HAProxy offers several options to stick to a session.
- always create cookie on its own using
cookie(alwaays set, bad if varnish in front) - prefix existing cookie using
cookie(but afaik no way to not set it) - reuse application session cookie
appsession(deprecated, do not use) - create a custom table with rules and values for stickiness
stick-table/stick(full flexibility!)
In Plone you probably want to stick to the current user or the language and in almost read-only intranets maybe to the a cluster of specific users (like of one department). Otherwise do not stick but do leastconn balancing.
The only way I know to achieve this is to use a stick ... rule.
Docs at https://cbonte.github.io/haproxy-dconv/configuration-1.6.html#4.2-stick%20match (and following stick*)