What are presentation, application and database servers in
SAP R/3?

Answer Posted / kapil

Presentation Layer :
These components are responsible for the presentation of
data for users and the receiving of user entries. When web
browsers are used as user interfaces, HTML pages prepared in
the presentation layer are accessed by application programs
through the classes and interfaces of the Internet
Connection Framework (ICF), which wrap the HTML pages. The
presentation in a web browser is based on Web Dynpro ABAP
(MVC-based technology for creating platform-independent
web-based interfaces) and also still Business Server Pages
(BSP, HTML pages that incorporate server-side scripts
written directly in ABAP). When you use the SAP GUI that is
available in versions for MS Windows, Java and Web Browser,
user interfaces are displayed in windows that are based on
the (classical) dynpro technology. The layout of a dynpro
can contain a set of predefined screen elements or GUI
controls (wrapped by the classes of a Control Framework, CFW).

Application Layer :
Here, application programs are executed. The application
layer contains a kernel written primarily in C/C++ that
serves as a platform for the application programs. The
kernel of AS ABAP provides Virtual Machines for ABAP with
ABAP Objects (main programming interface of AS ABAP for
business applications), XSLT and ST (transformation
languages for transforming ABAP data to XML and vice versa),
and still JavaScript (obsolete, server-side scripting in
BSPs beside ABAP).

Persistence Layer :
Here, persistent data is held in a database. The database is
accessed by ABAP programs through the database interface of
AS ABAP that is subdivided into an Open SQL interface and a
native SQL interface. Open SQL is a subset of the Structured
Query Language (SQL) realized directly by ABAP statements.
Native SQL are database-specific SQL instructions that are
passed directly to the database system (either statically or
dynamically via ADBC)

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