fix(likes): count unique visitors, not duplicate rows#89
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Parallel setLike races could leave multiple likes rows for one ipHash. Reads used row count (and backfill incremented per row), inflating counts. Dedup decrements could also desync the denormalized counter. Count distinct ipHashes, sync the counter after each mutation, and rebuild backfill totals per URL from unique visitors. Co-authored-by: Injoon Oh <injoon5@icloud.com>
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Bug and impact
Parallel
setLikemutations could leave multiplelikesrows for the sameurl+ipHash. The public count used row count (likes.lengthand backfillincrementLikeCountper row), so one visitor could inflate the displayed total (e.g. 2–3×). Dedup logic also calleddecrementLikeCountper removed duplicate, which could desync the denormalized counter.Root cause
The denormalized likes work in #85 counted table rows instead of distinct visitors. The earlier race fix branch (
4a4e7fc) usedSet(ipHash)for reads; the counter migration regressed to row-based counting.Fix
ipHashper URL for pre-backfill reads (uniqueLikeCount)syncLikeCountForUrlafter eachsetLikeso the denormalized counter matches realityValidation
scripts/like-unique-count-test.mjs(parallel POSTs; global count must not move by more than 1 for one visitor)Note: Production may still show stale inflated counts until affected pages get a like toggle or an admin re-runs
backfill.run.