Fixing Haproxy-Keepalived VIP Race Condition

To ensure both HAProxy instances are running, even if they do not have the floating IP, you need to configure HAProxy to bind to the virtual IP in a non-strict manner. Here are a few solutions to address your problem:

Solution 1: Configure HAProxy to Bind Non-Strictly

You can configure HAProxy to bind to the VIP even if it is not yet assigned to the interface. This can be done by setting transparent and interface options in HAProxy configuration.

Update your HAProxy configuration to include the transparent and interface options:

frontend http_front
    bind 172.22.21.36:80 transparent interface ens18
    ...

This will allow HAProxy to bind to the IP without it being present on the interface initially.

Solution 2: Dummy Interface with VIP

Create a dummy interface that always holds the VIP, ensuring HAProxy can start without issues. This interface will be overridden by keepalived when it assigns the VIP.

Create a dummy interface:

Add a dummy interface configuration on both nodes (e.g., ens19) and assign the VIP to it.

On Debian-based systems, you can add the following to /etc/network/interfaces:

auto dummy0
iface dummy0 inet static
   address 172.22.21.36
   netmask 255.255.255.0
   pre-up ip link add dummy0 type dummy
   post-down ip link del dummy0

On Red Hat-based systems, create a file /etc/sysconfig/network-scripts/ifcfg-dummy0 with:

DEVICE=dummy0
IPADDR=172.22.21.36
NETMASK=255.255.255.0
ONBOOT=yes

And then enable the dummy interface:

ip link add dummy0 type dummy
ip addr add 172.22.21.36/24 dev dummy0
ip link set dummy0 up

Ensure the dummy interface is up on boot:

 systemctl restart networking

Solution 3: Modify Keepalived to Start HAProxy

Ensure haproxy starts regardless of the VIP being present. You can modify your keepalived configuration to start haproxy if it's not running when the VIP is assigned.

Add start script for HAProxy:

Add a script to start haproxy if it's not running. Place the script at /usr/local/bin/start_haproxy.sh:
```
#!/bin/bash

if ! pgrep haproxy > /dev/null; then
    systemctl start haproxy
fi
```
Make sure the script is executable:
```
chmod +x /usr/local/bin/start_haproxy.sh
```
Modify keepalived configuration:

Update the notify_master and notify_backup sections to ensure haproxy starts as needed:

   vrrp_instance VI_01 {
       ...
       notify_master "/usr/local/bin/start_haproxy.sh && /usr/bin/echo 'Master Active' > /tmp/keepalived.state"
       notify_backup "/usr/local/bin/start_haproxy.sh && /usr/bin/echo 'Backup Active' > /tmp/keepalived.state"
       notify_fault "/usr/bin/echo 'Master-Backup Fault' > /tmp/keepalived.state"
   }

Summary

Non-strict binding: Adjust HAProxy configuration to bind in a transparent manner.
Dummy interface: Create a dummy interface holding the VIP to ensure HAProxy can start.
HAProxy start script: Ensure HAProxy starts regardless of VIP presence by modifying the keepalived configuration.

Choose the method that best fits your setup and preferences. The non-strict binding approach is often the simplest to implement.