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IVLE - System Architecture
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==========================
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This document describes the high-level system architecture of IVLE,
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specifically with respect to the "pluggable clients" interface.
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Users and authorization
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-----------------------
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We need some way to authenticate users and store information about a logged-in
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user. Whether they are stored in a database local to our system remains to be
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Importantly, we need some way to send user information to the clients. This is
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discussed in the "pluggable clients" section.
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The IVLE system is largely just a collection of various components, called
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"clients", such as the file browser, text editor, console, tutorial sheets,
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The architecture provides a common interface in which clients can be plugged
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Firstly, we want all HTML pages on the site to be generated with a common
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header. The easiest way to do this is to write our own Python handler which
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is common to the entire application (this replaces the standard handlers such
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This top-level handler handles all authentication (for instance,
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checking the session to see if a user is logged in properly and if not,
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redirecting to the login page). It then outputs the header, and calls the
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appropriate client based on the URL. The test of "whether the student is an
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Informatics student" is considered part of the authentication layer. (So
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students who are not enrolled in Informatics are treated the same way as a
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Note that some clients ("login" and "exec") do not require authentication.
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This will be one of the properties of the client in the global clients file.
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Note that the handler does *not* perform authorization - that is left up to
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One special feature of the handler will be the ability to write an XHTML
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header (which includes the user's name and links to profile page, IVLE logo,
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and tabs for all the clients). This is important to keep a consistent
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interface between the clients. This header will be available upon request from
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the client. It is up to the client to NOT request a header for non-HTML
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content (or it will be ruined), and also not to request a header when
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executing student's code (ie. the exec module will never request a header).
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### Plugin interface ###
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The top-level handler will keep a Python file (or a text, JSON, etc file)
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containing a list of valid clients. This is a dictionary mapping clients'
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internal names (the top-level directories, as described below in "URLs" and
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the "planned clients") to some other date about the clients (such as a
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friendly name to display in the tabs, and a boolean as to whether or not to
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display the client in the tabs).
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Part of the HTML header which the handler generates is a set of tabs linking
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to all of the clients in this list, or at least the ones with "show in tabs"
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turned on. Clients such as "exec" and "admin" will not have a tab.
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Each client will be located physically in a directory "clients", in a
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subdirectory of the client's name. (eg. the console is located in
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"clients/console"). There *must* be a file in this directory called
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**client.py**. This file is called by the handler for most requests.
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All requests will go through the handler. Note that there is some media (such
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as CSS, JavaScript and image files which are directly part of the application
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itself), which we do not want to pass through the handler. These will be
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placed in a special top-level directory, which Apache will be told to serve
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directly. (eg. "/media").
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This means that the contents of each client directory is a Python program
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*only*, and contains no files accessible by the browser. It consists of
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client.py, plus any Python files imported by client.py (but none of these
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files will directly serve web content).
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Inside client.py, there is a fixed interface which all clients must follow.
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Firstly, there is a set of information which the handler must pass to the
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client in numerous calls - such as username, URL, and nicely split up parts of
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the URL such as the path, the GET variables, and also the POST data, as well
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as mod_python's low-level Request object.
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This information is encapsulated into an object and passed as a single
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argument to the client handling functions.
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Note that as stated above, the handler may need to insert HTML contents into
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the output stream. Instead of having two separate function calls (a call to
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find the mime type and a call to get content), we'll simply provide a wrapper
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object to the client where the client can make callbacks to.
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To this end, the client receives an object containing all of the information,
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as well as an object with some methods to call. The handler passes this to a
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function in client.py, `handle`. The callback object contains the following
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* set_mime_type(string) - Sets the output mime type. May be called any number
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of times (including 0, will default to HTML), but may not be called after
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any writing has been done.
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* set_status(string) - Sets the HTTP response status. The string is a numeric
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code followed by a description, for example "404 File Not Found". May not be
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called after any writing has been done.
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* set_location(string) - Sets the Location field of the HTTP response to a new
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URL. For use with 300-level HTTP response codes. May not be called after any
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writing has been done.
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* write_html_headers() - Writes the general site headers to the output stream.
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May not be called after any writing has been done.
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* write(string) - Writes raw data to the output.
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Note that this is very similar to the CGI interface, but much higher level (we
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have functions to call instead of writing strings, and we send the GET and
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POST data in a packaged object instead of environment variables and stdin).
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Note that, as with CGI, there is a "cutoff point" during the processing
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(immediately when the first call to `write` or `write_html_headers` is made) -
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in which the response headers are written to the server.
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There will be a "help" app which is special in that it goes inside all of the
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other apps directories looking for a help file. So aside from "client.py",
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another special file is "help.html" which is a static help file for each
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module, sitting in that app's top-level directory.
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help.html is not to be served directly. The "help" app will embed it within
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another page. Therefore it is not a real HTML file - it should just be the
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inside of a body (it should not contain html or body tags).
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### Application directory hierarchy ###
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Due to the handler, we have a nice property that the application directory
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hierarchy is completely removed from the apparent hierarchy on the web. This
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has two opportunities: we can call the applications (in their directory
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hierarchy) a different name than the URL suggests, and also we can lay out the
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directory hierarchy with developers interests in mind.
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We capitalise on the first issue by mapping the "action" (url name) of a
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client to the actual name. (Clients are indexed by url-name so they can
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be looked up when a URL is requested).
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The proposed application directory hierarchy is:
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/clients - All clients go in here
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/clients/myclient - "actual" names of the clients
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/dispatch - Code files for the top-level dispatch
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/dispatch.py - Entrypoint for the top-level dispatch
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/media - Publically viewable files
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(Note that this directory hierarchy maps onto the web site)
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/media/myclient - media files specific to each client go in a subdir
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/media/dispatch - media files for the top-level dispatch
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/conf - Special .py files which hold configuration info (for the admin to
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edit, not the programmers).
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It would be good if we had full control of URLs and were able to make them
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"nice" at all times. The criteria for "nice" URLs are as follows:
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* The paths in the URLs reflect a sensible hierarchy of where you are in the
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program at the current time.
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* The URLs do not contain any file extensions for the pages (no .html or
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.py), although linked files such as CSS, JavaScript and image files should
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have appropriate file extensions.
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* The URLs do not contain unnecessary garbage arguments, and preferably no GET
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arguments at all (for instance, the file browser will specify the path to
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browse in the actual URL path, not the GET arguments.
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* The URL does not contain the student's login name. This is implicit in the
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browser session. (This requirement allows for us to link to URLs in
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documentation which will work for any student). (Note that URLs may contain
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other students login names for browsing their work - this is determined by
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the individual clients).
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The top-level directory given in the URL determines the client which the
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handler will pass off to. For instance,
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http://www.example.com/ivle/console
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Since IVLE is located at `http://www.example.com/ivle`, it will consider the
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"top-level directory" to be "console", and therefore will call the client
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whose action is "console". This may not be the actual name of the client. For
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example, the "edit" action maps onto the "editor" client, while the "serve"
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action maps onto the "exec" client. (Perhaps it is best for simplicity if
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these do in fact correspond).
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For another example, consider the file browser (action name "files"). The URL
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may have subdirectories after it which indicate the path to explore. This will
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be detailed in the clients section below. An example of a browse URL is:
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http://www.example.com/ivle/files/jdoe/151/proj1/
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In this instance, the handler will see the top-level directory as "files", and
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will therefore link to the file browser client. The file browser client will
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then receive the additional arguments passed to it in some way, which in this
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case are "jdoe/151/proj1/". The file browser client will then handle this path
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and serve up the correct directory.
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### Relative URLs inside HTML content ###
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It is a requirement that the application can be placed anywhere in a web
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server's directory hierarchy, not just at the top level. This means HTML
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should never contain absolute URLs (beginning with, eg, "/browse"). Then it
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would need to be in the site root.
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To solve the problem of how to generate URLs, one of the fields the handler
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will pass into the clients (which it will read from a config file somewhere)
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will be the "site root". This may be "/ivle", for instance. Therefore all
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absolute URLs generated by the applications must be prepended with the "site
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root". (In our case the site root will probably be "/", but it's a good
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### Student's directory hierarchy, common code ###
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Many clients share the concept of exploring the student's directory hierarchy,
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as explained above for the browser module. The common code for handling the
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student id or group name (etc) and authorization will be available as a
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separate module for all such clients (browser, editor, exec) to use.
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### File Browser, Text Editor and Executor ###
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Three of the most important clients are the file browser ("browser"), text
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editor ("editor") and executor ("exec"). These three share a commonality in
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that they all access the student's directory hierarchy and files. They all
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share a lot of code in common, and in particular, there is a common
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server-side handler for file access, directory listings and subversion.
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Firstly, every file and directory is classified into one of the following
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categories (based on its inferred MIME type and possibly whether it contains
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invalid Unicode characters):
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4. Text file (unless it fits the above, eg, SVG files)
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5. Any other binary file
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How each of these is handled depends on which of the 3 clients is accessing
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#### File Browser ####
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1. Directory - Displays a directory listing (this is its primary purpose).
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2. Image - Displays the image inside the main navigation interface.
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3. Audio - (non-core) Provides a streaming audio player within the main
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navigation interface.
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4. Text file - Redirect to edit.
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5. Binary file - Provides a download link within the main navigation
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Note that no matter what, using browser will remain within the navigation
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interface so you will never be "lost" inside a raw image or something. It also
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will not throw binary files as downloads directly to you.
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Note that the src of the image tag in (2) and the href of the download link in
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(5) will simply be links to the exec version of the same file.
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File browser will include the Python file which serves up JSON responses to
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requests for directory hierarchies, and performs SVN and file access commands.
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This file will be used by the text editor (at least) and possibly exec.
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#### Text Editor ####
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No matter what, editor provides a text area (with advanced editing
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capabilities and syntax highlighting) for any file, even if it is binary. The
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only exception is directories, which redirect to browser.
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Note that it will not be possible to click into the editor for a binary file
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(the browser will not offer an edit link). However, it will still be possible
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to manually nav there, and then you handle the shock yourself.
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Tab name: (not shown)
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The executor is used to directly serve files out of a student's directory, as
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if it was a standard web server. (It can be thought of as a little web server
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inside IVLE). This means that:
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* A whitelist of file types is kept which simply are served up raw. This
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includes HTML, JavaScript, CSS, all reasonable image and audio formats, etc.
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* Special "executable" file types (.py, .psp). Exec will call popen on a
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Python process which loads a mod_python handler, cgihandler or psphandler on
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* HTTP errors for banned files.
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* When presented with a directory, it first tries to execute `__init__.py`
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(the default item for the directory). It could also look for `index.html` or
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`index.psp` if that failed. Failing that, it returns an HTTP 403 Forbidden
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Action name: `console`
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### Tutorial Pages ###
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Action name: `tutorial`
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### Administration ###
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Tab name: (not shown)
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Client checks authorization for admin status. Tab is not shown so students
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will not normally know about this (but even if they find it they will be
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Tab name: (not shown)
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Authentication not required. Presents a login box.
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Other similar clients are "logout" (which just immediately logs the current
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user out and redirects to the main page), and "profile" (user settings).
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doc/notes/architecture.txt
