Strict Synchronous Replication Mode

The replication.mode strict-sync option instructs to create one or more cluster members as synchronous replicas. As indicated in the CRD reference the difference with the simple synchronous replication mode is that this option sets the Patroni synchronous_mode_strict and prevents from switching off the synchronous replication on the primary when no synchronous standby candidates are available. As a downside, the Leader is not enabled for writes (unless the Postgres transaction explicitly turns off synchronous_mode), blocking all client write requests until at least one synchronous replica comes up.

Setting up a Cluster with strict-sync replica

Move forward to the next item, creating a synchronous replication cluster:

$ cat << EOF | kubectl apply -f -
apiVersion: stackgres.io/v1
kind: SGCluster
metadata:
  namespace: failover
  name: sync-cluster
spec:
  postgres:
    version: '16.1'
  instances: 4
  sgInstanceProfile: 'size-s'
  pods:
    persistentVolume:
      size: '10Gi'
  configurations:
    sgPostgresConfig: 'pgconfig1'
    sgPoolingConfig: 'poolconfig1'
  prometheusAutobind: true
  nonProductionOptions:
    disableClusterPodAntiAffinity: true
  replication:
    mode: strict-sync
    role: ha-read
    syncInstances: 2
EOF        

$ kubectl -n failover exec -it sync-cluster-0 -c patroni -- patronictl list 
+ Cluster: sync-cluster (7373750354182599290) -----+-----------+----+-----------+
| Member         | Host             | Role         | State     | TL | Lag in MB |
+----------------+------------------+--------------+-----------+----+-----------+
| sync-cluster-0 | 10.244.0.11:7433 | Leader       | running   |  2 |           |
| sync-cluster-1 | 10.244.0.10:7433 | Sync Standby | streaming |  2 |         0 |
| sync-cluster-2 | 10.244.0.4:7433  | Sync Standby | streaming |  2 |         0 |
| sync-cluster-3 | 10.244.0.8:7433  | Replica      | streaming |  2 |         0 |
+----------------+------------------+--------------+-----------+----+-----------+