ProxySQL is a high-performance SQL proxy, which runs as a daemon watched by a monitoring process. The process monitors the daemon and restarts it in case of a crash to minimize downtime.
The daemon accepts incoming traffic from MySQL clients and forwards it to backend MySQL servers.
The proxy is designed to run continuously without needing to be restarted. Most configurations can be done at runtime using queries similar to SQL statements in the ProxySQL admin interface. These include runtime parameters, server grouping, and traffic-related settings.
Here, we will consider ProxySQL configured for async replication. Even when a replica is broken/stopped, ProxySQL still routes connections to replicas. It can be overcome by setting the appropriate value for mysql-monitor_slave_lag_when_null