Momen, this approach is still valid:
- In back-end ECC ABAP server I use t-code SEGW to create custom oData service.
- In Front-end NW 7.4 server I register the back-end custom oData service using t-code /n/IWFND/MAINT_SERVICE
- The custom oData service is used for a custom developed Fiori app in front-end NW.
On top, you have the option to utilize S4HANA optimizations by using so called CDS (core data services) views which also integrate to SEGW. I don't think using xsodata with direct access to HDB is the recommended approach.