Sharded counters

How this question breaks down

1. 1Hot countersCore19 minWhy a single counter row stops scaling when thousands of writers increment it at once, and how to tell a hot counter from a merely busy one.
2. 2Shard and sumCore28 minSplit a hot counter into N sub-counters that take writes in parallel, then add them up for reads, sizing N from the burst and keeping reads cheap with a rolled-up total.

Work through the parts in order and you’ve covered the full design. Each part is one topic.