Ditto (we had hand-rolled a coroutine system implemented in the most pedestrian manner you could imagine). Compared to the usual sea-of-threads and all the sync

Ditto (we had hand-rolled a coroutine system implemented in the most pedestrian manner you could imagine). Compared to the usual sea-of-threads and all the synchronization hell that falls out of that, a coroutine-based system was absolutely magical. Vastly fewer threading bugs. Being able to meaningfully support 500K users on a single mid-grade server was also great for the pocketbook. (When I saw the sheer scale of the Twitter servers a while back, I was gasping for air).