# 12. Web Application Development

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The HTTP Server
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## 12.11. Python Hosting

Virtuoso functionality can be enhanced through external libraries by loading shared objects or DLLs. The new functions are written in a language of 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 Python language is hosted within Virtuoso in this way. hosting_python.so (or hosting_python.dll) is the library used.

The Python hosting module requires the Python library, version 2.2.2 or above, to build and use the module, which then must be installed in the Plugins-Load-path defined in the Virtuoso ini file. The source code and build script (build.py) for building the hosting_python.so module are included in the Unix distribution in the custom/hosting/python directory. The Unix installer will offer to build it near the end of the installation process. The build.py is a kind of Makefile. On unix it requires libtool and cc in the path. On windows it requires cl.exe in the path.

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_python.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 Python hosting plugin supports the 'py' extension. Hence, upon initialization of the hosting plugin, Virtuoso defines the __http_handler_py(..) function according to the API for file type handlers in the Virtuoso HTTP server. With this handler in place, each hit on a .py 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 python interpreter has a global lock unrelated to the Virtuoso hosting module, thus no more than one thread can run python code at a time.

The handler will call the __hosting_http_handler VSE with a special set of parameters to represent the HTTP environment correctly. Virtuoso will, by default 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 Python Plugin

Executing Python code directly:

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

    returns : hello world

Executing a python script file (python/test_print.py in the Virtuoso working directory):

    python/test_print.py
    -------------------
    #!/usr/bin/python
    print "hello world file";

    select __hosting_http_handler ('py', 'python/test_print.py');

    returns : hello world file

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