How to write an alert people read

Three rungs, and each one costs something

Normal is a notification in the shade and needs no permission at all. Important is a heads-up banner with a sound, if the subscriber allowed this channel that. Urgent may take over the screen, override Do Not Disturb and be read aloud — each granted separately, each refusable. The rung is a ceiling rather than an outcome, and it is also a budget: a channel that spends Urgent on a car-park closure loses the permission the same afternoon, from the only people who could have granted it.

A picture, a sound or a document, when words are not enough

A diverted bus route is a map. A boil-water notice is a paragraph of instructions people will re-read. An alert can carry an image, a sound file or a document alongside the text. Documents are stored exactly as written and re-rendered from a sanitised copy rather than being run as markup, so an attachment cannot turn into a script on somebody’s lock screen. The bytes are uploaded once and attached by reference, which keeps a send cheap however large the audience.

Three numbers, and the one we will not print

You get: how many devices could be reached at the moment you sent, how many phones reported opening the alert, and how many acknowledged it. What you do not get is a delivery rate, because there is no per-device delivery record anywhere in this product — that absence is exactly what makes sending to four million people cost the same as sending to four. Reads and acknowledgements are self-reports from handsets that chose to send one, and they are labelled as such rather than dressed up as receipts.