feat(tags): tag-based cache invalidation#280
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Adds passive tag-based invalidation modeled on FusionCache: cache entries carry a set of tags, backends store invalidation timestamps per tag, and on read the FSM compares per-tag timestamps against the entry's creation time. No tag→keys index, no eager eviction — entries are treated as expired when any of their tags has a newer invalidation timestamp. FSM integration runs request-side tag invalidation prefetch in parallel with the cache read, attaches response-side tags at write time, and short-circuits the parallel fetch when no tags are configured (zero overhead for non-tag flows). Includes BDD coverage for invalidation behavior and backend-level tag round-trip / invalidate / invalidated tests across Moka, FeOxDb, and Redis.
Tag invalidation records previously shared the data cache's eviction budget. Under data write pressure, LRU could evict a tag's invalidation timestamp; the next read would then see no record and treat the entry as never invalidated, silently serving stale data. MokaBackend now keeps invalidation timestamps in a separate, unbounded Moka cache (no max_capacity, no expiry). invalidate / invalidated are overridden to use it directly instead of routing through write / read. Other backends are unaffected: FeOxDb has no eviction, Redis durability is operator-configured, and CompositionBackend already merges tag timestamps across layers. Adds a regression test asserting tag records survive heavy data eviction pressure.
Response tags were written onto CacheValue but never consulted on read.
PollCacheWithTagsFuture is consolidated into one future: cache read and
request-tag query run concurrently, then on a hit a lazy response-tag
query is merged into the same map the FSM scans.
Adds TagInvalidation { Check (default) | Skip } on EnabledCacheConfig.
Skip is a true off switch: cache_policy now takes Option<TE>, None
builds no extractor future at all. Tags are still written and
Backend::invalidate still records timestamps under Skip, so re-enabling
is safe.
Touches the cache_policy trait surface, both derive macros, and all
impls. Default behavior unchanged; existing suite passes.
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Adds passive tag-based invalidation modeled on FusionCache: cache entries carry a set of tags, backends store invalidation timestamps per tag, and on read the FSM compares per-tag timestamps against the entry's creation time. No tag→keys index, no eager eviction — entries are treated as expired when any of their tags has a newer invalidation timestamp.
FSM integration runs request-side tag invalidation prefetch in parallel with the cache read, attaches response-side tags at write time, and short-circuits the parallel fetch when no tags are configured (zero overhead for non-tag flows).
Includes BDD coverage for invalidation behavior and backend-level tag round-trip / invalidate / invalidated tests across Moka, FeOxDb, and Redis.