HAProxy health check

Question

I have three small questions that relate to the same topic:

1. With HAProxy, is there any way to load balance based on the current load of the different servers (ie. routing a request to the least loaded server).
2. If not, why isn't this behaviour built into HAProxy? Are there any major performance impacts of doing this? I imagine rather than for example, the instead of looking for the least connections (leastconn algorithm), that I'd be looking for the lowest load.
3. If there are no major performance impacts of doing this, is my assumption that one could build this functionality into a custom HAProxy distribution without affecting performance correct?

Answer 1

The only way to do this with haproxy is create a script that haproxy's health check checks. In the script you are checking the load average and if it's a valid load.. return a response code of 200. If it is too high return a 404 or 500 error. Your haproxy check would look something like

option httpchk HEAD /health_check.php HTTP/1.0

if haproxy sees any 2xx/3xx response it will be valid.. anything else it will take that node offline till the node's load goes down and the health_check.php scripts returns a 200