Interview notes: how I answered a question about the modding-books concurrency model
Most modding-books articles stop at "how to use it" and never cover "when not to use it". This is an attempt at the second half.
-- The query that broke: a full scan over 20M rows. -- A composite index took P99 from 1.8s down to 42ms. SELECT id, title, created_at FROM posts WHERE community_id = ? AND status = 1 ORDER BY score DESC LIMIT 20;
We also fixed monitoring along the way: replaced average-based alerts with percentiles and split them per endpoint. False alerts dropped by about seventy percent and the on-call rotation visibly cheered up.
On trade-offs, my view is this: if nobody on the team owns this area long-term, do not introduce a second mechanism. With two coexistence you first have to work out which one is even in play when things break, and that costs far more than the performance you saved.
What genuinely surprised me was the tail. The average looked great while P99 jumped by an order of magnitude past some threshold. The cause was not modding-books itself but our upstream connection reuse — the load test traffic was too clean and hid the long-tail requests.