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第 12 屆 iThome 鐵人賽
DAY 27
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DevOps

Docker獸 究極進化 ～～ Kubernetes獸系列 第 27 篇

Day-27 使用StatefulSet

12th鐵人賽 devops kubernetes
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前言

前面我們先介紹了Pod, 控管Pod的ReplicaSet與管理ReplicaSet的Deployment，也在前面章節解說了專門用於守護進程的DaemonSet，接下來則講述StatefulSet與其解決的問題。

What is StatefulSet ?

大家在使用Pod時一定會遇到一個問題，每種Pod有著許許多多的類別，並且每個種類的Pod都會有著一個以上的replica，但當其他Pod與外界在聯繫該種類的Pod時，無法識別它們。

或許在大部分情況之下，每個Pod都有著相同資料並執行著一樣的任務，但還是會遇到某些情形每個Pod需要有能夠識別彼此的識別證，就像是在Kafka中每個Node都是獨一無二的。

基本上StatefulSet管理Pod的方式與Deployment無異，但唯一不同的地方只有在StatefulSet Controller會為每個Pod生成獨一無二的識別資訊，並且這些資訊不會因為re-schedule而改變。

https://ithelp.ithome.com.tw/upload/images/20201012/20129737xpZvzODwUM.png

When to use StatefulSet ?

要如何判斷何時該使用StatefulSet呢？

  • 需要穩定且唯一的網路識別資訊
  • 有狀態的集群應用
  • 需要穩定且持久的儲存空間
  • 部署時，Pod需要有順序性地建立

也因此StatefulSet適合用來部署Kafka、MySQL、Redis與ZooKeeper等需要唯一識別身份與主機名的服務，至於其他無狀態服務請使用deployment。

What we need for StatefulSet ?

  1. Kubernetes v1.5以前的版本並不支援StatefulSet，v1.9後才正式資源。
  2. Storage部分必須要綁定到PVC，並配對至特定Storage Class或是PV，確保Pod被刪除後資料依然存在。(PV、PVC與Storage Class請參考Day-26 Kubernetes Storage)。
  3. 需要定義一個Headless Service與StatefuleSet進行配對，確保每個Pod都有其獨一無二的network identify。(Headless Service請參照Day-25 DaemonSet的Headless Service章節)

How to write StatefulSet ?

這裡我們以mongoDb為例，講述一次StatefulSet。

https://ithelp.ithome.com.tw/upload/images/20201012/2012973728agqHjEC0.png

在Mongo Cluster當中，我們會需要

  1. 可以識別每個MongoDB Pod的StatefulSet
  2. MongoDB 所連結的PVC與PV/StorageClass
  3. 創建能夠讓每個Pod有著網路識別資訊的Headless Service。

Starting with PV and StorageClass

ssd storage class

ssd.yaml

kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: fast
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-ssd
  • provisioner: 儲存卷分配器，這裡必需選擇你的儲存卷是由誰分配。若是GKE請使用kubernetes.io/gce-pd，其他請參考下方表格。
  • type: storage的種類，gce有提供pd-ssd、pd-standard兩種
Type Provisioner
AWS kubernetes.io/aws-ebs
GCE kubernetes.io/gce-pd
Glusterfs kubernetes.io/glusterfs
OpenStack Cinder kubernetes.io/cinder
vSphere kubernetes.io/vsphere-volume
Ceph RBD kubernetes.io/rbd
Quobyte kubernetes.io/quobyte
Azure disk kubernetes.io/azure-disk
Azure file kubernetes.io/azure-file
Portworx kubernetes.io/portworx-volume
ScaleIO kubernetes.io/scaleio

想知道更多詳細資訊請參照 https://k8smeetup.github.io/docs/concepts/storage/storage-classes/#gce

hhd storage class

hhd.yaml

kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: slow
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-standard

StatefulSet yaml

statefulset.yaml

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mongo
spec:
  selector:
    matchLabels:
      role: mongo
      environment: test
  serviceName: "mongo"
  replicas: 3
  template:
    metadata:
      labels:
        role: mongo
        environment: test
    spec:
      terminationGracePeriodSeconds: 10
      containers:
        - name: mongo
          image: mongo:3.4
          command:
            - mongod
            - "--replSet"
            - rs0
            - "--bind_ip"
            - 0.0.0.0
            - "--smallfiles"
            - "--noprealloc"
          ports:
            - containerPort: 27017
          volumeMounts:
            - name: mongo-persistent-storage
              mountPath: /data/db
        - name: mongo-sidecar
          image: cvallance/mongo-k8s-sidecar
          env:
            - name: MONGO_SIDECAR_POD_LABELS
              value: "role=mongo,environment=test"
  volumeClaimTemplates:
  - metadata:
      name: mongo-persistent-storage
      annotations:
        volume.beta.kubernetes.io/storage-class: "fast"
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 100Gi

這邊只講述deployment章節未提及的部分，若需要複習者請參考

  • kind: 必須指定為StatefulSet。
  • spec.template.spec.terminationGracePeriodSeconds: 每個pod依順序性刪除時間隔時間。
  • volumeClaimTemplates: 透過動態方式自動產生PVC，這裡需要指定一個已經存在並且合法的storageClass。

HeadlessService yaml

headless-service.yaml

apiVersion: v1
kind: Service
metadata:
  name: mongo
  labels:
    name: mongo
spec:
  ports:
  - port: 27017
    targetPort: 27017
  clusterIP: None
  selector:
    role: mongo

Deployment

  • 先創立所需的storage class
$ kubectl apply -f ssd.yaml
storageclass.storage.k8s.io/fast created

$ kubectl apply -f hhd.yaml
storageclass.storage.k8s.io/slow created
  • 創建stateful set並且綁定storage class
$ kubectl apply -f statefulset.yaml
statefulset.apps/mongo created
  • 最後則是創建headless service
$ kubectl apply -f headless-service.yaml 
service/mongo created

  • 我們可以透過GKE的控制台看到這些service都在創立，並且也顯示在了前台

    • storage class

    https://ithelp.ithome.com.tw/upload/images/20201012/20129737tzeNco62Uz.png

    • statefulset

    https://ithelp.ithome.com.tw/upload/images/20201012/201297372Pkc9wykjF.png

    • headless service

    https://ithelp.ithome.com.tw/upload/images/20201012/20129737DN70nZLLiF.png

Check mongo cluster

我們先去cluster中第一個mongodb查看資訊

$ kubectl exec -it mongo-0 mongo
Defaulting container name to mongo.
Use 'kubectl describe pod/mongo-0 -n default' to see all of the containers in this pod.
MongoDB shell version v3.4.24
connecting to: mongodb://127.0.0.1:27017
MongoDB server version: 3.4.24
Welcome to the MongoDB shell.
For interactive help, type "help".
For more comprehensive documentation, see
        http://docs.mongodb.org/
Questions? Try the support group
        http://groups.google.com/group/mongodb-user
Server has startup warnings: 
2020-10-10T12:54:28.505+0000 I STORAGE  [initandlisten] 
2020-10-10T12:54:28.506+0000 I STORAGE  [initandlisten] ** WARNING: Using the XFS filesystem is strongly recommended with the WiredTiger storage engine
2020-10-10T12:54:28.506+0000 I STORAGE  [initandlisten] **          See http://dochub.mongodb.org/core/prodnotes-filesystem
2020-10-10T12:54:28.586+0000 I CONTROL  [initandlisten] 
2020-10-10T12:54:28.586+0000 I CONTROL  [initandlisten] ** WARNING: Access control is not enabled for the database.
2020-10-10T12:54:28.586+0000 I CONTROL  [initandlisten] **          Read and write access to data and configuration is unrestricted.
2020-10-10T12:54:28.586+0000 I CONTROL  [initandlisten] ** WARNING: You are running this process as the root user, which is not recommended.
2020-10-10T12:54:28.587+0000 I CONTROL  [initandlisten]

> rs.initiate()
{
        "info2" : "no configuration specified. Using a default configuration for the set",
        "me" : "mongo-0:27017",
        "ok" : 1
}
rs0:OTHER> rs.conf()
{
        "_id" : "rs0",
        "version" : 1,
        "protocolVersion" : NumberLong(1),
        "members" : [
                {
                        "_id" : 0,
                        "host" : "mongo-0:27017",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 1,
                        "tags" : {
                                
                        },
                        "slaveDelay" : NumberLong(0),
                        "votes" : 1
                }
        ],
        "settings" : {
                "chainingAllowed" : true,
                "heartbeatIntervalMillis" : 2000,
                "heartbeatTimeoutSecs" : 10,
                "electionTimeoutMillis" : 10000,
                "catchUpTimeoutMillis" : 60000,
                "getLastErrorModes" : {
                        
                },
                "getLastErrorDefaults" : {
                        "w" : 1,
                        "wtimeout" : 0
                },
                "replicaSetId" : ObjectId("5f81b283e49202849539609a")
        }
}

再來去第二個mongo db看看

kubectl exec -ti mongo-1 mongo
Defaulting container name to mongo.
Use 'kubectl describe pod/mongo-1 -n default' to see all of the containers in this pod.
MongoDB shell version v3.4.24
^[[Aconnecting to: mongodb://127.0.0.1:27017
MongoDB server version: 3.4.24
Welcome to the MongoDB shell.
For interactive help, type "help".
For more comprehensive documentation, see
        http://docs.mongodb.org/
Questions? Try the support group
        http://groups.google.com/group/mongodb-user
Server has startup warnings: 
2020-10-10T12:55:06.160+0000 I STORAGE  [initandlisten] 
2020-10-10T12:55:06.160+0000 I STORAGE  [initandlisten] ** WARNING: Using the XFS filesystem is strongly recommended with the WiredTiger storage engine
2020-10-10T12:55:06.160+0000 I STORAGE  [initandlisten] **          See http://dochub.mongodb.org/core/prodnotes-filesystem
2020-10-10T12:55:06.227+0000 I CONTROL  [initandlisten] 
2020-10-10T12:55:06.227+0000 I CONTROL  [initandlisten] ** WARNING: Access control is not enabled for the database.
2020-10-10T12:55:06.227+0000 I CONTROL  [initandlisten] **          Read and write access to data and configuration is unrestricted.
2020-10-10T12:55:06.227+0000 I CONTROL  [initandlisten] ** WARNING: You are running this process as the root user, which is not recommended.

> rs.initiate()
{
        "info2" : "no configuration specified. Using a default configuration for the set",
        "me" : "mongo-1:27017",
        "ok" : 1
}
rs0:OTHER> rs.conf()
{
        "_id" : "rs0",
        "version" : 1,
        "protocolVersion" : NumberLong(1),
        "members" : [
                {
                        "_id" : 0,
                        "host" : "mongo-1:27017",
                        "arbiterOnly" : false,
                        "buildIndexes" : true,
                        "hidden" : false,
                        "priority" : 1,
                        "tags" : {
                                
                        },
                        "slaveDelay" : NumberLong(0),
                        "votes" : 1
                }
        ],
        "settings" : {
                "chainingAllowed" : true,
                "heartbeatIntervalMillis" : 2000,
                "heartbeatTimeoutSecs" : 10,
                "electionTimeoutMillis" : 10000,
                "catchUpTimeoutMillis" : 60000,
                "getLastErrorModes" : {
                        
                },
                "getLastErrorDefaults" : {
                        "w" : 1,
                        "wtimeout" : 0
                },
                "replicaSetId" : ObjectId("5f81b369ce23c2ce910e02d1")
        }
}

我們可以看到兩個mongoDB有著不同的host與id，這也表示我們成功了。

Scaling MongoDb replica

當replica數目大於1時，statefulSet會與deployment不同，statefulSet中的pod會同步且有順序的逐一產生。產生的流程如下：

  1. pod-0 —> pod-1 —> pod-2....etc，在每個pod後面都會加上sequence number並且按照順序產生。
  2. 當要對 pod 進行 scale 時，predecessor 的狀態必須是 Running & Ready，舉例來說：若pod要從1個scale up成2個時，pod-0的狀態必須是Running & Ready，pod-1才會開始create。
$ kubectl get pod
mongo-0                    2/2     Running   0          27m
mongo-1                    2/2     Running   0          26m
mongo-2                    2/2     Running   0          26m

$ kubectl scale --replicas=5 statefulset mongo
statefulset.apps/mongo scaled

$ kubectl get pod --watch
NAME                       READY   STATUS              RESTARTS   AGE
mongo-0                    2/2     Running             0          28m
mongo-1                    2/2     Running             0          27m
mongo-2                    2/2     Running             0          27m
mongo-3                    0/2     ContainerCreating   0          10s
mongo-3                    2/2     Running             0          16s
mongo-4                    0/2     Pending             0          0s
mongo-4                    0/2     Pending             0          0s
mongo-4                    0/2     Pending             0          5s
mongo-4                    0/2     ContainerCreating   0          5s
mongo-4                    2/2     Running             0          19s

我們可以發現當mongo statefulset scaling up時，pod數目確實會照順序產生並運行。

Clean up Mongo Db

與產生相同，刪除時也是有順序性的，但是他會反性刪除，意指從pod-2 —> pod-1 —> pod-0刪除。

Using Mongo Db cluster

Each pod in a StatefulSet backed by a Headless Service will have a stable DNS name. The template follows this format: .

This means the DNS names for the MongoDB replica set are:

mongo-0.mongo
mongo-1.mongo
mongo-2.mongo

We can use these names directly in the connection string URI of our app.

Using a database is outside the scope of this post, however for this case, the connection string URI would be:

"mongodb://mongo-0.mongo,mongo-1.mongo,mongo-2.mongo:27017/dbname_?"

Github Repo

本篇章所有程式碼將放在下面的github project當中的branch day-27

後記

這章節我們學會了運用StatefulSet，這也讓我們以後想部署類似Kafka、ZooKeeper或是Cluster類型的Database都有了方法，並且配合著StorageClass與PVC來持續化地保存資料。

https://ithelp.ithome.com.tw/upload/images/20201012/20129737gDeV40AOY1.png

https://ithelp.ithome.com.tw/upload/images/20201012/20129737QSVFnETTg9.png

Reference

https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/


上一篇
Day-26 瞭解 Kubernetes Storage
下一篇
Day-28 暸解 Namespace 與 Rbac
系列文
Docker獸 究極進化 ～～ Kubernetes獸 共 30 篇
  1. 26
    Day-26 瞭解 Kubernetes Storage
  2. 27
    Day-27 使用StatefulSet
  3. 28
    Day-28 暸解 Namespace 與 Rbac
  4. 29
    Day-29 了解 Kubernetes AutoScaler
  5. 30
    Day-30 學習 Helm (上)
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