Stateful Set是Kubernetes 1.9版本新引入的一個概念，用于管理有狀态的應用。
Kubernetes官方文檔：
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由以下幾個部分組成：
1. 用于定義網絡标志（DNS domain）的Headless Service
2. 用于建立PersistentVolumes的volumeClaimTemplates
3. 定義具體應用的StatefulSet
下面我給出了一個實際應用中的StatefulSet的yaml檔案：
使用kubectl get statefulset檢視生成的statefulset:
生成的headless service：
生成的pod：
當我把statefulset yaml檔案裡的replicas從1改成3之後，果然觀察到有兩個新的pod正在啟動，并且名稱滿足命名規範-X。
使用kubectl describe檢視建立的statefulset明細：
statefulSet自動建立的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
使用下面的指令登入到statefulset提供的postgreSQL伺服器上：
1. kubectl run tester -it --rm --image=postgres:9.6 --env=“PGCONNECT_TIMEOUT=5” --command – bash
看到root$之後，說明我們已經連接配接上pod了。
使用如下指令行連接配接postgreSQL伺服器：
psql -h ads-db-statefulset-0.ads-db-service -p 5432 -U adsuser -W ads
當然如果不用指令行，也可以使用pgadmin，以圖形化界面連接配接statefulSet裡的postgreSQL伺服器：
sudo apt install pgadmin3
進行端口轉發，這樣我們可以使用localhost：5432進行連接配接：
kubectl port-forward ads-db-statefulset-0 5432:5432
也能成功連接配接：