HAProxy 1.7 have a very nice feature called Stream Processing Offload Engine (SPOE).
This feature makes it possible to handle some expensive checks in a dedicated external process or on a server(-farm).
The full documentation can be found here.
Due to the fact that I like the visual presentation of such a process I have created a picture how the SPOE works.
It looks a little bit complicated but when you have understand the concept behind the SPOE you will see that this could be a very powerful tool.