This repository contains sample config files and scripts used for setting up a context aware MySQL replication pool on HAProxy.

A context aware pool is a pool of backend MySQL servers, such that each tells HAproxy whether it should be included in the pool or not.

HAProxy-wise, the pool is fixed, and MySQL server state changes are not reflected in its config. This setup is resilient to service haproxy reload/restart.

A backend server is able to tell HAProxy:

• I'm good to participate in a pool (HTTP 200)
• I'm in bad state; don't send traffic my way (HTTP 503)
• I'm in maintenance mode. No error on my side, but don't send traffic my way (HTTP 404)

The servers respond to an explicit request (check) type sent by HAProxy, such as /check-lag or /ignore-lag.

HAProxy uses a main backend pool and a backup backend pool, such that the backup pool is activated when the main pool runs out of capacity. HAProxy uses different check types for the two pools.