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// @author Sven van der Meer (sven.van.der.meer@ericsson.com)
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== Introduction to APEX Engine and Applications
The core of APEX is the APEX Engine, also known as the APEX Policy Engine or the APEX PDP (since it is in fact a Policy Decision Point).
Beside this engine, an APEX system comes with a few applications intended to help with policy authoring, deployment, and execution.

The engine itself and most applications are started from the command line with command line arguments.
This is called a Command Line Interface (CLI).
Some applications require an installation on a webserver, as for instance the REST Editor.
Those applications can be accessed via a web browser.

You can also use the available APEX APIs and applications to develop other applications as required.
This includes policy languages (and associated parsers and compilers / interpreters), GUIs to access APEX or to define policies, clients to connect to APEX, etc.

For this documentation, we assume an installation of APEX as a full system based on a current ONAP release.

== CLI on Unix, Windows, and Cygwin
A note on APEX CLI applications: all applications and the engine itself have been deployed and tested on different operating systems: Red Hat, Ubuntu, Debian, Mac OSX, Windows, Cygwin.
Each operating system comes with its own way of configuring and executing Java.
The main items here are:

- For UNIX systems (RHL, Ubuntu, Debian, Mac OSX), the provided bash scripts work as expected
  with absolute paths (e.g. `/opt/app/policy/apex-pdp/apex-pdp-{release-version}/examples`),
  indirect and linked paths (e.g. `../apex/apex`),
  and path substitutions using environment settings (e.g. `$APEX_HOME/bin/`)
- For Windows systems, the provided batch files (`.bat`) work as expected with 
  with absolute paths (e.g. `C:\apex\apex-{release-version}\examples`),
  and path substitutions using environment settings (e.g. `%APEX_HOME%\bin\`)
- For Cygwin system we assume a standard Cygwin installation with standard tools (mainly bash) using a Windows Java installation.
  This means that the bash scripts can be used as in UNIX, however any argument pointing to files and directories need to use either a DOS path (e.g. `C:\apex\apex-{release-version}\examples\config\...`)
  or the command `cygpath` with a mixed option.
  The reason for that is: Cygwin executes Java using UNIX paths but then runs Java as a DOS/WINDOWS process, which requires DOS paths for file access.