP: None
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selector:
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app: nginx
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---
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apiVersion: apps/v1beta1
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kind: StatefulSet
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metadata:
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name: web
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spec:
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serviceName: "nginx"
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replicas: 2
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template:
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metadata:
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labels:
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app: nginx
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spec:
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containers:
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- name: nginx
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image: gcr.io/google_containers/nginx-slim:0.8
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ports:
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- containerPort: 80
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name: web
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volumeMounts:
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- name: www
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mountPath: /usr/share/nginx/html
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volumeClaimTemplates:
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- metadata:
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name: www
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annotations:
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volume.alpha.kubernetes.io/storage-class: anything
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spec:
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accessModes: [ "ReadWriteOnce" ]
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resources:
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requests:
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storage: 1Gi
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```
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```sh
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$ kubectl create -f web.yaml
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service "nginx" created
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statefulset "web" created
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# 查看创建的headless service和statefulset
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$ kubectl get service nginx
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NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
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nginx None <none> 80/TCP 1m
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$ kubectl get statefulset web
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NAME DESIRED CURRENT AGE
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web 2 2 2m
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# 根据volumeClaimTemplates自动创建PVC（在GCE中会自动创建kubernetes.io/gce-pd类型的volume）
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$ kubectl get pvc
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NAME STATUS VOLUME CAPACITY ACCESSMODES AGE
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www-web-0 Bound pvc-d064a004-d8d4-11e6-b521-42010a800002 1Gi RWO 16s
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www-web-1 Bound pvc-d06a3946-d8d4-11e6-b521-42010a800002 1Gi RWO 16s
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# 查看创建的Pod，他们都是有序的
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$ kubectl get pods -l app=nginx
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NAME READY STATUS RESTARTS AGE
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web-0 1/1 Running 0 5m
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web-1 1/1 Running 0 4m
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# 使用nslookup查看这些Pod的DNS
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$ kubectl run -i --tty --image busybox dns-test --restart=Never --rm /bin/sh
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/ # nslookup web-0.nginx
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Server: 10.0.0.10
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Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
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Name: web-0.nginx
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Address 1: 10.244.2.10
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/ # nslookup web-1.nginx
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Server: 10.0.0.10
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Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
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Name: web-1.nginx
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Address 1: 10.244.3.12
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/ # nslookup web-0.nginx.default.svc.cluster.local
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Server: 10.0.0.10
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Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
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Name: web-0.nginx.default.svc.cluster.local
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Address 1: 10.244.2.10
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```
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还可以进行其他的操作
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```sh
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# 扩容
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$ kubectl scale statefulset web --replicas=5
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# 缩容
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$ kubectl patch statefulset web -p '{"spec":{"replicas":3}}'
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# 镜像更新（目前还不支持直接更新image，需要patch来间接实现）
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$ kubectl patch statefulset web --type='json' -p='[{"op": "replace", "path": "/spec/template/spec/containers/0/image", "value":"gcr.io/google_containers/nginx-slim:0.7"}]'
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# 删除StatefulSet和Headless Service
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$ kubectl delete statefulset web
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$ kubectl delete service nginx
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# StatefulSet删除后PVC还会保留着，数据不再使用的话也需要删除
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$ kubectl delete pvc www-web-0 www-web-1
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```
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## zookeeper
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另外一个更能说明StatefulSet强大功能的示例为[zookeeper.yaml](../manifests/test/zookeeper.yaml)。
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```yaml
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---
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apiVersion: v1
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kind: Service
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metadata:
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name: zk-headless
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labels:
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app: zk-headless
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spec:
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ports:
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- port: 2888
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name: server
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- port: 3888
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name: leader-election
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clusterIP: None
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selector:
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app: zk
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---
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apiVersion: v1
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kind: ConfigMap
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metadata:
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name: zk-config
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data:
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ensemble: "zk-0;zk-1;zk-2"
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jvm.heap: "2G"
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tick: "2000"
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init: "10"
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sync: "5"
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client.cnxns: "60"
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snap.retain: "3"
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purge.interval: "1"
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---
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apiVersion: policy/v1beta1
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kind: PodDisruptionBudget
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metadata:
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name: zk-budget
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spec:
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selector:
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matchLabels:
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app: zk
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minAvailable: 2
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---
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apiVersion: apps/v1beta1
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kind: StatefulSet
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metadata:
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name: zk
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spec:
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serviceName: zk-headless
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replicas: 3
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template:
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metadata:
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labels:
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app: zk
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annotations:
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pod.alpha.kubernetes.io/initialized: "true"
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scheduler.alpha.kubernetes.io/affinity: >
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{
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"podAntiAffinity": {
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"requiredDuringSchedulingRequiredDuringExecution": [{
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"labelSelector": {
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"matchExpressions": [{
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"key": "app",
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"operator": "In",
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"values": ["zk-headless"]
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}]
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},
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"topologyKey": "kubernetes.io/hostname"
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}]
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}
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}
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spec:
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containers:
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- name: k8szk
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imagePullPolicy: Always
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image: gcr.io/google_samples/k8szk:v1
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resources:
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requests:
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memory: "4Gi"
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cpu: "1"
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ports:
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- containerPort: 2181
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name: client
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- containerPort: 2888
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name: server
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- containerPort: 3888
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name: leader-election
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env:
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- name : ZK_ENSEMBLE
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valueFrom:
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configMapKeyRef:
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name: zk-config
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key: ensemble
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- name : ZK_HEAP_SIZE
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valueFrom:
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configMapKeyRef:
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name: zk-config
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key: jvm.heap
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- name : ZK_TICK_TIME
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valueFrom:
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configMapKeyRef:
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name: zk-config
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key: tick
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- name : ZK_INIT_LIMIT
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valueFrom:
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configMapKeyRef:
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name: zk-config
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key: init
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- name : ZK_SYNC_LIMIT
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valueFrom:
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configMapKeyRef:
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name: zk-config
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key: tick
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- name : ZK_MAX_CLIENT_CNXNS
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valueFrom:
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configMapKeyRef:
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name: zk-config
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key: client.cnxns
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- name: ZK_SNAP_RETAIN_COUNT
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valueFrom:
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configMapKeyRef:
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name: zk-config
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key: snap.retain
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- name: ZK_PURGE_INTERVAL
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valueFrom:
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configMapKeyRef:
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name: zk-config
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key: purge.interval
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- name: ZK_CLIENT_PORT
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value: "2181"
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- name: ZK_SERVER_PORT
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value: "2888"
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- name: ZK_ELECTION_PORT
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value: "3888"
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command:
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- sh
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- -c
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- zkGenConfig.sh && zkServer.sh start-foreground
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readinessProbe:
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exec:
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command:
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- "zkOk.sh"
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initialDelaySeconds: 15
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timeoutSeconds: 5
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livenessProbe:
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exec:
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command:
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- "zkOk.sh"
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initialDelaySeconds: 15
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timeoutSeconds: 5
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volumeMounts:
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- name: datadir
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mountPath: /var/lib/zookeeper
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securityContext:
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runAsUser: 1000
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fsGroup: 1000
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volumeClaimTemplates:
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- metadata:
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name: datadir
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annotations:
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volume.alpha.kubernetes.io/storage-class: anything
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spec:
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accessModes: [ "ReadWriteOnce" ]
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resources:
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requests:
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storage: 20Gi
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```
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```sh
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kubectl create -f zookeeper.yaml
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```
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详细的使用说明见[zookeeper stateful application](https://kubernetes.io/docs/tutorials/stateful-application/zookeeper/)。
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## StatefulSet注意事项
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1. 还在beta状态，需要kubernetes v1.5版本以上才支持
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2. 所有Pod的Volume必须使用PersistentVolume或者是管理员事先创建好
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3. 为了保证数据安全，删除StatefulSet时不会删除Volume
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4. StatefulSet需要一个Headless Service来定义DNS domain，需要在StatefulSet之前创建好
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5. 目前StatefulSet还没有feature complete，比如更新操作还需要手动patch。
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更多可以参考[Kubernetes文档](https://kubernetes.io/docs/concepts/abstractions/controllers/statefulsets/)。
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