Create a read-only replica

To increase read throughput and availability, you can create an AlloyDB Omni replica server for an existing AlloyDB Omni server. A replica server provides a read-only clone of your primary database server. It continuously updates its own data to reflect changes to your primary server's data.

The following sections provide steps to configure your AlloyDB Omni primary server for replication, configure the replica server, and verify replication status on the primary server.

For details, see Install AlloyDB Omni on Kubernetes.

Configure your primary server for replication

Update the network or firewall settings of your primary server's host so that it allows ingress traffic from the replica server's host through the primary server's Postgres port.
Create a replication user:
```
docker exec CONTAINER_NAME psql -h localhost -U postgres -c "create user REPLICA_USER with replication password 'PASSWORD'"
```
Replace the following:
- REPLICA_USER: the name of the replica user.
- PASSWORD: the password for the replica user.
- CONTAINER_NAME: the name that you assigned to the AlloyDB Omni container when you installed it.
Add the following lines in the /DATA_DIR/pg_hba.conf file before host all all all scram-sha-256 that exists in the file:
```
host alloydbmetadata    alloydbmetadata   IP_RANGE trust
host replication        REPLICA_USER    IP_RANGE scram-sha-256
```
Replace the following:
- IP_RANGE: the IP range, in CIDR notation, of the subnet where your replica machine is located. For example, 203.0.113.0/24.
- DATA_DIR: the file system path you want this replica to use for its data directory.
Restart your primary server:
```
docker restart CONTAINER_NAME
```

Create and configure the replica server

To create and configure an AlloyDB Omni replica server, complete the following steps:

Ensure the replica has network connectivity to the primary server:
```
ping SOURCE_IP
```
Replace SOURCE_IP with the IP address of the primary AlloyDB Omni instance to replicate from.

Ensure the replica can connect to the primary server:

docker run -it --rm google/alloydbomni psql "host=SOURCE_IP user=REPLICA_USER replication=1" -c "IDENTIFY_SYSTEM;"

The output should look similar to the following:

       systemid       | timeline |  xlogpos  | dbname
 ---------------------+----------+-----------+--------
  7376500460465963036 |        1 | 0/454B670 |
 (1 row)

The replica must have a persistent storage location on disk:

 mkdir alloydb && docker run --rm -it \
   -e PGDATA=/var/lib/postgresql/data/pgdata \
   -v "$PWD/alloydb":/var/lib/postgresql/data \
   -p 5432:5432 \
   --user $(id -u):$(id -g) \
   google/alloydbomni \
   /usr/lib/postgresql/15/bin/pg_basebackup \
     --pgdata=/var/lib/postgresql/data/pgdata \
     --checkpoint=fast \
     --host="SOURCE_IP" \
     --port="SOURCE_PORT" \
     --username=REPLICA_USER \
     --create-slot \
     --write-recovery-conf \
     --slot="SLOT_NAME"

Replace the following:

SOURCE_PORT: the TCP port of the primary AlloyDB Omni instance to replicate from. If you don't provide this value, then AlloyDB Omni applies a default value of 5432.
SLOT_NAME: the label for the name of the replication slot.

Start your replica instance:

docker run --detach \
  --name pg-service \
  -e POSTGRES_PASSWORD=a \
  -e PGDATA=/var/lib/postgresql/data/pgdata \
  -v "$PWD/alloydb":/var/lib/postgresql/data \
  -v /dev/shm:/dev/shm \
  -p 5432:5432 \
  --user $(id -u):$(id -g) \
  google/alloydbomni

Verify replication status on the primary server

To verify replication is configured properly, run the following command on the primary server's host:

  docker exec -it CONTAINER_NAME psql -h localhost -U postgres -c "select * from pg_stat_replication"

The output table contains one row for every replica connected to your primary database server.

After you set up replication, all inserts, updates, and deletions to rows in your primary database server become readable on your replica within seconds.