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IVLE Dispatch Module Design
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===========================
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This is the design for the dispatch module (the top-level app which handles
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all requests in the web app and passes them on to other apps).
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Request handling sequence
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-------------------------
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The request handling sequence consists of two parts. The first part is
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dispatch being called. It autonomously performs auth and some housekeeping,
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and then passes control to another specified app.
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The second part of request handling is done by the app in question. The object
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which dispatch passes the app has several methods in it which the app can
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call. In this way, dispatch code is still used in the second part of the
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request handling, but it is used passively instead of actively.
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Finally, dispatch cleans up a little bit (writing footers, etc) when the app
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1. dispatch loads. It imports config modules, so it has access to
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admin-configured information about the plugged in apps and other
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2. Breaks the URL and request data down and builds the IVLE request object
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(which is higher level than the Apache request object and independent of
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mod_python). This includes breaking the path into the "first directory"
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(the app name) and the rest of the path (which is passed to the app as the
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3. Decides which app to load by looking it up in the app list. If no path is
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specified, selects the default app (does not redirect just yet). If the
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app is not found in the app list, throws a 404 error and exits.
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4. If the app requires authentication, checks the session information to see
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if a user is logged in. If not, passes control to a special "app" (built
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into dispatch, but with the same interface as other apps), the login page,
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5. If no app was specified, performs a redirect to the default app, and
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6. Passes control to the specified app.
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7. After the app completes, performs cleanup (such as writing HTML footer).
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The dispatch also provides several callbacks.
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The dispatch "request" object could just be the Apache request object, but
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augmented with additional methods and attributes, and an overridden "write"
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### Writing callbacks ###
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One important callback sequence is writing to the output. The dispatch does
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not provide the app with an Apache request object. This is to ensure that none
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of the apps are dependent on mod_python (even though dispatch, itself, is).
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Each app therefore calls the dispatch request object to do all the writing.
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(Note the apps should be given access to the Apache request object but
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recommended not to use it. This is so that "exec" can pass it to the executed
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The writing sequence takes place in two parts: writing the HTTP headers, and
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The HTTP headers are written by dispatch, not the app. The app just sets
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fields in the request object, such as "content_type", "title" and
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"write_html_head_foot". (This is similar to how Apache works but has some
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When the first actual write call is made, the app looks at the fields and
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writes out the HTTP headers. The "write_html_head_foot" field, if True, causes
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dispatch to also write out the IVLE HTML header bar section.
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When doing the cleanup step, if write_html_head_foot is set, dispatch then
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writes the HTML footer.
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The header and footer includes the html and body tags. Therefore, any app
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which sets "write_html_head_foot" should just write body content, no html or
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The login screen is a special part of IVLE because it is not part of any
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specific app, and does not have its own URL (there is no `/login`). It is
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actually part of the dispatch module.
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The login screen is displayed instead of whichever page was requested if the
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app or page requested requires login. It does not navigate away from that page
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(it keeps the same URL as requested). This is so that once the login is
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successful, it can reload that URL and be on the requested page.
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The login screen gracefully handles browsers without JavaScript, giving a nice
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error message. The other pages won't necessarily be so graceful, as the user
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is expected to have JavaScript if they passed the login screen.
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The login interaction will be done with Ajax, because it might as well be.
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This simplifies the system because the primary (non-ajax) requests do not
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contain login POSTdata. The login is made to a special url which returns JSON,
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and sets cookies/sessions. Upon success, the JavaScript itself performs a
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hard refresh, which then succeeds because the cookie has been set to log in.
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The login screen has the login box on the right, and displays other system
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information on the rest of the screen. (This information is stuff the students
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should probably see once in awhile even though they won't navigate explicitly
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to it - system and subject announcements, and a random "tip of the day").
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The IVLE Request object
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-----------------------
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The IVLE request object is mostly a wrapper around the Apache request object,
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but it's a bit higher level and does not depend upon mod_python.
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The file dispatch/request.py (or documentation generated from it) formally
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specifies the attributes and methods of this object.
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removed
doc/notes/apps/dispatch.txt
