Working with no signal

Trace assumes there is no connection. That is the normal case on a job site, not the exception.

What happens at the shutter

The file is written to the phone's storage, then a row is written to the phone's own database, and only then is a network call attempted. That order is deliberate: the photo exists the moment the shutter fires, whatever the connection is doing.

Nothing removes a queued photo until the server has confirmed it.

Retries

A failed upload backs off and tries again — quickly at first, then more slowly. If the app is killed mid-upload, the interrupted item is put back in the queue when it next starts.

Duplicates

Every capture carries an identifier generated on the phone at the moment of the shutter, and it is sent unchanged on every retry. If a response is lost on a dying connection and the app tries again, the server recognizes the identifier and returns the photo it already has. It cannot create a second copy.

Watching the queue

The app shows a per-photo state. In the office, the sidebar shows any device holding a backlog — because "did the documentation actually make it up" should never be a question anybody has to go looking for.