Single Page Applications (JSON over the wire) are applications that run on a single web page. After loading an HTML page and some JavaScript, they rely on Ajax ("Asynchronous JavaScript and XML") requests to pass JSON data objects between the server and client, via JavaScript and the Document Object Model (DOM) API to update the HTML page, while No need to reload the whole page.
In a hypermedia-driven application (HDA), the browser is limited to displaying HTML, processing JavaScript and CSS, but has no knowledge of the state of the application (that is, what happened before and what will happen next). The logic of the application is maintained entirely on the server. In contrast, in a single-page application (SPA), all routes and potential actions are predefined and implemented on the front end. Therefore, any modification to the application requires rebuilding the front-end application.
The idea of separation of concerns is not a high priority in HDA. CSS, JavaScript, and HTML are not completely separated, and the backend is not only responsible for data, but also responsible for generating UI and program flow. However, the advantage of this approach is that we can maintain and customize everything in one place, like we are used to in the past, for example in previous SAP GUI applications.
In the end, bringing all these concepts together, we believe that it is possible to develop applications of much lower complexity, but with only slightly lower UI fidelity compared to SPAs. Or, when we try to illustrate it visually, HDA applications are designed to find the "sweet spot" between MPA and SPA:
UI5 is very different from frameworks like htmx and unpoly. In a UI5 application, all logic is handled on the front end, while the back end is implemented using OData. This means that ABAP is only used to pass data and has no chance to implement its own logic or UI.