"Service upgrades are even more important because, while some products are designed from the ground to support config updates without having to be reloaded, they cannot be upgraded at all without a stop/start sequence. Consequently, admins leave bogus versions of those software components running in production because it is never the right time to do it."
All in all, HAproxy is a brilliant piece of software and Willy is running it in an exemplary way. Kudos!
I know that HAProxy is not an apples to apples comparison with something like apache, but I don't see how something similar would be a huge burden to add.
It's not that surprising to me, especially given that haproxy is 17 years old. Expectations of a load balancer where pretty light when it was invented, so the internals weren't built for hot reload.
Bingo. The internals will eventually be rewritten to support things like this (and I think that's what Willy was hinting at towards the end of the article) in time. But it's a big project, and it's a complicated piece of software, and there are a lot of conflicting demands on dev time.
That said: it's a welcoming community, and if you want to help... do it!
It reloads just fine, same as all software.
Or if you prefer the pessimistic version: HAProxy, nginx, Apache and Varnish all suck at reloading configurations.
The difference is that HAProxy 1) tests it and 2) have a TCP mode.
To quote the issue: They manage to achieve 1 error per 40 000 connections... only if pinning to specifics CPU (typical in high performance environment to achieve 100% usage of all cores) while doing 10 reloads per second and creating 80k new connections per second.
Do you think any of apache/nginx/varnish, would do better than that in these circumstances? If you do, you are not very realistic ;)
Surely you don't need to fork: Just parse the new config, create the necessary internal data structures, and let traffic flow into the new ruleset while keeping all the sockets (except for those that are superfluous, and of course let in-flight requests finish). Is it because HAProxy's internals weren't designed to do that and that it would too big of a rewrite?
I always found Varnish's design very cool: It compiles the configuration (which is a DSL called VCL) to C and loads it as a dynamically loaded library. I don't know how it does hot reloads, but I believe it does do them seamlessly.