Tips and FAQs

Connection strings

PostgreSQL client libraries like libpq and jdbc support client-connection failover. The connection string contains multiple servers, for example, host=srv1,srv2. The client library loops over the available hosts to find a connection that's available and capable of read-write or read-only operations. This configuration allows clients to follow the primary cluster during a switchover.

Example:

psql "host=srv1,srv2 dbname=postgres user=admin target_session_attrs=read-write" -c "SELECT pg_is_in_recovery();"

Output

pg_is_in_recovery
-------------------
 f
(1 row)

psql "host=srv1,srv2 dbname=postgres user=admin target_session_attrs=read-only" -c "SELECT pg_is_in_recovery();"

Output

pg_is_in_recovery
-------------------
 t
(1 row)

VIP & Keepalived

When using multiple HAProxy instances and you need a way to decide which proxy host to connect to, the easiest solution is to use Keepalived and a VIP (virtual IP).

RHEL8 example

This example shows installing and configuring on RHEL8.

First, install Keepalived on each hosts running HAProxy:

sudo dnf install -y keepalived
sudo cp /etc/keepalived/keepalived.conf /etc/keepalived/keepalived.conf.bck

For this example, use 192.168.121.121 as the virtual IP. Configure Keepalived:

VIP=192.168.121.121
MY_IP=$(hostname -I | awk ' {print $1}')
DEVICE=$(ip -br address | grep $MY_IP | awk '{print $1}')
cat <<EOF | sudo tee /etc/keepalived/keepalived.conf
global_defs {
  vrrp_garp_master_delay 3
  vrrp_garp_master_repeat 4
  vrrp_garp_master_refresh 60
  vrrp_garp_master_refresh_repeat 4
}
vrrp_instance patroni {
  state BACKUP
  interface $DEVICE
  virtual_router_id 100
  priority 100
  advert_int 1
  virtual_ipaddress {
    $VIP
  }
  authentication {
    auth_type PASS
    auth_pass secret
  }
}
EOF

Configure the firewall if needed:

sudo firewall-cmd --add-rich-rule='rule protocol value="vrrp" accept' --permanent
sudo firewall-cmd --reload

Finally, enable and start the Keepalived service:

sudo systemctl enable keepalived
sudo systemctl start keepalived

Once started, the virtual IP is attached to one of the hosts. To verify, try to reach it:

ping -c 3 $VIP

Output

PING 192.168.121.121 (192.168.121.121) 56(84) bytes of data.
64 bytes from 192.168.121.121: icmp_seq=1 ttl=64 time=0.196 ms
64 bytes from 192.168.121.121: icmp_seq=2 ttl=64 time=0.410 ms
64 bytes from 192.168.121.121: icmp_seq=3 ttl=64 time=0.322 ms

--- 192.168.121.121 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2052ms
rtt min/avg/max/mdev = 0.196/0.309/0.410/0.087 ms

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