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DOC: config: prefer-last-server: add notes for non-deterministic algorithms

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Add some notes which load-balancing algorithm can be considered as
deterministic or non-deterministic and add some examples for each type.
This was asked via mailing list to clarify the usage of
prefer-last-server option.

This can be backported to all stable versions.
This commit is contained in:
Valentine Krasnobaeva 2025-06-17 13:33:12 +00:00 committed by Olivier Houchard
parent 8fc0d2fbd5
commit cdb2f8d780
1 changed files with 17 additions and 2 deletions
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19
doc/configuration.txt
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@ -10639,8 +10639,23 @@ no option prefer-last-server
desirable in these environments as well, to avoid redistributing the traffic
after every other response.
If this option has been enabled in a "defaults" section, it can be disabled
in a specific instance by prepending the "no" keyword before it.
It may be useful to precise here, which load balancing algorithms are
considered deterministic. Deterministic algorithms will always select the same
server for a given client data, assuming the set of available servers has not
changed. In general, deterministic algorithms involve hasing or lookups on the
incoming requests to choose the target server. However, this is not always the
case; "static-rr", for example, can be also considered as deterministic because
the server choice is based on the server's static weight, making the selection
predictable. "sticky" algorithm provides deterministic routing for the
returning clients.
As for non-deterministic algorithms, these algorithms select a server based on
dynamic server state or simple rotation, so two consecutive requests are not
guaranteed to land on the same server. option prefer-last-server is designed
specifically for these. roundrobin, leastconn are examples of such algorithms.
If this option has been enabled in a "defaults" section, it can be
disabled in a specific instance by prepending the "no" keyword before it.
See also: "option http-keep-alive"