Kubernetes stateful set讲解以及一个基于postgreSQL的具体例子

Stateful Set是Kubernetes 1.9版本新引入的一个概念，用于管理有状态的应用。

Kubernetes官方文档：

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

Manages the deployment and scaling of a set of Pods, and provides guarantees about the ordering and uniqueness of these Pods.

Like a Deployment, a StatefulSet manages Pods that are based on an identical container spec. Unlike a Deployment, a StatefulSet maintains a sticky identity for each of their Pods. These pods are created from the same spec, but are not interchangeable: each has a persistent identifier that it maintains across any rescheduling.

StatefulSet由以下几个部分组成：

1. 用于定义网络标志（DNS domain）的Headless Service

2. 用于创建PersistentVolumes的volumeClaimTemplates

3. 定义具体应用的StatefulSet

下面我给出了一个实际应用中的StatefulSet的yaml文件：

---
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: ads-db-statefulset
labels:
component: ads
module: db
spec:
serviceName: ads-db-service
replicas: 1
selector:
matchLabels:
component: ads
module: db
template:
metadata:
labels:
component: ads
module: db
spec:
volumes:
- name: init
secret:
secretName: ads-db-secret
items:
- key: initdb.sql
path: initdb.sql
containers:
- name: ads-db-pod
image: postgres:9.6
ports:
- containerPort: 5432
name: ads-db-port
volumeMounts:
- name: ads-db-volume
mountPath: /var/lib/postgresql/data/
- name: init
mountPath: /docker-entrypoint-initdb.d/
env:
- name: PGDATA
valueFrom:
configMapKeyRef:
name: ads-db-configmap
key: pgdata_value
- name: POSTGRES_PASSWORD
valueFrom:
secretKeyRef:
name: ads-db-secret
key: postgres_password_value
volumeClaimTemplates:
- metadata:
name: ads-db-volume
labels:
component: ads
module: db
spec:
accessModes: [ "ReadWriteOnce" ]
resources:
requests:
storage: 1Gi

使用kubectl get statefulset查看生成的statefulset:

生成的headless service：

生成的pod：

当我把statefulset yaml文件里的replicas从1改成3之后，果然观察到有两个新的pod正在启动，并且名称满足命名规范-X。

使用kubectl describe查看创建的statefulset明细：

statefulSet自动创建的persistentVolumeClaim：

The files belonging to this database system will be owned by user "postgres".

This user must also own the server process.

The database cluster will be initialized with locale "en_US.utf8".

The default database encoding has accordingly been set to "UTF8".

The default text search configuration will be set to "english".

Data page checksums are disabled.

fixing permissions on existing directory /var/lib/postgresql/data/pgdata ... ok

creating subdirectories ... ok

selecting default max_connections ... 100

selecting default shared_buffers ... 128MB

selecting dynamic shared memory implementation ... posix

creating configuration files ... ok

running bootstrap script ... ok

performing post-bootstrap initialization ... ok

syncing data to disk ... ok

Success. You can now start the database server using:

pg_ctl -D /var/lib/postgresql/data/pgdata -l logfile start

使用下面的命令登录到statefulset提供的postgreSQL服务器上：

1. kubectl run tester -it --rm --image=postgres:9.6 --env="PGCONNECT_TIMEOUT=5" --command -- bash

看到root$之后，说明我们已经连接上pod了。

使用如下命令行连接postgreSQL服务器：

psql -h ads-db-statefulset-0.ads-db-service -p 5432 -U adsuser -W ads

当然如果不用命令行，也可以使用pgadmin，以图形化界面连接statefulSet里的postgreSQL服务器：

sudo apt install pgadmin3

进行端口转发，这样我们可以使用localhost：5432进行连接：

kubectl port-forward ads-db-statefulset-0 5432:5432

也能成功连接：

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