Virtuoso functionality can be enhanced through external libraries by loading shared objects or DLLs. The new functions are written in a language of the developer's choice and compiled to produce a shared library appropriate to the operating system. The path to the shared library must be declared in the Virtuoso INI file and the server restarted before it can be used.

The Virtuoso INI file uses a [Plugins] configuration section for listing shared libraries for the server to load upon startup. The layout is as follows:

[Plugins]
LoadPath = <module path> (example : /home/virtuoso/hosting)
Load1 = <module type>, <module name> (example : hosting_perl.so)
Load2 = <module type 2>, <module name 2>
..
LoadN = <module type N>, <module name N>

Virtuoso reads the Load1, Load2, ... LoadN lines from the [Plugins] section and attaches them according to their type.

<module path> is the directory containing shared modules for use with Virtuoso. (e.g. /home/virtuoso/hosting )

<module type> specifies the type of module that is to be loaded, and hence how Virtuoso is to use it. So far only the "Hosting" type exists.

<module name> is the file name of the modules shared library or object. (e.g. hosting_perl.so )

The "Hosting" type defines entry points for initialization of the runtime hosting environment, destruction of the user environment and execution of a file or string containing commands in the hosted language. It also returns a list of file extensions that it is capable of processing. Virtuoso dynamically defines memory-resident (no disk image) HTTP server handlers for each specified type. For example, the Perl hosting plugin supports 'pl' extension. Hence, upon initialization of the hosting plugin, Virtuoso defines the __http_handler_pl(..) function according to the API for file type handlers in the Virtuoso HTTP server. With this handler in place, each hit on a .pl file (file system or WebDAV) with appropriate execute permissions will cause the HTTP server to execute the code within it and return the result instead of simple the file contents.

The handler will call the __hosting_http_handler VSE with a special set of parameters to represent the HTTP environment correctly.

There's a VSE to call each of the hosting modules:

__hosting_http_handler ( in extension varchar, in content varchar, [in params varchar, ] [in lines any, ] [inout filename_head_ret varchar, ] [in options any, ] [out diag_ret varchar]) returns varchar

such that:

The function will return a varchar containing the HTTP body if in HTTP mode (__VIRT_CGI = 1 ) or the messages printed to STDOUT.

Virtuoso will normally call to memory each plugin as required, and expel it when finished. This behavior can be controlled by the INI file parameter:

[HTTPServer]
PersistentHostingModules = 1/0 default 0

Setting PersistentHostingModules to "1" prevents Virtuoso from removing the interpreters from the HTTP threads after each request.

select __hosting_http_handler ('pl', 'print "hello world"; ', '', vector (), 'helloworld.pl');

returns : hello world

/perl/test_print.pl
-------------------
#!/usr/bin/perl
print "hello world file";

select __hosting_http_handler ('pl', 'perl/test_print.pl');

returns : hello world file

	Note:
	The hosting_perl hosting module uses the perl tie() function to "tie" up the STDIN, STDOUT, STDERR, exit() and %ENV perl objects. Untying any of these may lead to unpredictable results.