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‎README.md

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#postgres-on-kubernetes
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PostgreSQL on Kubernetes
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##Prerequisites
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```bash
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# install minikube to create a single-node cluster
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brew install minikube
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# start cluster using VMs
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minikube start --vm=true
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# create a custom namespace and context
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kubectl create namespace postgres
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kubectl config set-context postgres --namespace postgres --cluster minikube --user minikube
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kubectl config use-context postgres
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```
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##Persistent Volumes
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In order to persist the data stored in PostgreSQL it’s necessary to create[Persistent Volumes](https://kubernetes.io/docs/concepts/storage/persistent-volumes/) that have a pod-independent lifecycle. Within a Stateful Set a so called_Persistent Volume Claim_ with a specific[Storage Class](https://kubernetes.io/docs/concepts/storage/storage-classes/) can be configured.
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There are two ways to create Persistent Volumes. Either you manually create a volume per replica of PostgreSQL or you configure[dynamic provisioning](https://minikube.sigs.k8s.io/docs/handbook/persistent_volumes/#dynamic-provisioning-and-csi). For simplicity we choose the manual approach first.
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```bash
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# create 3 persistent volumes
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kubectl apply -f pv-0.yaml
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kubectl apply -f pv-1.yaml
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kubectl apply -f pv-2.yaml
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# list persistent volumes
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kubectl get pv
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NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE
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pv-postgresql-0 1Gi RWO Retain Available default 9s
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pv-postgresql-1 1Gi RWO Retain Available default 7s
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pv-postgresql-2 1Gi RWO Retain Available default 3s
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```
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##Headless Service
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A[Headless Service](https://kubernetes.io/docs/concepts/services-networking/service/#headless-services) is specified with`clusterIP: None` and omits to use a L4 load balancer. By also defining a selector, the endpoints controller creates`Endpoint` records and also modifies the DNS configuration so that`A` records are returned that point directly to the pods.
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```bash
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# create headless service
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kubectl apply -f svc.yaml
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# describe headless service
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kubectl describe svc postgresql-svc
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Name: postgresql-svc
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Namespace: postgres
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Labels: sfs=postgresql-sfs
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Annotations:<none>
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Selector: sfs=postgresql-sfs
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Type: ClusterIP
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IP Family Policy: SingleStack
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IP Families: IPv4
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IP: None
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IPs: None
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Port: postgresql-port 5432/TCP
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TargetPort: 5432/TCP
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Endpoints:<none>
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Session Affinity: None
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Events:<none>
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```
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##Secrets
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PostgreSQL uses environment variables for configuration. The most important one for the official[PostgreSQL Docker image](https://hub.docker.com/_/postgres) is`POSTGRES_PASSWORD`. We utilize[Secrets](https://kubernetes.io/docs/concepts/configuration/secret/) to inject the respective value into the container later on.
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```bash
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# create secret from literal
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kubectl create secret generic postgresql-secrets --from-literal=POSTGRES_PASSWORD=tes6Aev8
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# describe secret
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kubectl describe secrets postgresql-secrets
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Name: postgresql-secrets
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Namespace: postgres
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Labels:<none>
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Annotations:<none>
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Type: Opaque
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Data
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====
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POSTGRES_PASSWORD: 8 bytes
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```
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##Stateful Sets
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A[Stateful Set](https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/) is similar to a_Replica Set_ in a sense that it also handles pods for the configured number of replicas. In contrast to a_Replica Set_, it maintains a sticky identity for each of them. This means they are created in a fixed, sequential order and deleted counterwise. Their network identity is stable as well, what enables us to reference them by the automatically assigned DNS host inside of the cluster.
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```bash
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# create stateful set with 3 replicas
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kubectl apply -f sfs.yaml
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# list stateful sets
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kubectl get statefulsets
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NAME READY AGE
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postgresql-sfs 3/3 16s
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# list pods
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kubectl get pods
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NAME READY STATUS RESTARTS AGE
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postgresql-sfs-0 1/1 Running 0 86s
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postgresql-sfs-1 1/1 Running 0 83s
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postgresql-sfs-2 1/1 Running 0 80s
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# inspect logs of a random pod
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kubectl logs postgresql-sfs-0
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PostgreSQL Database directory appears to contain a database; Skipping initialization
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2021-08-04 08:19:50.832 UTC [1] LOG: starting PostgreSQL 13.3 (Debian 13.3-1.pgdg100+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit
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2021-08-04 08:19:50.832 UTC [1] LOG: listening on IPv4 address"0.0.0.0", port 5432
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2021-08-04 08:19:50.832 UTC [1] LOG: listening on IPv6 address"::", port 5432
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2021-08-04 08:19:50.835 UTC [1] LOG: listening on Unix socket"/var/run/postgresql/.s.PGSQL.5432"
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2021-08-04 08:19:50.838 UTC [26] LOG: database system was shut down at 2021-08-03 14:33:17 UTC
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2021-08-04 08:19:50.843 UTC [1] LOG: database system is ready to accept connections
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# describe the service to see that 3 endpoints were created automatically
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kubectl describe svc postgresql-svc
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Name: postgresql-svc
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Namespace: postgres
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Labels: sfs=postgresql-sfs
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Annotations:<none>
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Selector: sfs=postgresql-sfs
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Type: ClusterIP
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IP Family Policy: SingleStack
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IP Families: IPv4
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IP: None
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IPs: None
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Port: postgresql-port 5432/TCP
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TargetPort: 5432/TCP
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Endpoints: 172.17.0.3:5432,172.17.0.4:5432,172.17.0.5:5432
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Session Affinity: None
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Events:<none>
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```
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##Connect to
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You are able to directly connect to PostgreSQL by starting`bash` within a particular pod.
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```bash
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kubectlexec -it postgresql-sfs-0 -- bash
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root@postgresql-sfs-0:/# PGPASSWORD=tes6Aev8 psql -U postgres
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psql (13.3 (Debian 13.3-1.pgdg100+1))
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Type"help"for help.
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postgres=# exit
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root@postgresql-sfs-0:/# exit
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exit
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```
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An alternative approach is running a temporary PostgreSQL container and using the included`psql` to connect to one of the database instances. Where the hostname is the automatically created DNS host of the service we deployed earlier. The format of that hostname is:`<service-name>.<namespace>.svc.cluster.local` and will be resolved to a random pod running a database server.
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```bash
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kubectl run -it --rm pg-psql --image=postgres:13.3 --restart=Never --env="PGPASSWORD=tes6Aev8" -- psql -h postgresql-svc.postgres.svc.cluster.local -U postgres
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If you don't see a command prompt, try pressing enter.
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postgres=# \q
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pod "pg-psql" deleted
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```
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To check that the DNS hostname works we deploy a busybox instance.
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```bash
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kubectl run -it --rm busybox --image=busybox --restart=Never -- sh
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If you don't see acommand prompt, try pressing enter.
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/# ping postgresql-svc.postgres.svc.cluster.local
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PING postgresql-svc.postgres.svc.cluster.local (172.17.0.3): 56 data bytes
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64 bytes from 172.17.0.3: seq=0 ttl=64 time=0.828 ms
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64 bytes from 172.17.0.3: seq=1 ttl=64 time=0.080 ms
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64 bytes from 172.17.0.3: seq=2 ttl=64 time=0.080 ms
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^C
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--- postgresql-svc.postgres.svc.cluster.local ping statistics ---
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3 packets transmitted, 3 packets received, 0% packet loss
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round-trip min/avg/max = 0.080/0.329/0.828 ms
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/# nslookup postgresql-svc.postgres.svc.cluster.local
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Server:10.96.0.10
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Address:10.96.0.10:53
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Name:postgresql-svc.postgres.svc.cluster.local
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Address: 172.17.0.5
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Name:postgresql-svc.postgres.svc.cluster.local
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Address: 172.17.0.4
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Name:postgresql-svc.postgres.svc.cluster.local
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Address: 172.17.0.3
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*** Can't find postgresql-svc.postgres.svc.cluster.local: No answer
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/ # exit
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pod "busybox" deleted
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```

‎pv-0.yaml

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apiVersion:v1
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kind:PersistentVolume
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metadata:
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name:pv-postgresql-0
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spec:
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storageClassName:default
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accessModes:
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-ReadWriteOnce
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capacity:
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storage:1Gi
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hostPath:
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path:/data/pv-postgresql-0/

‎pv-1.yaml

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apiVersion:v1
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kind:PersistentVolume
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metadata:
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name:pv-postgresql-1
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spec:
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storageClassName:default
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accessModes:
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-ReadWriteOnce
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capacity:
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storage:1Gi
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hostPath:
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path:/data/pv-postgresql-1/

‎pv-2.yaml

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apiVersion:v1
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kind:PersistentVolume
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metadata:
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name:pv-postgresql-2
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spec:
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storageClassName:default
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accessModes:
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-ReadWriteOnce
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capacity:
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storage:1Gi
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hostPath:
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path:/data/pv-postgresql-2/

‎sfs.yaml

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apiVersion:apps/v1
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kind:StatefulSet
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metadata:
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name:postgresql-sfs
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spec:
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selector:
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matchLabels:
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sfs:postgresql-sfs
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serviceName:postgresql-svc
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replicas:3
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template:
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metadata:
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labels:
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sfs:postgresql-sfs
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spec:
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terminationGracePeriodSeconds:10
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containers:
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-name:postgresql-db
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image:postgres:13.3
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env:
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-name:POSTGRES_PASSWORD
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valueFrom:
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secretKeyRef:
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name:postgresql-secrets
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key:POSTGRES_PASSWORD
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-name:PGDATA
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value:/var/lib/postgresql/data/pgdata
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ports:
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-containerPort:5432
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name:postgresql-port
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volumeMounts:
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-name:data
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mountPath:/var/lib/postgresql/data
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volumeClaimTemplates:
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-metadata:
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name:data
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spec:
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accessModes:
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-ReadWriteOnce
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storageClassName:default
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resources:
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requests:
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storage:1Gi

‎svc.yaml

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apiVersion:v1
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kind:Service
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metadata:
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name:postgresql-svc
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labels:
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sfs:postgresql-sfs
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spec:
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ports:
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-port:5432
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name:postgresql-port
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clusterIP:None
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selector:
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sfs:postgresql-sfs

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