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The high-level architecture of the website, including the controllers for different parts of the website (REST API, html, ...) and connections to the database.


The source code is available in the 'website' repository:

Important files and folders

In this section we go through all important files and folder of the system.

Root directory

The root directory of OpenML contains the following files and folders.

  • system: This folder contains all files provided by CodeIgniter 2.1.3. The contents of this folder is beyond the scope of this document, and not relevant for extending OpenML. All the files in this folder are in the same state as they were provided by Ellislabs, and none of these files should ever be changed.

  • sparks: Sparks is a package management system for Codeigniter that allows for instant installation of libraries into the application. This folder contains two libraries provided by third party software developers, oauth1 (based on version 1 the oauth protocol) and oauth2 (similarly, based on version 2 of the oauth protocol). The exact contents of this folder is beyond the scope of this document and not relevant for extending OpenML.

  • openml_OS: All files in this folder are written specifically for OpenML. When extending the functionality OpenML, usually one of the files in this folder needs to be adjusted. As a thorough understanding of the contents of this folder is vital for extending OpenML, we will discuss the contents of this folder in [[URL Mapping]] in more detail.

  • index.php: This is the “bootstrap” file of the system. Basically, every page request on OpenML goes through this file (with the css, images and javascript files as only exception). It then determines which CodeIgniter and OpenML files need to be included. This file should not be edited.

  • .htaccess: This file (which configures the Apache Rewrite Engine) makes sure that all URL requests will be directed to index.php. Without this file, we would need to include index.php explicitly in every URL request. This file makes sure that all other URL requests without index.php embedded in it automatically will be transformed to index.php. Eg., will be rewritten to This will be explained in detail in [[URL Mapping]].

  • css: A folder containing all stylesheets. These are important for the layout of OpenML.

  • data: A folder containing data files, e.g., datasets, implementation files, uploaded content. Please note that this folder does not necessarily needs to be present in the root directory. The OpenML Base Config file determines the exact location of this folder.

  • downloads: Another data folder, containing files like the most recent database snapshot.

  • img: A folder containing all static images shown on the webpage.

  • js: A folder containing all used Javascript files and libraries, including third party libraries like jQuery and datatables.

  • Various other files, like .gitignore, favicon.ico, etc.


This folder is (in CodeIgniter jargon) the “Application folder”, and contains all files relevant to OpenML. Within this folder, the following folders should be present: (And also some other folders, but these are not used by OpenML)

  • config: A folder containing all config files. Most notably, it contains the file BASE_CONFIG.php, in which all system specific variables are set; the config items within this file differs over various installations (e.g., on localhost, Most other config files, like database.php, will receive their values from BASE_CONFIG.php. Other important config files are autoload.php, determining which CodeIgniter / OpenML files will be loaded on any request, openML.php, containing config items specific to OpenML, and routes.php, which will be explained in [[URL Mapping]].

  • controllers: In the Model/View/Controller design pattern, all user interaction goes through controllers. In a webapplication setting this means that every time a URL gets requested, exactly one controller gets invoked. The exact dynamics of this will be explained in [[URL Mapping]].

  • core: A folder that contains CodeIgniter specific files. These are not relevant for the understanding of OpenML.

  • helpers: This folder contains many convenience functions. Wikipedia states: “A convenience function is a non-essential subroutine in a programming library or framework which is intended to ease commonly performed tasks”. For example the file_upload_helper.php contains many functions that assist with uploading of files. Please note that a helper function must be explicitly loaded in either the autoload config or the files that uses its functions.

  • libraries: Similar to sparks, this folder contains libraries specifically written for CodeIgniter. For example, the library used for all user management routines is in this folder.

  • models: In the Model/View/Controller design pattern, models represent the state of the system. In a webapplication setting, you could say that a model is the link to the database. In OpenML, almost all tables of the database are represented by a model. Each model has general functionality applicable to all models (e.g., retrieve all records, retrieve record with constraints, insert record) and functionality specific to that model (e.g., retrieve a dataset that has certain data properties). Most models extend an (abstract) base class, located in the abstract folder. This way, all general functionality is programmed and maintained in one place.

  • third_party: Although the name might suggests differently, this folder contains all OpenML Java libraries.

  • views: In the Model/View/Controller design pattern, the views are the way information is presented on the screen. In a webapplication setting, a view usually is a block of (PHP generated) HTML code. The most notable view is frontend_main.php, which is the template file determining the main look and feel of OpenML. Every single page also has its own specific view (which is parsed within frontend_main.php). These pages can be found (categorized by controller and name) in the pages folder. More about this structure is explained in [[URL Mapping]].


Architecture and libraries involved in generating the frontend functions.



All pages are generated by first loading frontend_main.php. This creates the 'shell' in which the content is loaded. It loads all css and javascript libraries, and contains the html for displaying headers and footers.

Create new page

The preferred method is creating a new folder into the folder <root_directory>/openml_OS/views/pages/frontend This page can be requested by<folder_name> or just<folder_name> This method is preferred for human readable webpages, where the internal actions are simple, and the output is complex. We will describe the files that can be in this folder.

  • pre.php: Mandatory file. Will be executed first. Do not make this file produce any output! Can be used to pre-render data, or set some variables that are used in other files.

  • body.php: Highly recommended file. Intended for displaying the main content of this file. Will be rendered at the right location within the template file (frontend_main.php).

  • javascript.php: Non-mandatory file. Intended for javascript function on which body.php relies. Will be rendered within a javascript block in the header of the page.

  • post.php: Non mandatory file. Will only be executed when a POST request is done (e.g., when a HTML form was send using the POST protocol). Will be executed after pre.php, but before the rendering process (and thus, before body.php and javascript.php). Should handle the posted input, e.g., file uploads.

It is also recommended to add the newly created folder to the mapping in the routes.php config file. This way it can also be requested by the shortened version of the URL. (Note that we deliberately avoided to auto-load all pages into this file using a directory scan, as this makes the webplatform slow. )

For more information, see [[URL Mapping]].