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StatefulSet
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
- 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
- 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
- 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序进行（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
- 有序收缩，有序删除（即从N-1到0）
从上面的应用场景可以发现，StatefulSet由以下几个部分组成：
- 用于定义网络标志（DNS domain）的Headless Service
- 用于创建PersistentVolumes的volumeClaimTemplates
- 定义具体应用的StatefulSet
StatefulSet中每个Pod的DNS格式为statefulSetName-{0..N-1}.serviceName.namespace.svc.cluster.local，其中
serviceName为Headless Service的名字0..N-1为Pod所在的序号，从0开始到N-1statefulSetName为StatefulSet的名字namespace为服务所在的namespace，Headless Service和StatefulSet必须在相同的namespace.cluster.local为Cluster Domain
注意
- Kubernetes v1.7及以前API版本使用
extensions/v1beta1 - Kubernetes v1.8的API版本升级到
apps/v1beta2
简单示例
以一个简单的nginx服务web.yaml为例：
---
apiVersion: v1
kind: Service
metadata:
name: nginx
labels:
app: nginx
spec:
ports:
- port: 80
name: web
clusterIP: None
selector:
app: nginx
---
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
name: web
spec:
serviceName: "nginx"
replicas: 2
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: gcr.io/google_containers/nginx-slim:0.8
ports:
- containerPort: 80
name: web
volumeMounts:
- name: www
mountPath: /usr/share/nginx/html
volumeClaimTemplates:
- metadata:
name: www
annotations:
volume.alpha.kubernetes.io/storage-class: anything
spec:
accessModes: [ "ReadWriteOnce" ]
resources:
requests:
storage: 1Gi
$ kubectl create -f web.yaml
service "nginx" created
statefulset "web" created
# 查看创建的headless service和statefulset
$ kubectl get service nginx
NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
nginx None <none> 80/TCP 1m
$ kubectl get statefulset web
NAME DESIRED CURRENT AGE
web 2 2 2m
# 根据volumeClaimTemplates自动创建PVC（在GCE中会自动创建kubernetes.io/gce-pd类型的volume）
$ kubectl get pvc
NAME STATUS VOLUME CAPACITY ACCESSMODES AGE
www-web-0 Bound pvc-d064a004-d8d4-11e6-b521-42010a800002 1Gi RWO 16s
www-web-1 Bound pvc-d06a3946-d8d4-11e6-b521-42010a800002 1Gi RWO 16s
# 查看创建的Pod，他们都是有序的
$ kubectl get pods -l app=nginx
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 5m
web-1 1/1 Running 0 4m
# 使用nslookup查看这些Pod的DNS
$ kubectl run -i --tty --image busybox dns-test --restart=Never --rm /bin/sh
/ # nslookup web-0.nginx
Server: 10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
Name: web-0.nginx
Address 1: 10.244.2.10
/ # nslookup web-1.nginx
Server: 10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
Name: web-1.nginx
Address 1: 10.244.3.12
/ # nslookup web-0.nginx.default.svc.cluster.local
Server: 10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
Name: web-0.nginx.default.svc.cluster.local
Address 1: 10.244.2.10
还可以进行其他的操作
# 扩容
$ kubectl scale statefulset web --replicas=5
# 缩容
$ kubectl patch statefulset web -p '{"spec":{"replicas":3}}'
# 镜像更新（目前还不支持直接更新image，需要patch来间接实现）
$ 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"}]'
# 删除StatefulSet和Headless Service
$ kubectl delete statefulset web
$ kubectl delete service nginx
# StatefulSet删除后PVC还会保留着，数据不再使用的话也需要删除
$ kubectl delete pvc www-web-0 www-web-1
更新StatefulSet
v1.7+支持StatefulSet的自动更新，通过spec.updateStrategy设置更新策略。目前支持两种策略
- OnDelete：当
.spec.template更新时，并不立即删除旧的Pod，而是等待用户手动删除这些旧Pod后自动创建新Pod。这是默认的更新策略，兼容v1.6版本的行为 - RollingUpdate：当
.spec.template更新时，自动删除旧的Pod并创建新Pod替换。在更新时，这些Pod是按逆序的方式进行，依次删除、创建并等待Pod变成Ready状态才进行下一个Pod的更新。
Partitions
RollingUpdate还支持Partitions，通过.spec.updateStrategy.rollingUpdate.partition来设置。当partition设置后，只有序号大于或等于partition的Pod会在.spec.template更新的时候滚动更新，而其余的Pod则保持不变（即便是删除后也是用以前的版本重新创建）。
# 设置partition为3
$ kubectl patch statefulset web -p '{"spec":{"updateStrategy":{"type":"RollingUpdate","rollingUpdate":{"partition":3}}}}'
statefulset "web" patched
# 更新StatefulSet
$ 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"}]'
statefulset "web" patched
# 验证更新
$ kubectl delete po web-2
pod "web-2" deleted
$ kubectl get po -lapp=nginx -w
NAME READY STATUS RESTARTS AGE
web-0 1/1 Running 0 4m
web-1 1/1 Running 0 4m
web-2 0/1 ContainerCreating 0 11s
web-2 1/1 Running 0 18s
Pod管理策略
v1.7+可以通过.spec.podManagementPolicy设置Pod管理策略，支持两种方式
- OrderedReady：默认的策略，按照Pod的次序依次创建每个Pod并等待Ready之后才创建后面的Pod
- Parallel：并行创建或删除Pod（不等待前面的Pod Ready就开始创建所有的Pod）
Parallel示例
---
apiVersion: v1
kind: Service
metadata:
name: nginx
labels:
app: nginx
spec:
ports:
- port: 80
name: web
clusterIP: None
selector:
app: nginx
---
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
name: web
spec:
serviceName: "nginx"
podManagementPolicy: "Parallel"
replicas: 2
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: gcr.io/google_containers/nginx-slim:0.8
ports:
- containerPort: 80
name: web
volumeMounts:
- name: www
mountPath: /usr/share/nginx/html
volumeClaimTemplates:
- metadata:
name: www
spec:
accessModes: [ "ReadWriteOnce" ]
resources:
requests:
storage: 1Gi
可以看到，所有Pod是并行创建的
$ kubectl create -f webp.yaml
service "nginx" created
statefulset "web" created
$ kubectl get po -lapp=nginx -w
NAME READY STATUS RESTARTS AGE
web-0 0/1 Pending 0 0s
web-0 0/1 Pending 0 0s
web-1 0/1 Pending 0 0s
web-1 0/1 Pending 0 0s
web-0 0/1 ContainerCreating 0 0s
web-1 0/1 ContainerCreating 0 0s
web-0 1/1 Running 0 10s
web-1 1/1 Running 0 10s
zookeeper
另外一个更能说明StatefulSet强大功能的示例为zookeeper.yaml。
---
apiVersion: v1
kind: Service
metadata:
name: zk-headless
labels:
app: zk-headless
spec:
ports:
- port: 2888
name: server
- port: 3888
name: leader-election
clusterIP: None
selector:
app: zk
---
apiVersion: v1
kind: ConfigMap
metadata:
name: zk-config
data:
ensemble: "zk-0;zk-1;zk-2"
jvm.heap: "2G"
tick: "2000"
init: "10"
sync: "5"
client.cnxns: "60"
snap.retain: "3"
purge.interval: "1"
---
apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
name: zk-budget
spec:
selector:
matchLabels:
app: zk
minAvailable: 2
---
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
name: zk
spec:
serviceName: zk-headless
replicas: 3
template:
metadata:
labels:
app: zk
annotations:
pod.alpha.kubernetes.io/initialized: "true"
scheduler.alpha.kubernetes.io/affinity: >
{
"podAntiAffinity": {
"requiredDuringSchedulingRequiredDuringExecution": [{
"labelSelector": {
"matchExpressions": [{
"key": "app",
"operator": "In",
"values": ["zk-headless"]
}]
},
"topologyKey": "kubernetes.io/hostname"
}]
}
}
spec:
containers:
- name: k8szk
imagePullPolicy: Always
image: gcr.io/google_samples/k8szk:v1
resources:
requests:
memory: "4Gi"
cpu: "1"
ports:
- containerPort: 2181
name: client
- containerPort: 2888
name: server
- containerPort: 3888
name: leader-election
env:
- name : ZK_ENSEMBLE
valueFrom:
configMapKeyRef:
name: zk-config
key: ensemble
- name : ZK_HEAP_SIZE
valueFrom:
configMapKeyRef:
name: zk-config
key: jvm.heap
- name : ZK_TICK_TIME
valueFrom:
configMapKeyRef:
name: zk-config
key: tick
- name : ZK_INIT_LIMIT
valueFrom:
configMapKeyRef:
name: zk-config
key: init
- name : ZK_SYNC_LIMIT
valueFrom:
configMapKeyRef:
name: zk-config
key: tick
- name : ZK_MAX_CLIENT_CNXNS
valueFrom:
configMapKeyRef:
name: zk-config
key: client.cnxns
- name: ZK_SNAP_RETAIN_COUNT
valueFrom:
configMapKeyRef:
name: zk-config
key: snap.retain
- name: ZK_PURGE_INTERVAL
valueFrom:
configMapKeyRef:
name: zk-config
key: purge.interval
- name: ZK_CLIENT_PORT
value: "2181"
- name: ZK_SERVER_PORT
value: "2888"
- name: ZK_ELECTION_PORT
value: "3888"
command:
- sh
- -c
- zkGenConfig.sh && zkServer.sh start-foreground
readinessProbe:
exec:
command:
- "zkOk.sh"
initialDelaySeconds: 15
timeoutSeconds: 5
livenessProbe:
exec:
command:
- "zkOk.sh"
initialDelaySeconds: 15
timeoutSeconds: 5
volumeMounts:
- name: datadir
mountPath: /var/lib/zookeeper
securityContext:
runAsUser: 1000
fsGroup: 1000
volumeClaimTemplates:
- metadata:
name: datadir
annotations:
volume.alpha.kubernetes.io/storage-class: anything
spec:
accessModes: [ "ReadWriteOnce" ]
resources:
requests:
storage: 20Gi
kubectl create -f zookeeper.yaml
详细的使用说明见zookeeper stateful application。
StatefulSet注意事项
- 还在beta状态，需要kubernetes v1.5版本以上才支持
- 所有Pod的Volume必须使用PersistentVolume或者是管理员事先创建好
- 为了保证数据安全，删除StatefulSet时不会删除Volume
- StatefulSet需要一个Headless Service来定义DNS domain，需要在StatefulSet之前创建好