Local-first health data: what it means and why it matters

"Local-first" has become a term in software development circles, but its implications for health data are significant enough to be worth explaining clearly.

A local-first app stores your data on your own device first — not on a company's server. The app works without an internet connection. Your data belongs to you in a practical sense, not just a legal one.

How most health apps store your data

Most health apps follow the same architecture: you enter data in the app, it's sent to the company's servers, and a copy is kept in your device cache for performance. The authoritative copy lives on their infrastructure.

This means: if the company shuts down, you lose your data. If they're acquired, the acquirer gets your data. If they change their terms of service, you have to accept or lose access. If they're breached, your health data is in the breach.

None of this requires malicious intent — it's just what happens when someone else holds your data.

How local-first apps work

A local-first health app uses your browser's built-in storage APIs (localStorage or IndexedDB) to persist data directly on your device. There is no server component — the app doesn't have a backend to send data to.

Your data exists in your browser. When you open the app, it reads from local storage. When you save an entry, it writes to local storage. No network request is made. The app works identically offline.

What you give up

Local-first has real trade-offs:

• No automatic sync across devices. Your phone and laptop each have separate data unless you manually import/export.
• No cloud backup. If you lose your device without having exported a backup, your data is gone. (This is why good local-first apps make export easy and remind you to do it.)
• No sharing. You can't easily share data with a healthcare provider through the app — you export and send.

For most people tracking personal health symptoms, none of these are dealbreakers. You probably use one primary device for health tracking. You can export a backup monthly. You can email a report to your GP.

Ownership vs. access

The distinction that matters most: with a cloud app, you have access to your data — contingent on the company remaining operational and the terms remaining acceptable. With a local-first app, you own your data. It's a file on your device. You can copy it, move it, open it in any text editor, or import it into a different app entirely.

Health data is worth keeping for years. Symptom logs, medication histories, and cycle data have clinical value long after the period they were recorded. That data should be in a format you control.