The problem

Ines runs a climbing club of 480 members. Every Friday at 18:00 she posts the weekend meeting point in the club group, and within a minute 60 people reply with "in" or "out".

If her post reaches half the members five minutes late, ten people drive to the wrong crag. If a reply vanishes, someone loses their seat in a car. Members scroll back to last month to find the gear list.

You design the app that carries those messages for a million people a day.