My IoT company would like to use client certificate authentication to secure communications between each "thing" and a central server. We deploy about 30K things per year, and they have about a 5-year lifetime, so our server-side solution needs to be able to support 150-200k certificates at a time. From reading and asking other questions, it seems like the best-of-breed solution is EJBCA, which appears to scale quite well, but I also see that haproxy (theoretically) has the ability to do it as well.
My question is this: how well does haproxy scale to handle a large number of client certs and connections?