When we often talk about ABAP Application Server, generally the kind of impresison or the focus people have in just on the ABAP/ABAP Objects . But here I would like to take a holistic approach and will discuss how the one layer is stack on top of the other layer.
You all might be heard of the open SQL and the native SQL i.e the database interaface . This eases our ABAP development by directly embedding the SQL inline with code.But for this there is seperate layer of services which are working in backgroung to supply us with such a fantastic and easy way to interact with databases. This is whole layer which sits on top of the open and native SQL layer , known as Object Services which was added in recent releases.This layer utilises only the native SQL but exposes them in object oriented manner , things you normally do with open SQL like delete, modification ..updates and thus directly connects to ABAP / ABAP OO layer.
Next to ABAP / ABAP OO layer , there is XSLT/ST layer. This idea behind creating this XSLT/ST processor is for execution of standard base XSLT (Extended Tranformation )or own Proprietary ST ( Simple Tranformations )and important for people who wants – to and from – transformation between datatypes of ABAP and XML , which was well required for Web Services also.
Moving up we have the comunication Framework i.e the Control framework which sits on the RFC framework of SAP GUI and allow us to enjoy the controls on our classic dynpro screens .. majoritiy of you might have worked on CL_GUI classes which tie it to the SAP GUI smart client. The another framwrok which runs on the same layer is the ICF (Internet Communication Framework) which sit on top of the ICM (Internet Communication Manager) . The ICM works over HTTP and HTTPs and thus open the path to have new UI technologies like BSP and Webdynpro , help in rendering the screens on Web Browsers. BSP(Business Server Pages) UI model is the first attempt to create web UIs using HTTP connector. It only support the UI programming with ABAP and not in the JAVA . But on the other side Webdynpro provides Java and ABAP stack for UI programming.