View counter - counting a million clicks without a million writes

Why this design

Every video, post and article shows a number under it. Writing that number looks like one UPDATE per view, and the interview question is what goes wrong when one item gets 50,000 views a second. The answer is not "the database is too slow" - it is that every increment of the same row waits for the same row lock, so the hottest item is limited to a few thousand increments a second no matter how big the database is. The fix is to stop writing per view, and to be honest about what that costs.

What you will learn

• Why a hot row serialises writes, and why adding database replicas does nothing for it.
• How in-memory batching turns 50,000 increments a second on one row into 10.
• Exactly how many views a crash can lose, computed from the flush window.
• Why dedupe (one view per viewer per 30 seconds) is a separate, cheap store.
• Why the displayed counter and the analytics numbers are two different pipelines.

How to read this board

1. Top: COUNT A VIEW. Viewer, player, load balancer, the View API hex, the dedupe store and the red in-memory batcher.
2. Right: STORES. The counts table the batcher flushes into, and the count cache the read path uses.
3. Bottom: READ THE COUNT. The page asks for the number; it comes from cache, a few seconds stale.
4. Bottom right: ANALYTICS. Raw view events, a rollup job, and daily numbers per video.
5. Press play, then read pages 1 to 6, then the tables. The conversation to have is the red box.

Functional

• Count a view when a video starts playing (or after 3 seconds of play - a product rule, decide it once).
• Count at most one view per viewer per video per 30 seconds, so refresh-spamming does not inflate counts.
• Show the view count on the video page and in listings.
• Daily views per video for the creator dashboard.

Non-functional

• Recording a view must never slow playback: the beacon answers in under 10 ms and the player does not wait for it.
• The displayed count may lag reality by up to ~10 seconds.
• The displayed count may undercount by a small, bounded amount after a crash. It must never overcount.
• Handle 50,000 views a second on a single video without degrading other videos.
• If views are ever billable (paying creators per view), this board is not enough - see page 5.

Out of scope

• Bot and fraud filtering beyond the dedupe window.
• Watch time, likes and other counters - the same design repeated.