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The progressive adoption of OpenSSL 3 and its abysmal handshake performance has started to reveal situations where it simply isn't possible anymore to succesfully run health checks on many servers, because between the moment all the checks are started and the moment the handshake finally completes, the timeout has expired! This also has consequences on production traffic which gets significantly delayed as well, all that for lots of checks. While it's possible to increase the check delays, it doesn't solve everything as checks still take a huge amount of time to converge in such conditions. Here we take a different approach by permitting to enforce the maximum concurrent checks per thread limitation and implementing an ordered queue. Thanks to this, if a thread about to start a check has reached its limit, it will add the check at the end of a queue and it will be processed once another check is finished. This proves to be extremely efficient, with all checks completing in a reasonable amount of time and not being disturbed by the rest of the traffic from other checks. They're just cycling slower, but at the speed the machine can handle. One must understand however that if some complex checks perform multiple exchanges, they will take a check slot for all the required duration. This is why the limit is not enforced by default. Tests on SSL show that a limit of 5-50 checks per thread on local servers gives excellent results already, so that could be a good starting point.
The HAProxy documentation has been split into a number of different files for ease of use. Please refer to the following files depending on what you're looking for : - INSTALL for instructions on how to build and install HAProxy - BRANCHES to understand the project's life cycle and what version to use - LICENSE for the project's license - CONTRIBUTING for the process to follow to submit contributions The more detailed documentation is located into the doc/ directory : - doc/intro.txt for a quick introduction on HAProxy - doc/configuration.txt for the configuration's reference manual - doc/lua.txt for the Lua's reference manual - doc/SPOE.txt for how to use the SPOE engine - doc/network-namespaces.txt for how to use network namespaces under Linux - doc/management.txt for the management guide - doc/regression-testing.txt for how to use the regression testing suite - doc/peers.txt for the peers protocol reference - doc/coding-style.txt for how to adopt HAProxy's coding style - doc/internals for developer-specific documentation (not all up to date)