Strava. Race day: thousands of activity uploads at once. Save a run or ride, and never lose it once it is accepted. Show the athlete their activity within a few seconds. Time it on every segment it covered and rank it, within tens of seconds. Put it on followers' feeds within tens of seconds.
Its design on a normal day: p99 152ms, 0% of requests failing, 598/s answered.
What Tailspin shows: Answering an upload at once and doing the heavy work later, from a log; Two readers of one log, each at its own pace: segments and feeds; The segment matcher as the hungriest step, and partitions capping it; A feed read as one list and a batch of cached summaries; Feed reads starving uploads when they share a database with no cache.
Following Design Strava, Engineering Atlas.