~azzar1/unity/add-show-desktop-key

IVLE Dispatch Module Design

===========================

    Author: Matt Giuca

    Date: 11/12/2007

This is the design for the dispatch module (the top-level app which handles

all requests in the web app and passes them on to other apps).

Request handling sequence

-------------------------

The request handling sequence consists of two parts. The first part is

dispatch being called. It autonomously performs auth and some housekeeping,

and then passes control to another specified app.

The second part of request handling is done by the app in question. The object

which dispatch passes the app has several methods in it which the app can

call. In this way, dispatch code is still used in the second part of the

request handling, but it is used passively instead of actively.

Finally, dispatch cleans up a little bit (writing footers, etc) when the app

exits.

1. dispatch loads. It imports config modules, so it has access to

    admin-configured information about the plugged in apps and other

    settings.

2. Breaks the URL and request data down and builds the IVLE request object

    (which is higher level than the Apache request object and independent of

    mod_python). This includes breaking the path into the "first directory"

    (the app name) and the rest of the path (which is passed to the app as the

    "path").

3. Decides which app to load by looking it up in the app list. If no path is

    specified, selects the default app (does not redirect just yet). If the

    app is not found in the app list, throws a 404 error and exits.

4. If the app requires authentication, checks the session information to see

    if a user is logged in. If not, passes control to a special "app" (built

    into dispatch, but with the same interface as other apps), the login page,

    and skips to step 7.

5. If no app was specified, performs a redirect to the default app, and

    exits.

6. Passes control to the specified app.

7. After the app completes, performs cleanup (such as writing HTML footer).

The dispatch also provides several callbacks.

The dispatch "request" object could just be the Apache request object, but

augmented with additional methods and attributes, and an overridden "write"

method.

### Writing callbacks ###

One important callback sequence is writing to the output. The dispatch does

not provide the app with an Apache request object. This is to ensure that none

of the apps are dependent on mod_python (even though dispatch, itself, is).

Each app therefore calls the dispatch request object to do all the writing.

(Note the apps should be given access to the Apache request object but

recommended not to use it. This is so that "exec" can pass it to the executed

code apps).

The writing sequence takes place in two parts: writing the HTTP headers, and

writing the response.

The HTTP headers are written by dispatch, not the app. The app just sets

fields in the request object, such as "content_type", "title" and

"write_html_head_foot". (This is similar to how Apache works but has some

extra fields).

When the first actual write call is made, the app looks at the fields and

writes out the HTTP headers. The "write_html_head_foot" field, if True, causes

dispatch to also write out the IVLE HTML header bar section.

When doing the cleanup step, if write_html_head_foot is set, dispatch then

writes the HTML footer.

The header and footer includes the html and body tags. Therefore, any app

which sets "write_html_head_foot" should just write body content, no html or

body tags.

Login screen

------------

The login screen is a special part of IVLE because it is not part of any

specific app, and does not have its own URL (there is no `/login`). It is

actually part of the dispatch module.

The login screen is displayed instead of whichever page was requested if the

app or page requested requires login. It does not navigate away from that page

(it keeps the same URL as requested). This is so that once the login is

successful, it can reload that URL and be on the requested page.

The login screen gracefully handles browsers without JavaScript, giving a nice

error message. The other pages won't necessarily be so graceful, as the user

is expected to have JavaScript if they passed the login screen.

The login interaction will be done with Ajax, because it might as well be.

This simplifies the system because the primary (non-ajax) requests do not

contain login POSTdata. The login is made to a special url which returns JSON,

and sets cookies/sessions. Upon success, the JavaScript itself performs a

hard refresh, which then succeeds because the cookie has been set to log in.

The login screen has the login box on the right, and displays other system

information on the rest of the screen. (This information is stuff the students

should probably see once in awhile even though they won't navigate explicitly

to it - system and subject announcements, and a random "tip of the day").

The IVLE Request object

-----------------------

The IVLE request object is mostly a wrapper around the Apache request object,

but it's a bit higher level and does not depend upon mod_python.

The file dispatch/request.py (or documentation generated from it) formally

specifies the attributes and methods of this object.