The LAMP Architecture

Moodle is developed on the open-source LAMP framework consisting of Linux (operating system), Apache (web server), MySQL (database), and PHP (programming language). Due to the portability of these components and the modularity of Moodle itself (that's what the "M" stands for), it can support a wide range of operating systems, database systems, and web servers. A simple overview of the LAMP architecture can be shown as follows:

The lowest level is the Operating System. While Linux is the preferred platform, other UNIX derivatives such as Solaris and AIX are supported as well, along with Windows and Mac OS X. Certain libraries will have to be installed, a topic we covered in the previous chapter on installation.

PHP is the Programming Language in which Moodle is developed (accompanied by HTML and CSS files). It is the only component that cannot be replaced with any counterpart.

MySQL is the database of choice for most open-source applications, but other database systems such as PostgreSQL, Oracle, and Microsoft SQL Server too work without problems. Again, detailed information has been provided in the chapter on installation.

Apache has become the de-facto standard for the large-scale web applications, closely followed by Microsoft IIS. Both the web servers are supported like any others offering a PHP support.

The interaction of the elements in the Moodle architecture is shown in the following image:

The user makes Requests via the web browser interface (for example, display a learning resource). The web browser passes the request on to the Web Server, which calls the PHP module that is responsible for the call. The PHP module Calls the Database with an action (query, update, insert, or delete operation) that Returns the requested data. Based on this information, the PHP module returns data (usually in the form of HTML code) to the web server, which passes the information to be displayed back to the user's browser.

The Moodle Layer

Now, let's look at the Moodle layer in more detail. The main building blocks of Moodle are shown in the following image:

Moodle distinguishes between Code (mostly written in PHP, HTML, and CSS) and Data (values added in the configuration files via the Moodle interface, or manually).

Moodle Libraries, Modules (such as resources and activities), Blocks, Plug-ins, and other units are represented in a code. The code is always stored in files within the Moodle directory structure. The code includes all the elements that deal with the back-end (server) and front-end (user interface) operations.

The data regarding Moodle courses, users, roles, groups, and so on (for example, grades) is mostly stored in the Moodle database. However, certain data, such as user pictures, is stored in specific files or directories.

Now let's have a look at how the Moodle files area, that is, the directory structure is organized.

Code and Data Locations

Though Moodle takes care of the organization of its code and data, it is good to know where a file is located in your learning system.

All the system files, that is, the files that are required to run Moodle, are located in a number of directories under $CFG->dirroot (the root directory of your Moodle installation). These files are potentially replaced when updating Moodle. The structure is as follows (All directories are located on the same level. However, some folders contain sub-directories.):

Moodle data that includes everything the users have added to the system such as learning resources by teachers, forum posts, or assignments by students, and system settings by the administrator, is stored in two locations:

Moodle stores almost all data in the used database, except for the files. Only a link for the files is stored in the database, and the actual files are stored in a directory specified by the $CFG->dataroot variable in config.php. This is called as moodledata directory and is organized as follows:

1 Front page files Moodle architecturemoodledata directory, organizing2 Files for course number 2 3 Files for course number 3 cache Caching data sessions User session data temp Temporary file upgradelogs Log file for each Moodle upgrade that has been performed user User-related data

At creation, each course is allocated a unique number, which is also used as the name for the sub-directory. The exception is the front page, which is treated as a course but is always given the number "one".