Thank you very much for your response. It's interesting to learn that round-robin is considered a non-deterministic algorithm. We assumed it was deterministic because of its definition that requests are sent to the next backend/server one after the other. During our experiments with HAProxy 2.8.14, we could observe that this option has an impact on the load balancing.
Is it possible to improve the HAProxy documentation by clarifying which algorithms are deterministic and which are not? We have a further question regarding the http-keep-alive (https://www.haproxy.com/documentation/haproxy-configuration-manual/2-8r1/#4.2-option%20http-keep-alive) property. The documentation states it's enabled by default. If the client sends a request with the "Connection: Close" header, will HAProxy close the connection to the backend or keep it idle? Best regards, Tamara From: Valentine Krasnobaeva <vkrasnoba...@haproxy.com> Date: Tuesday, 10. June 2025 at 14:32 To: haproxy@formilux.org <haproxy@formilux.org>, Boehm, Tamara <tamara.bo...@sap.com> Subject: Re: prefer-last-server with round robin lb algo You don't often get email from vkrasnoba...@haproxy.com. Learn why this is important<https://aka.ms/LearnAboutSenderIdentification> Hi Tamara! Thanks for reporting this. Non, round-robin is not deterministic, thus compatible with prefer-last-server and this option should not be ignored. With which version are you observing this behaviour? "prefer-last-server" is mainly used for keepalive with servers as the doc states and I see, that you are also using "option srvtcpka". If you do tests with some external clients in parallel, make sure that these clients do not close its connections with haproxy between sending requests. Could you provide your configuration, if possible + could you describe in which conditions you are observing this ? As for doc, this info about load balancing alogorithms will be added explicitly. In general, all supported algorithms, where hashing is envolved are deterministic. Kind regards, On 03/06/2025 14:39, Boehm, Tamara wrote: Dear community, We are using round-robin in HAProxy as a load-balancing algorithm. In our backend config, we use prefer-last-server as an option. But according to the documentation (https://www.haproxy.com/documentation/haproxy-configuration-manual/latest/#4.2-option%20prefer-last-server), the option is only applied when the load balancing algorithm in use is not deterministic. We could not find any examples in the documentation of what load balancing algorithms HAProxy considers as deterministic. According to our understanding, the round robin algorithm is a deterministic one. Can you confirm whether our understanding is correct that this option prefer-last-server is fully ignored in our case? Our config looks like: backend http-routers-http1 mode http balance roundrobin option srvtcpka no option http-server-close option prefer-last-server Thanks a lot and best regards, Tamara -- Bien cordialement, Valentine
