You add 24 more workers to fix Kafka lag — and they all sit idle. Meanwhile one huge message is stuck at the front of a partition, and nothing behind it moves.
“We sold 3 tickets but only had 1.” The code is inside a transaction, looks right, works a hundred times locally, and only breaks in production under load.
A beginner-friendly walkthrough: what “concurrent” means, what the C10K and C10M problems actually are, how the industry usually fixes them (event loops vs whole-system design), and what still goes wrong in production —…