docs: document disable_surrogate_key - #141
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The option has been parsed since the souin v1.7.7 bump but is absent from the README, so it is effectively undiscoverable — in caddyserver#118 and caddyserver#127 it only surfaced through maintainer comments. It matters most on high-cardinality workloads, where maintaining the surrogate index dominates the cost of a cache store, so document both what it does and when reaching for it is worthwhile.
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This PR and all of the following text is written by Claude, after debugging a production incident as mentioned.
disable_surrogate_keyhas been parsed since the souin v1.7.7 bump (configuration.go→cfg.SurrogateKeyDisabled) but is not mentioned in the README, so it is effectively undiscoverable. In #118 and #127 it only surfaced through maintainer comments.It is the right escape hatch for anyone who never invalidates by surrogate key, and it matters most on high-cardinality workloads: maintaining the index is a read-modify-write of an index entry on every cache store, so the cost of storing a response grows with the number of responses already indexed. That is the mechanism behind #114, #118 and #127 — I hit it in production and wrote up the measurements in #127 (comment).
Setting it resolved the problem for me with no change to caching behaviour: responses are still stored and served from cache, only the surrogate bookkeeping is skipped.
This adds the option to the global options example and a short section covering what it does and when to reach for it.
Documented under Global Option Syntax only. The parser also accepts it in the per-directive
cache {}block, but I have only verified the global form in production, so I have not claimed the other.Related: darkweak/souin#840 fixes the underlying cause — responses carrying no surrogate-key header are currently all indexed under a single empty tag, which is what makes that index grow without bound.