The core of APEX is a collection of PL/SQL packages, written using the PL/SQL Web Toolkit, and several hundred database tables, in which all the metadata regarding developed applications are stored.
The PL/SQL Web Toolkit is a collection of Oracle supplied database packages, which allow us to produce Web page code, directly in the database, using PL/SQL stored procedures. During run-time, and in real-time, the APEX engine queries the metadata tables and retrieves all the relevant information for a specific application page. Next, it uses the PL/SQL Web Toolkit to generate the application page's HTML code. This page's code also includes the proper linkage to the CSS and JavaScript resources that support proper page functionality, layout, and styles. Some internal tests run by the APEX development team, showed that the APEX engine performs all the real-time rendering processes, with an average overhead of less than 0.04 seconds per page. Pretty impressive in my opinion, and the APEX engine includes a server side caching mechanism, that can help us reduce that further.
It is important to understand that although the final product of APEX Application Builder is an HTML page code, no static files with any related HTML file extensions, are stored on the server. Each page is created upon demand a specific URI request. That, of course, gives us considerable flexibility with constructing the pages, e.g. the same page can be displayed somewhat differently for various users, or include conditioned components, depending on the user's security privileges, or role in the company, etc.
So, on one side we have the APEX engine, a collection of PL/SQL packages, running inside an Oracle database, and on the other side we have the application user, using a Web browser. How can we connect, and communicate between the sides? As we are in a Web environment, we are going to use the HTTP protocol, implemented in two technologies.
The first way is by using the Oracle HTTP Server (OHS) technology. OHS is based on the well-known (open source) Apache HTTP server, in its 1.3 or 2.0 versions. What interests us, for the APEX environment, is the Oracle plug-in module for this HTTP server, called mod_plsql.
OHS, with the mod_plsql module, is located between the client's Web browser and the Oracle database server. The mod_plsql module communicates with the database by mapping the Web browser request into PL/SQL stored procedures in the database. In turn, the PL/SQL stored procedures can manipulate the data in the database tables and generate HTTP responses, which can include HTML code, to be displayed on the client Web browser.
In order to communicate with the database, mod_plsql is using a DAD (Database Access Descriptor) file, which contains configuration parameters on how to connect with the database, which user and password to use, NLS parameters, and others.
The second technology to implement the HTTP communication protocol utilizes the Embedded PL/SQL Gateway running on the XML DB HTTP server, which is an integrated feature of the Oracle Database. Using the DBMS_EPG package, the Embedded PL/SQL Gateway can implement the core functionality of the OHS mod_plsql module, without installing it. This technology was first supported by APEX in the embedded version (2.1) of the Oracle XE database. Today, it's also supported by APEX 3.0 and above versions, running on Oracle 11g databases. APEX 3.0, which uses the embedded PL/SQL gateway, is installed by default as part of the 11g database installation process, and can be upgraded to any higher APEX version.
