5.1 What an Application Is
An Atrium application is a directory of web files with a manifest. The contract has two independent axes rather than kinds:
- Where the UI comes from. Either the package ships static files
and the session host serves them (
entry, the default), or the frame points at a URL Atrium does not serve — an application that is its own web server, or someone else's. - What runs behind it. Nothing; commands the session runs on
request (
exec, §5.5); a terminal (pty, §5.5); or, in a later version, a long-running backend process.
The useful shapes fall out of the grid: a pure page (About), a wrapper app — static UI whose buttons run allowlisted commands, most of the Toolbox's natural contents — a terminal, and an embedded external web application.
Two properties hold across all of them:
- Everything an application does on the machine happens as the user, inside the session host's process tree and the session job's cgroup. The manifest's capabilities narrow what an application may ask the session to do; they never widen who it is.
- An application that loads the SDK is a citizen; one that does not still works. The SDK is how a frame gets the handshake, the theme, requests and shortcuts. A plain page without it is shown after a grace period and swallows global chords while focused — degraded, deliberately, rather than broken.