Hi Shyam,
These are the answers as per my understanding. hope it will clear some of the doubts
1) what is the difference between a Front end ABAP and backend ABAP server?
Ans: Both are technically ABAP servers. front end server is the one which act as hub for the odata services and UI5 pages. However the same can be achieved in the backend server also (netwever version should be greater than 7.31 for this). SAP recommends using a seperate server for frontend and odata services so that it can act as a hub to multiple backend systems and user will have one fiori launchpad with apps fetching data from different backend systems.
2)when UI5 and Fiori is a HTML5 based technology and The business logic is not part of Fiori UX , why the Front end application server is called a
ABAP front end server ?
Fiori applications are HTML5 applications using UI5 libraries. SAPUI5 applications can run on non abap systems also like SAP Netweaver portal or hana cloud portal.
However Fiori applications (which are also UI5 applications ) are packaged as BSP applications and are available in the front end components for the particular application.
Other than UI5 pages, Fiori application need odata services also to fetch data these services are also published and accessible from ABAP frontend service catalog.
3)does front end ABAP server have any relevance when the backend is a HANA system with HANA XS Application server ?
Same as above, irrespective of backend server, front end is yoru choice, fiori applications are available as abap components so you need abap frontend in that case.
Regards,
Sarbjeet Singh