The targets endpoint runs >1 s cold — pulls 30 days of measurements
for every type plus medications, intakes, mood and glucose to build
the consistency strips, streaks and target-met-this-week rollup. It's
read on every Insights mount and on every sub-page click, so the
cumulative cost dominates a power user's session.
Wrap the body in the existing analytics-style server cache with a 60 s
TTL and a per-user key. The invalidation helpers for measurement,
mood and medication writes evict the user bucket so the next read
paints fresh data without the cache lagging the underlying state.
The bfcache-friendly Cache-Control header lets a back-forward
navigation restore from the BFCache instead of repaying the cost.
Extends the v1.4.33 Coach snapshot LRU shape into a reusable
ServerCache<T> with TTL expiry, capacity-bounded LRU eviction,
single-flight read coalescing, prefix delete, and per-instance
hit/miss/eviction/stampede counters. Adds five invalidation helpers
keyed by the per-user prefix scheme so write endpoints can flush every
cache that may reflect their changed rows in a single call.
The cached() wrapper attaches cache.<name>.outcome + cache.<name>.key_hash
to the active wide event so production logs carry the hit ratio without
leaking the userId portion of the cache key.
Hand-rolled to keep the dependency surface at zero — same recipe as the
snapshot cache the dashboard already relies on. Process-local; multi-
instance staleness is bounded by TTL, the Redis migration path stays a
future option.
Marc-André Bombeck