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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 minikube.

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

$ kubectl get ns demo
NAME    STATUS  AGE
demo    Active  5s

Note: Yaml files used in this tutorial are stored in docs/examples/postgres folder in github repository kubedb/cli.

Create PostgreSQL with Streaming replication

The example below demonstrates KubeDB PostgreSQL for Streaming Replication

apiVersion: kubedb.com/v1alpha1
kind: Postgres
metadata:
  name: ha-postgres
  namespace: demo
spec:
  version: 9.6
  replicas: 3
  storage:
    storageClassName: "standard"
    accessModes:
    - ReadWriteOnce
    resources:
      requests:
        storage: 50Mi

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.standby

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

$ kubedb create -f https://raw.githubusercontent.com/kubedb/cli/0.8.0-beta.2/docs/examples/postgres/clustering/ha-postgres.yaml
validating "https://raw.githubusercontent.com/kubedb/cli/0.8.0-beta.2/docs/examples/postgres/clustering/ha-postgres.yaml"
postgres "ha-postgres" created

KubeDB operator creates three Pod as PostgreSQL server.

$ kubectl get pods -n demo --selector="kubedb.com/name=ha-postgres" --show-labels
NAME            READY     STATUS    RESTARTS   AGE       LABELS
ha-postgres-0   1/1       Running   0          48s       kubedb.com/role=primary,kubedb.com/kind=Postgres,kubedb.com/name=ha-postgres,statefulset.kubernetes.io/pod-name=ha-postgres-0,controller-revision-hash=ha-postgres-69c84579bb
ha-postgres-1   1/1       Running   0          47s       kubedb.com/role=replica,kubedb.com/kind=Postgres,kubedb.com/name=ha-postgres,statefulset.kubernetes.io/pod-name=ha-postgres-1,controller-revision-hash=ha-postgres-69c84579bb
ha-postgres-2   1/1       Running   0          45s       kubedb.com/role=replica,kubedb.com/kind=Postgres,kubedb.com/name=ha-postgres,statefulset.kubernetes.io/pod-name=ha-postgres-2,controller-revision-hash=ha-postgres-69c84579bb

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="kubedb.com/name=ha-postgres"
NAME                  TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
ha-postgres           ClusterIP   10.106.9.219    <none>        5432/TCP   4m
ha-postgres-replicas  ClusterIP   10.104.95.105   <none>        5432/TCP   4m

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

Here,

• Service ha-postgres targets Pod ha-postgres-0, which is primary server, by selector kubedb.com/kind=Postgres,kubedb.com/name=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 kubedb.com/kind=Postgres,kubedb.com/name=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:

• 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
• database: postgres
• username: postgres

Run following command to get postgres superuser password

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

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.

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="kubedb.com/name=ha-postgres" --show-labels
NAME            READY     STATUS    RESTARTS   AGE       LABELS
ha-postgres-0   1/1       Running   0          9s        kubedb.com/role=replica,kubedb.com/kind=Postgres,kubedb.com/name=ha-postgres,statefulset.kubernetes.io/pod-name=ha-postgres-0,controller-revision-hash=ha-postgres-69c84579bb
ha-postgres-1   1/1       Running   0          6m        kubedb.com/role=primary,kubedb.com/kind=Postgres,kubedb.com/name=ha-postgres,statefulset.kubernetes.io/pod-name=ha-postgres-1,controller-revision-hash=ha-postgres-69c84579bb
ha-postgres-2   1/1       Running   0          6m        kubedb.com/role=replica,kubedb.com/kind=Postgres,kubedb.com/name=ha-postgres,statefulset.kubernetes.io/pod-name=ha-postgres-2,controller-revision-hash=ha-postgres-69c84579bb

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/v1alpha1
kind: Postgres
metadata:
  name: hot-postgres
  namespace: demo
spec:
  version: 9.6
  replicas: 3
  standbyMode: hot
  storage:
    storageClassName: "standard"
    accessModes:
    - ReadWriteOnce
    resources:
      requests:
        storage: 50Mi

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.standby

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

$ kubedb create -f https://raw.githubusercontent.com/kubedb/cli/0.8.0-beta.2/docs/examples/postgres/clustering/hot-postgres.yaml
validating "https://raw.githubusercontent.com/kubedb/cli/0.8.0-beta.2/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="kubedb.com/name=hot-postgres" --show-labels
NAME             READY     STATUS    RESTARTS   AGE       LABELS
hot-postgres-0   1/1       Running   0          25s       kubedb.com/role=primary,kubedb.com/kind=Postgres,kubedb.com/name=hot-postgres,statefulset.kubernetes.io/pod-name=hot-postgres-0,controller-revision-hash=hot-postgres-6799bc9d4
hot-postgres-1   1/1       Running   1          24s       kubedb.com/role=replica,kubedb.com/kind=Postgres,kubedb.com/name=hot-postgres,statefulset.kubernetes.io/pod-name=hot-postgres-1,controller-revision-hash=hot-postgres-6799bc9d4
hot-postgres-2   1/1       Running   0          23s       kubedb.com/role=replica,kubedb.com/kind=Postgres,kubedb.com/name=hot-postgres,statefulset.kubernetes.io/pod-name=hot-postgres-2,controller-revision-hash=hot-postgres-6799bc9d4

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:

• address: use Pod IP ($ kubectl get pods hot-postgres-2 -n demo -o yaml | grep podIP)
• port: 5432
• database: postgres
• username: postgres

Run following command to get postgres superuser password

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

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:

$ kubedb delete pg,drmn,snap -n demo --all --force
$ kubectl delete ns demo

Next Steps

• Setup Continuous Archiving in PostgreSQL using wal-g
• Monitor your PostgreSQL database with KubeDB using built-in Prometheus.
• Monitor your PostgreSQL database with KubeDB using CoreOS Prometheus Operator.
• Wondering what features are coming next? Please visit here.
• Want to hack on KubeDB? Check our contribution guidelines.