Most apps stop the moment the signal drops. You tap a button, a spinner turns, and your work is gone. Some apps keep going: you can still read, write and save, and everything catches up later. This article explains how those apps are built, in plain words first and in engineering detail after.

What an offline app is, and what it is made of

An offline app is an app that keeps its most important features working when there is no internet. It does not pretend the internet is there. It saves your work on the device, tells you honestly that it has not been sent yet, and sends it when the connection comes back.

People often call this "offline-first". The idea is simple: the app talks to the device first and to the server second. The server is still the final record, but the screen never waits for it.

An offline app is made of these parts:

A local database on the device. The app keeps its own copy of the data it needs, on the phone or in the browser. On a phone this is often SQLite.…