To handle 25k qps with Rails, assuming 500ms per request, you need 12k Ruby processes. That probably means you run on 200 boxes with 64 CPUs each. Maybe you get

To handle 25k qps with Rails, assuming 500ms per request, you need 12k Ruby processes. That probably means you run on 200 boxes with 64 CPUs each. Maybe you get a little bit of oversubscribing of processes to CPUs due to I/O. Conservatively it's still 100 boxes. To handle 25k qps with Rust, you can run 10 boxes. Maybe less. Deployments are complicated (even when we wrap them in simple abstractions). A "rare" delay that impacts 1% of boxes during a deploy will become common on 100 boxes but still somewhat rare on only 10. Running a single rust process per box allows a connection pooler to be in-process. It's more efficient, it supports postgres prepared statements, it's one less operational headache, etc.