有状态服务的概念

既然名叫有状态，那么就与之相对，会有我们很熟悉的无状态。无状态的概念，就是只负责运算，不负责任何数据的存储，这样就能很轻松地做到水平扩展。

之前写的关于无状态的例子

那么，有状态的概念又是什么呢，简单来说，就是会有数据的存储，需要持久化。

k8s的petset

简单来说，pod是用来跑无状态服务，petset就是跑有状态服务。
1.3之前k8s大多是用于无状态的web应用，但是我们实际业务却有很多有状态的服务，对于谷歌来说，绝对不会放弃这一块的机会，所以petset就应运而生。

那么，作为有状态服务的基石，petset需要具备哪些特征呢：

有唯一的编号
在网络上有个不会改变的标识，k8s是通过域名实现的。pod，则是名字后面还有随机数，所以需要有service来做转发
每个有状态服务，都需要有自己的卷，这样就能保证数据可靠存储

petset的典型场景

MySQL
Zookeeper
Cassandra
redis

小试验

# A headless service to create DNS records
apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  # *.nginx.default.svc.cluster.local
  clusterIP: None
  selector:
    app: nginx
---
apiVersion: apps/v1alpha1
kind: PetSet
metadata:
  name: web
spec:
  serviceName: "nginx"
  replicas: 2
  template:
    metadata:
      labels:
        app: nginx
      annotations:
        pod.alpha.kubernetes.io/initialized: "true"
    spec:
      terminationGracePeriodSeconds: 0
      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

提示

$ ./kubectl describe pvc www-web-0
Name:		www-web-0
Namespace:	default
Status:		Pending
Volume:
Labels:		app=nginx
Capacity:
Access Modes:
Events:
  FirstSeen	LastSeen	Count	From				SubobjectPath	Type		Reason			Message
  ---------	--------	-----	----				-------------	--------	------			-------
  5m		10s		22	{persistentvolume-controller }			Warning		ProvisioningFailed	No provisioner plugin found for the claim!

这是因为需要设置Persistent Volume Provisioning
方法是在controller manager的启动参数加上--enable-hostpath-provisioner=true，然后重启controller，再create就OK了。

redis

https://github.com/kubernetes/kubernetes/tree/master/test/e2e/testing-manifests/petset/redis

service.yml

# A headless service to create DNS records
apiVersion: v1
kind: Service
metadata:
  annotations:
    service.alpha.kubernetes.io/tolerate-unready-endpoints: "true"
  name: redis
  labels:
    app: redis
spec:
  ports:
  - port: 6379
    name: peer
  # *.redis.default.svc.cluster.local
  clusterIP: None
  selector:
    app: redis

petset.yml

apiVersion: apps/v1alpha1
kind: PetSet
metadata:
  name: rd
spec:
  serviceName: "redis"
  replicas: 3
  template:
    metadata:
      labels:
        app: redis
      annotations:
        pod.alpha.kubernetes.io/initialized: "true"
        pod.alpha.kubernetes.io/init-containers: '[
            {
                "name": "install",
                "image": "gcr.io/google_containers/redis-install-3.2.0:e2e",
                "imagePullPolicy": "Always",
                "args": ["--install-into=/opt", "--work-dir=/work-dir"],
                "volumeMounts": [
                    {
                        "name": "opt",
                        "mountPath": "/opt"
                    },
                    {
                        "name": "workdir",
                        "mountPath": "/work-dir"
                    }
                ]
            },
            {
                "name": "bootstrap",
                "image": "debian:jessie",
                "command": ["/work-dir/peer-finder"],
                "args": ["-on-start=\"/work-dir/on-start.sh\"", "-service=redis"],
                "env": [
                  {
                      "name": "POD_NAMESPACE",
                      "valueFrom": {
                          "fieldRef": {
                              "apiVersion": "v1",
                              "fieldPath": "metadata.namespace"
                          }
                      }
                   }
                ],
                "volumeMounts": [
                    {
                        "name": "opt",
                        "mountPath": "/opt"
                    },
                    {
                        "name": "workdir",
                        "mountPath": "/work-dir"
                    }
                ]
            }
        ]'
    spec:
      terminationGracePeriodSeconds: 0
      containers:
      - name: redis
        image: debian:jessie
        ports:
        - containerPort: 6379
          name: peer
        command:
        - /opt/redis/redis-server
        args:
        - /opt/redis/redis.conf
        readinessProbe:
          exec:
            command:
            - sh
            - -c
            - "/opt/redis/redis-cli -h $(hostname) ping"
          initialDelaySeconds: 15
          timeoutSeconds: 5
        volumeMounts:
        - name: datadir
          mountPath: /data
        - name: opt
          mountPath: /opt
      volumes:
      - name: opt
        emptyDir: {}
      - name: workdir
        emptyDir: {}
  volumeClaimTemplates:
  - metadata:
      name: datadir
      annotations:
        volume.alpha.kubernetes.io/storage-class: anything
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi

参考

本博客欢迎转发,但请保留原作者信息
github:codejuan
博客地址:http://blog.decbug.com/

Continuous Learning

kubernetes尝试有状态服务