Little's law — how many are inside

This line goes inside the server, starting with Timetable, a transit-times API. Riders' apps and the screens at stations ask it one thing all day: “When's my next train?”

Each question is a lookup. The Timetable API asks its timetable database for the next departures. Each lookup uses one database connection while it runs, then frees it for the next. The database reports how many connections are in use, second by second.

On a weekday afternoon about 200 lookups arrive a second. Each spends about 5 ms in the database, and riders have their answer in about 10 ms. Rush hour is on its way.