Hi, I'm trying to determine if haproxy can be configured to solve a rate limiting based problem I have. I believe that it can, but that I am not seeing how to put the configuration together to get it done. Here's what I'm trying to do:
I have a set of servers (backends) that can each handle a specific number of requests per second (the same rate for each backend). I'd like haproxy to accept requests and farm them out to these backends so that each request is sent to the first backend that isn't over its rate limit. If all backends are over their rate limits, ideally the client connection would just block and wait, but if haproxy has to return a rejection, I think I can deal with this. My first thought was to use frontend's rate-limit sessions, setting it to n*rate-limit where n is the number of backends I have to serve these requests. Additionally, those backends would be balanced round-robin. The problem with this is that if a backend falls out, the front end rate limit is then too high since there are less backends available than there were when it was originally configured. The only way I see that I could dynamically change the frontend rate-limit as backends rise and fall is to write something that watches the logs for rise/fall messages and uses the global rate limit setting via the haproxy socket. This might work, but the biggest drawback is that one instance of haproxy could only handle requests of a single rate limit, since modifications after starting would have to be global (not per frontend). I guess in other words, I am trying to apply rate limits to individual backend servers, and to have a front end cycle through all available backend servers until it either finds one that can handle the request, or exhausts them all, at which time it'd ideally just block and keep trying, or less ideally send some sort of failure/rejection to the client. I feel like there's a simple solution here that I'm not seeing. Any help is appreciated. Thanks! Andy
