Create a read-only replica

Preview

This feature is subject to the "Pre-GA Offerings Terms" in the General Service Terms section of the Service Specific Terms. Pre-GA features are available "as is" and might have limited support. For more information, see the launch stage descriptions.

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Select a documentation version: 18.1.0

• 18.1.0

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.

Before you begin

Before you proceed, install and set up AlloyDB Omni using the RPM package. For more information, see Install AlloyDB Omni for Linux.

Configure your primary server for replication

1. 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.

2. Create a replication user:

   psql -h localhost -U postgres -c "create user REPLICA_USER with replication password 'PASSWORD';
   grant pg_read_all_settings to REPLICA_USER;"

3. Add the following lines in the /DATA_DIR/pg_hba.conf file of the primary server 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 data directory path of the primary server.

4. Restart your primary server:

   sudo systemctl restart alloydbomniPG_VERSION

Create and configure the replica server

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

1. 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.

2. Ensure the replica can connect to the primary server:

   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)
   

3. The replica must have a persistent storage location on disk:

   pg_basebackup \
     --pgdata=DATA_DIR \
     --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.
   • DATA_DIR: the data directory path for the replica server.

4. Start your replica instance:

   sudo systemctl start alloydbomniPG_VERSION

Verify replication status on the primary server

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

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.