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Streaming Replication

Streaming Replication provides asynchronous replication to one or more standby servers.

Before You Begin

At first, you need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using kind.

Now, install KubeDB cli on your workstation and KubeDB operator in your cluster following the steps here.

To keep things isolated, this tutorial uses a separate namespace called demo throughout this tutorial.

$ kubectl create ns demo
namespace/demo created

Note: YAML files used in this tutorial are stored in docs/examples/postgres folder in GitHub repository kubedb/docs.

Create PostgreSQL with Streaming replication

The example below demonstrates KubeDB PostgreSQL for Streaming Replication

apiVersion: kubedb.com/v1alpha2
kind: Postgres
metadata:
  name: ha-postgres
  namespace: demo
spec:
  version: "13.2"
  replicas: 3
  storageType: Durable
  storage:
    storageClassName: "standard"
    accessModes:
    - ReadWriteOnce
    resources:
      requests:
        storage: 1Gi

In this examples:

• This Postgres object creates three PostgreSQL servers, indicated by the replicas field.
• One server will be primary and two others will be warm standby servers, default of spec.standbyMode

What is Streaming Replication

Streaming Replication allows a standby server to stay more up-to-date by shipping and applying the WAL XLOG records continuously. The standby connects to the primary, which streams WAL records to the standby as they’re generated, without waiting for the WAL file to be filled.

Streaming Replication is asynchronous by default. As a result, there is a small delay between committing a transaction in the primary and the changes becoming visible in the standby.

Streaming Replication setup

Following parameters are set in postgresql.conf for both primary and standby server

wal_level = replica
max_wal_senders = 99
wal_keep_segments = 32

Here,

• wal_keep_segments specifies the minimum number of past log file segments kept in the pg_xlog directory.

And followings are in recovery.conf for standby server

standby_mode = on
trigger_file = '/tmp/pg-failover-trigger'
recovery_target_timeline = 'latest'
primary_conninfo = 'application_name=$HOSTNAME host=$PRIMARY_HOST'

Here,

• trigger_file is created to trigger a standby to take over as primary server.
• $PRIMARY_HOST holds the Kubernetes Service name that targets primary server

Now create this Postgres object with Streaming Replication support

$ kubectl create -f https://github.com/kubedb/docs/raw/v2021.12.21/docs/examples/postgres/clustering/ha-postgres.yaml
postgres.kubedb.com/ha-postgres created

KubeDB operator creates three Pod as PostgreSQL server.

$ kubectl get pods -n demo --selector="app.kubernetes.io/instance=ha-postgres" --show-labels
NAME            READY   STATUS    RESTARTS   AGE   LABELS
ha-postgres-0   1/1     Running   0          20s   controller-revision-hash=ha-postgres-6b7998ccfd,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=ha-postgres,kubedb.com/role=primary,statefulset.kubernetes.io/pod-name=ha-postgres-0
ha-postgres-1   1/1     Running   0          16s   controller-revision-hash=ha-postgres-6b7998ccfd,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=ha-postgres,kubedb.com/role=replica,statefulset.kubernetes.io/pod-name=ha-postgres-1
ha-postgres-2   1/1     Running   0          10s   controller-revision-hash=ha-postgres-6b7998ccfd,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=ha-postgres,kubedb.com/role=replica,statefulset.kubernetes.io/pod-name=ha-postgres-2

Here,

• Pod ha-postgres-0 is serving as primary server, indicated by label kubedb.com/role=primary
• Pod ha-postgres-1 & ha-postgres-2 both are serving as standby server, indicated by label kubedb.com/role=replica

And two services for Postgres ha-postgres are created.

$ kubectl get svc -n demo --selector="app.kubernetes.io/instance=ha-postgres"
NAME                   TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)    AGE
ha-postgres            ClusterIP   10.102.19.49   <none>        5432/TCP   4m
ha-postgres-replicas   ClusterIP   10.97.36.117   <none>        5432/TCP   4m

$ kubectl get svc -n demo --selector="app.kubernetes.io/instance=ha-postgres" -o=custom-columns=NAME:.metadata.name,SELECTOR:.spec.selector
NAME                   SELECTOR
ha-postgres            map[app.kubernetes.io/name:postgreses.kubedb.com app.kubernetes.io/instance:ha-postgres kubedb.com/role:primary]
ha-postgres-replicas   map[app.kubernetes.io/name:postgreses.kubedb.com app.kubernetes.io/instance:ha-postgres]

Here,

• Service ha-postgres targets Pod ha-postgres-0, which is primary server, by selector app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=ha-postgres,kubedb.com/role=primary.
• Service ha-postgres-replicas targets all Pods (ha-postgres-0, ha-postgres-1 and ha-postgres-2) with label app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=ha-postgres.

These standby servers are asynchronous warm standby server. That means, you can only connect to primary sever.

Now connect to this primary server Pod ha-postgres-0 using pgAdmin installed in quickstart tutorial.

Connection information:

• Host name/address: you can use any of these

  • Service: ha-postgres.demo
  • Pod IP: ($kubectl get pods ha-postgres-0 -n demo -o yaml | grep podIP)

• Port: 5432

• Maintenance database: postgres

• Username: Run following command to get username,

  $ kubectl get secrets -n demo ha-postgres-auth -o jsonpath='{.data.\POSTGRES_USER}' | base64 -d
  postgres
  

• Password: Run the following command to get password,

  $ kubectl get secrets -n demo ha-postgres-auth -o jsonpath='{.data.\POSTGRES_PASSWORD}' | base64 -d
  MHRrOcuyddfh3YpU
  

You can check pg_stat_replication information to know who is currently streaming from primary.

postgres=# select * from pg_stat_replication;

Here, both ha-postgres-1 and ha-postgres-2 are streaming asynchronously from primary server.

Lease Duration

Get the postgres CRD at this point.

$ kubectl get pg -n demo   ha-postgres -o yaml
apiVersion: kubedb.com/v1alpha2
kind: Postgres
metadata:
  creationTimestamp: "2019-02-07T12:14:05Z"
  finalizers:
  - kubedb.com
  generation: 2
  name: ha-postgres
  namespace: demo
  resourceVersion: "44966"
  selfLink: /apis/kubedb.com/v1alpha2/namespaces/demo/postgreses/ha-postgres
  uid: dcf6d96a-2ad1-11e9-9d44-080027154f61
spec:
  authSecret:
    name: ha-postgres-auth
  leaderElection:
    leaseDurationSeconds: 15
    renewDeadlineSeconds: 10
    retryPeriodSeconds: 2
  podTemplate:
    controller: {}
    metadata: {}
    spec:
      resources: {}
  replicas: 3
  storage:
    accessModes:
    - ReadWriteOnce
    dataSource: null
    resources:
      requests:
        storage: 1Gi
    storageClassName: standard
  storageType: Durable
  terminationPolicy: Halt
  version: "10.2"-v5
status:
  observedGeneration: 2$4213139756412538772
  phase: Running

There are three fields under Postgres CRD’s spec.leaderElection. These values defines how fast the leader election can happen.

• leaseDurationSeconds: This is the duration in seconds that non-leader candidates will wait to force acquire leadership. This is measured against time of last observed ack. Default 15 secs.
• renewDeadlineSeconds: This is the duration in seconds that the acting master will retry refreshing leadership before giving up. Normally, LeaseDuration * 2 / 3. Default 10 secs.
• retryPeriodSeconds: This is the duration in seconds the LeaderElector clients should wait between tries of actions. Normally, LeaseDuration / 3. Default 2 secs.

If the Cluster machine is powerful, user can reduce the times. But, Do not make it so little, in that case Postgres will restarts very often.

Automatic failover

If primary server fails, another standby server will take over and serve as primary.

Delete Pod ha-postgres-0 to see the failover behavior.

kubectl delete pod -n demo ha-postgres-0

$ kubectl get pods -n demo --selector="app.kubernetes.io/instance=ha-postgres" --show-labels
NAME            READY     STATUS    RESTARTS   AGE       LABELS
ha-postgres-0   1/1       Running   0          10s       controller-revision-hash=ha-postgres-b8b4b5fc4,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=ha-postgres,kubedb.com/role=replica,statefulset.kubernetes.io/pod-name=ha-postgres-0
ha-postgres-1   1/1       Running   0          52m       controller-revision-hash=ha-postgres-b8b4b5fc4,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=ha-postgres,kubedb.com/role=primary,statefulset.kubernetes.io/pod-name=ha-postgres-1
ha-postgres-2   1/1       Running   0          51m       controller-revision-hash=ha-postgres-b8b4b5fc4,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=ha-postgres,kubedb.com/role=replica,statefulset.kubernetes.io/pod-name=ha-postgres-2

Here,

• Pod ha-postgres-1 is now serving as primary server
• Pod ha-postgres-0 and ha-postgres-2 both are serving as standby server

And result from pg_stat_replication

postgres=# select * from pg_stat_replication;

You can see here, now ha-postgres-0 and ha-postgres-2 are streaming asynchronously from ha-postgres-1, our primary server.

Streaming Replication with hot standby

Streaming Replication also works with one or more hot standby servers.

apiVersion: kubedb.com/v1alpha2
kind: Postgres
metadata:
  name: hot-postgres
  namespace: demo
spec:
  version: "13.2"
  replicas: 3
  standbyMode: Hot
  storageType: Durable
  storage:
    storageClassName: "standard"
    accessModes:
    - ReadWriteOnce
    resources:
      requests:
        storage: 1Gi

In this examples:

• This Postgres object creates three PostgreSQL servers, indicated by the replicas field.
• One server will be primary and two others will be hot standby servers, as instructed by spec.standbyMode

hot standby setup

Following parameters are set in postgresql.conf for standby server

hot_standby = on

Here,

• hot_standby specifies that standby server will act as hot standby.

Now create this Postgres object

$ kubectl create -f https://github.com/kubedb/docs/raw/v2021.12.21/docs/examples/postgres/clustering/hot-postgres.yaml
postgres "hot-postgres" created

KubeDB operator creates three Pod as PostgreSQL server.

$ kubectl get pods -n demo --selector="app.kubernetes.io/instance=hot-postgres" --show-labels
NAME             READY     STATUS    RESTARTS   AGE       LABELS
hot-postgres-0   1/1       Running   0          1m        controller-revision-hash=hot-postgres-6c48cfb5bb,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=hot-postgres,kubedb.com/role=primary,statefulset.kubernetes.io/pod-name=hot-postgres-0
hot-postgres-1   1/1       Running   0          1m        controller-revision-hash=hot-postgres-6c48cfb5bb,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=hot-postgres,kubedb.com/role=replica,statefulset.kubernetes.io/pod-name=hot-postgres-1
hot-postgres-2   1/1       Running   0          48s       controller-revision-hash=hot-postgres-6c48cfb5bb,app.kubernetes.io/name=postgreses.kubedb.com,app.kubernetes.io/instance=hot-postgres,kubedb.com/role=replica,statefulset.kubernetes.io/pod-name=hot-postgres-2

Here,

• Pod hot-postgres-0 is serving as primary server, indicated by label kubedb.com/role=primary
• Pod hot-postgres-1 & hot-postgres-2 both are serving as standby server, indicated by label kubedb.com/role=replica

These standby servers are asynchronous hot standby servers.

That means, you can connect to both primary and standby sever. But these hot standby servers only accept read-only queries.

Now connect to one of our hot standby servers Pod hot-postgres-2 using pgAdmin installed in quickstart tutorial.

Connection information:

• Host name/address: you can use any of these

  • Service: hot-postgres-replicas.demo
  • Pod IP: ($kubectl get pods hot-postgres-2 -n demo -o yaml | grep podIP)

• Port: 5432

• Maintenance database: postgres

• Username: Run following command to get username,

  $ kubectl get secrets -n demo hot-postgres-auth -o jsonpath='{.data.\POSTGRES_USER}' | base64 -d
  postgres
  

• Password: Run the following command to get password,

  $ kubectl get secrets -n demo hot-postgres-auth -o jsonpath='{.data.\POSTGRES_PASSWORD}' | base64 -d
  ZZgjjQMUdKJYy1W9
  

Try to create a database (write operation)

postgres=# CREATE DATABASE standby;
ERROR:  cannot execute CREATE DATABASE in a read-only transaction

Failed to execute write operation. But it can execute following read query

postgres=# select pg_last_xlog_receive_location();
 pg_last_xlog_receive_location
-------------------------------
 0/7000220

So, you can see here that you can connect to hot standby and it only accepts read-only queries.

Cleaning up

To cleanup the Kubernetes resources created by this tutorial, run:

$ kubectl patch -n demo pg/ha-postgres pg/hot-postgres -p '{"spec":{"terminationPolicy":"WipeOut"}}' --type="merge"
$ kubectl delete -n demo pg/ha-postgres pg/hot-postgres

$ kubectl delete ns demo

Next Steps

• Monitor your PostgreSQL database with KubeDB using built-in Prometheus.
• Monitor your PostgreSQL database with KubeDB using Prometheus operator.
• Want to hack on KubeDB? Check our contribution guidelines.