Can we create complex transactional FIORI apps using RAP?

[coishr]

We have to migrate ESS' webdynpro applications to UI5 Fiori apps. But our webdynpro applications are very complex and bulky. We are receiving a lot of input data in non-tabular format and then processing through complex logic. There are numerous screen level validations as well. Upon processing, the data is written down in multiple standard as well as custom tables. 

Can we create such complex Fiori apps using SAP RAP Model or we should follow some other model for this task. One approach is to create multiple ODATA services without CDS view and Annotations and then process the data in individual function modules. CDS/BOPF/RAP are appropriate for tiny applications where not much front end coding is required.

For example, can we create travel expense application using SAP RAP model ?

[thomas_mller13]

If you have a complicated validation logic and you want to encapsulate this logic in a ABAP model class, this class is not persistent over serveral roundtrips due to the REST API conecpt. Thus, the validation has to be made on client side, which is very bad, since all the information for validation is on server side. E.g. the concept of restful applications in the context of complicted business transactions with user interaction is a stupid idea. 

[coishr]

so what should be the approach to develop fiori apps for such scenarios? should we remove client side validations, take all the input from the user and send it to backend through multiple odata services. And process the input data in backend and display the result back to client ?

[Andre_Fischer]

It is not a stupid idea. The whole development of SAP S/4HANA is based on the ABAP RESTful application programming model now.