Count the views on a video or post. It sounds like count++; it is one of the harder write-heavy distributed-systems problems in practice. The work is dominated by three pressures the naive version ignores: dedup (the same viewer reloading must not inflate the number), bot / invalid-traffic filtering (a large fraction of raw "views" are not humans, and the public number must be reported net of them), and batched aggregation (you cannot synchronously increment a row per view at 250k views/sec — you log an idempotent event and roll it up asynchronously). The system is also read-dominated for the display of the count and hot-keyed on the write of a viral video, so it pulls in opposite directions at once.

This canonical models the main flows: the ingest/write path, the read path that displays the count, the speed-layer aggregation that produces the fast approximate number, and the batch "verified-views" layer that produces the authoritative one — the layer behind YouTube's famous 301+ freeze and the reason counts sometimes go down.