# 12. Web Application Development

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Web Application Development
The HTTP Server
Web Services ACL (Access Control List)
Virtuoso Server Pages (VSP)
Virtuoso Server Pages for XML (VSPX)
Writing ASP.Net Web Applications
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Writing PHP Applications
Writing JSP Applications
Perl Hosting
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## 12.10. Perl Hosting

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 Perl language is hosted within Virtuoso in this way. hosting_perl.so is the library used.

The Virtuoso INI file uses a [Plugins] configuration section for listing shared libraries for the server to load upon startup. An example of this is:

    [Plugins]
    LoadPath = /home/virtuoso/hosting
    Load1 = Hosting, hosting_perl.so
    ..

The "Hosting" type defines the 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.

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 simply the file's contents.

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

Virtuoso will normally call the plugin to memory 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.

See Also:

VSEI Plugins

Using the Perl Plugin

Executing Perl code directly:

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

    returns : hello world

Executing a perl script file (perl/test_print.pl in the Virtuoso working directory):

    /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.

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