# Server Side Includes

## Overview of SSI

SSI (Server Side Includes) are directives that are placed in HTML pages, and evaluated on the server while the pages are being served. They let you add dynamically generated content to an existing HTML page, without having to serve the entire page via a CGI program, or other dynamic technology such as a mod_python handler.

SSI directives have the following syntax::

    <!--#element attribute=value attribute=value ... -->

It is formatted like an HTML comment, so if you don’t have SSI correctly enabled, the browser will ignore it, but it will still be visible in the HTML source. If you have SSI correctly configured, the directive will be replaced with its results.

For a more thorough description of the SSI mechanism and how to enable it, see the SSI tutorial provided with the Apache documentation.

Version 3.3 of mod_python introduces support for using Python code within SSI files. Note that mod_python honours the intent of the Apache `IncludesNOEXEC` option to the `Options` directive. That is, if `IncludesNOEXEC` is enabled, then Python code within a SSI file will not be executed.

## Using Python Code

The extensions to mod_python to allow Python code to be used in conjunction with SSI introduces the new SSI directive called `'python'`. This directive can be used in two forms, these being `'eval'` and `'exec'` modes. In the case of `'eval'`, a Python expression is used and it is the result of that expression which is substituted in place into the page.:

    <!--#python eval="10*'HELLO '" -->
    <!--#python eval="len('HELLO')" -->

Where the result of the expression is not a string, the value will be automatically converted to a string by applying `'str()'` to the value.

In the case of `'exec'` a block of Python code may be included. For any output from this code to appear in the page, it must be written back explicitly as being part of the response. As SSI are processed by an Apache output filter, this is done by using an instance of the mod_python `filter` object which is pushed into the global data set available to the code.:

    <!--#python exec="
    filter.write(10*'HELLO ')
    filter.write(str(len('HELLO')))
    " -->

Any Python code within the `'exec'` block must have a zero first level indent. You cannot start the code block with an arbitrary level of indent such that it lines up with any indenting used for surrounding HTML elements.

Although the mod_python `filter` object is not a true file object, that it provides the `write()` method is sufficient to allow the `print` statement to be used on it directly. This will avoid the need to explicitly convert non string objects to a string before being output.:

    <!--#python exec="
    print >> filter, len('HELLO')
    " -->

## Scope of Global Data

Multiple instances of `'eval'` or `'exec'` code blocks may be used within the one page. Any changes to or creation of global data which is performed within one code block will be reflected in any following code blocks. Global data constitutes variables as well as module imports, function and class definitions.:

    <!--#python exec="
    import html, time, os
    def _escape(object):
        return html.escape(str(object))
    now = time.time()
    " -->
    <html>
    <body>
    <pre>
    <!--#python eval="_escape(time.asctime(time.localtime(now)))"-->

    <!--#python exec="
    keys = os.environ.keys()
    keys.sort()
    for key in keys:
        print >> filter, _escape(key),
        print >> filter, '=',
        print >> filter, _escape(repr(os.environ.get(key)))
    " -->
    </pre>
    </body>
    </html>

The lifetime of any global data is for the current request only. If data must persist between requests, it must reside in external modules and as necessary be protected against multithreaded access in the event that a multithreaded Apache MPM is used.

## Pre-populating Globals

Any Python code which appears within the page has to be compiled each time the page is accessed before it is executed. In other words, the compiled code is not cached between requests. To limit such recompilation and to avoid duplication of common code amongst many pages, it is preferable to pre-populate the global data from within a mod_python handler prior to the page being processed.

In most cases, a fixup handler would be used for this purpose. For this to work, first need to configure Apache so that it knows to call the fixup handler.:

    PythonFixupHandler _handlers

The implementation of the fixup handler contained in `_handlers.py` then needs to create an instance of a Python dictionary, store that in the mod_python request object as `ssi_globals` and then populate that dictionary with any data to be available to the Python code executing within the page.:

    from mod_python import apache

    import html, time

    def _escape(object):
       return html.escape(str(object))

    def _header(filter):
       print >> filter, '...'

    def _footer(filter):
       print >> filter, '...'

    def fixuphandler(req):
       req.ssi_globals = {}
       req.ssi_globals['time'] = time
       req.ssi_globals['_escape'] = _escape
       req.ssi_globals['_header'] = _header
       req.ssi_globals['_footer'] = _footer
       return apache.OK

This is most useful where it is necessary to insert common information such as headers, footers or menu panes which are dynamically generated into many pages.:

    <!--#python exec="
    now = time.time()
    " -->
    <html>
    <body>
    <!--#python exec="_header(filter)" -->
    <pre>
    <!--#python eval="_escape(time.asctime(time.localtime(now)))"-->
    </pre>
    <!--#python exec="_footer(filter)" -->
    </body>
    </html>

## Conditional Expressions

SSI allows for some programmability in its own right through the use of conditional expressions. The structure of this conditional construct is::

    <!--#if expr="test_condition" -->
    <!--#elif expr="test_condition" -->
    <!--#else -->
    <!--#endif -->

A test condition can be any sort of logical comparison, either comparing values to one another, or testing the ‘truth’ of a particular value.

The source of variables used in conditional expressions is distinct from the set of global data used by the Python code executed within a page. Instead, the variables are sourced from the `subprocess_env` table object contained within the request object. The values of these variables can be set from within a page using the SSI `'set'` directive, or by a range of other Apache directives within the Apache configuration files such as `BrowserMatchNoCase` and `SetEnvIf`.

To set these variables from within a mod_python handler, the `subprocess_env` table object would be manipulated directly through the request object.:

    from mod_python import apache

    def fixuphandler(req):
       debug = req.get_config().get('PythonDebug', '0')
       req.subprocess_env['DEBUG'] = debug
       return apache.OK

If being done from within Python code contained within the page itself, the request object would first have to be accessed via the filter object.:

    <!--#python exec="
    debug = filter.req.get_config().get('PythonDebug', '0')
    filter.req.subprocess_env['DEBUG'] = debug
    " -->
    <html>
    <body>
    <!--#if expr="${DEBUG} != 0" -->
    DEBUG ENABLED
    <!--#else -->
    DEBUG DISABLED
    <!--#endif -->
    </body>
    </html>

## Enabling INCLUDES Filter

SSI is traditionally enabled using the `AddOutputFilter` directive in the Apache configuration files. Normally this would be where the request mapped to a static file.:

    AddOutputFilter INCLUDES .shtml

When mod_python is being used, the ability to dynamically enable output filters for the current request can instead be used. This could be done just for where the request maps to a static file, but may just as easily be carried out where the content of a response is generated dynamically. In either case, to enable SSI for the current request, the `request.add_output_filter()` method of the mod_python request object would be used.:

    from mod_python import apache

    def fixuphandler(req):
       req.add_output_filter('INCLUDES')
       return apache.OK