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Run as Synchronous Replication Cluster

KubeDB supports Synchronous Replication for PostgreSQL Cluster. This tutorial will show you how to use KubeDB to run PostgreSQL database with Replication Mode as Synchronous.

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.

Configure Synchronous Replication Cluster

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.

Now, create Postgres crd specifying spec.streamingMode with Synchronous field.

$ kubectl apply -f https://github.com/kubedb/docs/raw/v2022.05.24/docs/examples/postgres/synchronous/postgres.yaml
postgres.kubedb.com/demo-pg created

Below is the YAML for the Postgres crd we just created.

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

By default, KubeDB create a Synchronous Replication where one Replica Postgres server out of all the replicas will be in sync with Current primary. And others are potential candidate to be in sync with primary if the synchronous replica failed in any case.

Let’s check in the postgres cluster that we have deployed. Now, exec into the current primary, in our case it is Pod demo-pg-0.

$ kubectl exec -it -n demo demo-pg-0 -c postgres  -- bash
bash-5.1$ psql
psql (14.2)
Type "help" for help.

postgres=# select application_name, client_addr, state, sent_lsn, write_lsn, flush_lsn, replay_lsn, sync_state from pg_stat_replication;
 application_name | client_addr |   state   | sent_lsn  | write_lsn | flush_lsn | replay_lsn | sync_state 
------------------+-------------+-----------+-----------+-----------+-----------+------------+------------
 demo-pg-1        | 10.244.0.22 | streaming | 0/5000060 | 0/5000060 | 0/5000060 | 0/5000060  | sync
 demo-pg-2        | 10.244.0.24 | streaming | 0/5000060 | 0/5000060 | 0/5000060 | 0/5000060  | potential

But Users can also configure a Synchronous replication cluster where all the replica are in sync with current primary. Let’s see how a user can do so, Users need to provide custom configuration with setting the config for synchronous_standby_names.

For example, If there are 3 nodes in a Postgres cluster where 1 node is a primary and other 2 are acting as replicas. In this scenario, We can set all the 2 replicas server as synchronous replica with the current primary. We need to provide synchronous_standby_names = 'FIRST 2 (*)' inside custom configuration. That`s all, Then you can see that all the replicas are configured as synchronous replica.

$ kubectl exec -it -n demo demo-pg-0 -c postgres  -- bash
bash-5.1$ psql
psql (14.2)
Type "help" for help.

postgres=# select application_name, client_addr, state, sent_lsn, write_lsn, flush_lsn, replay_lsn, sync_state from pg_stat_replication;
 application_name | client_addr |   state   | sent_lsn  | write_lsn | flush_lsn | replay_lsn | sync_state 
------------------+-------------+-----------+-----------+-----------+-----------+------------+------------
 demo-pg-1        | 10.244.0.22 | streaming | 0/5000060 | 0/5000060 | 0/5000060 | 0/5000060  | sync
 demo-pg-2        | 10.244.0.24 | streaming | 0/5000060 | 0/5000060 | 0/5000060 | 0/5000060  | sync

To know how to set custom configuration for postgres please check here.

synchronous_commit

remote_write: By default KubeDB Postgres uses remote_write for synchronous_commit, which is the least sufficient option for replication in terms of data preservation as it only guarantees that transaction was replicated over the network and saved into the standby’s WAL(write-ahead-log) without fsync. KubeDB is using it to ensure minimum latency.

remote_apply: which means that the transaction upon completion will be both: persisted to a durable storage and visible to a user on standby server(s). Note that this will cause much larger commit delays than other options.

on: is a quite safe option when dealing with synchronous replication. on which in context of synchronous replication might be better referred to as remote_flush. Commits will wait until replies from the current synchronous standby(s) indicate they have received the commit record of the transaction and flushed it to disk. Although the output of the transaction will not be immediately visible to the users on the standby server(s).

Cleaning up

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

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

kubectl delete ns demo

If you would like to uninstall KubeDB operator, please follow the steps here.

Next Steps

• Learn about backup and restore PostgreSQL database using Stash.
• Learn about initializing PostgreSQL with Script.
• 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.