#6 The Frontend Knows Nothing
The first GET request delivers a UI5 app to the browser. Worth asking what is actually inside it.
Not the screen. Not the fields, not the table, not the buttons, and not what any of them do. The app that arrives is a shell: enough UI5 to take a view and a model from the server, render them, and send events back. It cannot tell whether it is currently showing a table, a form or a wizard, and it has no idea which action follows the next click. Open the network tab and you can watch it find out.
Everything it needs arrives per request — the two strings of #5. Everything it decides is decided on the server.
The pattern has a name: a Hypermedia-Driven Application. Between the multi-page app, where every interaction fetches a whole document, and the single-page app, which holds routes and state in the browser and needs a rebuild to change either, an HDA renders what the server sends and knows nothing else. It is the sweet spot those two leave open, and it is where htmx, Hotwire and LiveView all sit.
Multi-page, single-page, hypermedia-driven — and what each has to rebuild.
If that division of labor feels oddly familiar, it should: it is SAP GUI's. PBO builds the screen, the user acts, PAI receives the event and decides what happens next. The dialog step moved to AJAX and the screen became an XML view, but the shape of the conversation did not change — which is why the flow reads as obvious to anyone who has written a module pool, and as faintly heretical to anyone who has only written SPAs.
The consequence is what matters here. There is one shell, and every app in the system shares it. Not one deployed frontend per app, each drifting to a different UI5 version, a different bootstrap, a different set of libraries, each pinned to whenever somebody last had time to touch it. One artifact to keep current, and every app is current with it.
Anyone who has ever hunted down which of forty deployed apps is still on an ancient UI5 version knows what that is worth.
A frontend that knows nothing about the app is a frontend that never needs to be redeployed when the app changes.