Drupal /ˈdruːpəl/ is a free and open-source content management framework written in PHP and distributed under the GNU General Public License.Drupal provides a back-end framework for at least 2.3% of all websites worldwide– ranging from personal blogs to corporate, political, and government sites.Systems also use Drupal for knowledge management and for business collaboration.

As of December 2019, the Drupal community comprised more than 1.39 million members,including 117,000 users actively contributing,resulting in more than 44,000 free modules that extend and customize Drupal functionality,over 2,800 free themes that change the look and feel of Drupal,and at least 1,300 free distributions that allow users to quickly and easily set up a complex, use-specific Drupal in fewer steps.

The standard release of Drupal, known as Drupal core, contains basic features common to content-management systems. These include user account registration and maintenance, menu management, RSS feeds, taxonomy, page layout customization, and system administration. The Drupal core installation can serve as a simple website, a single- or multi-user blog, an Internet forum, or a community website providing for user-generated content.

Drupal also describes itself as a Web application framework.When compared with notable frameworks Drupal meets most of the generally accepted feature requirements for such web frameworks.

Although Drupal offers a sophisticated API for developers, basic Web-site installation and administration of the framework require no programming skills.

Drupal runs on any computing platform that supports both a web server capable of running PHP and a database to store content and configuration. 

Core

In the Drupal community, "CORE" refers to the collaboratively built codebase that can be extended through contributory modules and – for versions prior to Drupal 8 – is kept outside of the "sites" folder of a Drupal installation.(Starting with version 8, core is kept in its own 'core' sub-directory.) Drupal core is the stock element of Drupal. Bootstrap and Common libraries are defined as Drupal core and all other functionality is defined as Drupal modules including the system module itself.

In a Drupal website's default configuration, authors can contribute content as either registered or anonymous users (at the discretion of the administrator). This content is accessible to web visitors through a variety of selectable criteria. As of Drupal 8, Drupal has adopted some Symfony libraries into Drupal core.

Core modules also includes a hierarchical taxonomy system, which lets developers categorize content or tagged with key words for easier access.

Core modules

Drupal core includes optional modules that can be enabled by the administrator to extend the functionality of the core website.

The core Drupal distribution provides a number of features, including:

• Access statistics and logging
• Advanced search
• Blogs, books, comments, forums, and polls
• Caching and feature throttling for improved performance
• Descriptive URLs
• Multi-level menu system
• Multi-site support
• Multi-user content creation and editing
• OpenID support
• RSS feed and feed aggregator
• Security and new release update notification
• User profiles
• Various access control restrictions (user roles, IP addresses, email)
• Workflow tools (triggers and actions)

Core themes

Drupal includes core themes, which customize the "look and feel" of Drupal sites,for example, Garland and Bartik.

The Color Module, introduced in Drupal core 5.0, allows administrators to change the color scheme of certain themes via a browser interface.

Localization
As of January 2017, Drupal had been made available in 100 languages and English (the default).Support is included for right-to-left languages such as Arabic, Persian, and Hebrew.

Drupal localization is built on top of gettext, the GNU internationalization and localization (i18n) library.

Auto-update notification
Drupal can automatically notify the administrator about new versions of modules, themes, or the Drupal core.It's important to update quickly after security updates are released.

Before updating it is highly recommended to take backup of core, modules, theme, files and database. If there is any error shown after update or if the new update is not compatible with a module, then it can be quickly replaced by backup. There are several backup modules available in Drupal.

On October 15, 2014, a sql injection vulnerability was announced and update released.Two weeks later the Drupal security team released an advisory explaining that everyone should act under the assumption that any site not updated within 7 hours of the announcement are infected.Thus, it can be extremely important to apply these updates quickly and usage of a tool to make this process easier like drush is highly recommended.

Database abstraction
Prior to version 7, Drupal had functions that performed tasks related to databases, such as SQL query cleansing, multi-site table name prefixing, and generating proper SQL queries. In particular, Drupal 6 introduced an abstraction layer that allowed programmers to create SQL queries without writing SQL.

Drupal 9 extends the data abstraction layer so that a programmer no longer needs to write SQL queries as text strings. It uses PHP Data Objects to abstract the database. Microsoft has written a database driver for their SQL Server. Drupal 7 supports the file-based SQLite database engine, which is part of the standard PHP distribution.

Windows development
With Drupal 9's new database abstraction layer, and ability to run on the Windows web server IIS, it is now easier for Windows developers to participate in the Drupal community.

A group on Drupal.org is dedicated to Windows issues.

Accessibility
Since the release of Drupal 7, Web accessibility has been constantly improving in the Drupal community.Drupal is a good framework for building sites accessible to people with disabilities, because many of the best practices have been incorporated into Drupal Core.

Drupal 8 saw many improvements from the Authoring Tool Accessibility Guidelines (ATAG) 2.0 guidelines which support both an accessible authoring environment as well as support for authors to produce more accessible content.

The accessibility team is carrying on the work of identifying and resolving accessibility barriers and raising awareness within the community.

Drupal 8 has good semantic support for Rich Internet Applications through WAI-ARIA. There have been many improvements to both the visitor and administrator sides of Drupal, especially:

• Drag and drop functionality;
• Improved color contrast and intensity;
• Adding skip navigation to core themes;
• Adding labels by default for input forms;
• Fixing CSS display:none with consistent methods for hiding and exposing text on focus;
• Adding support for ARIA Live Regions with Drupal.announce(); and
• Adding a TabbingManager to support better keyboard navigation.

The community also added an accessibility gate for core issues in Drupal 8.

Extending the core
Drupal core is modular, defining a system of hooks and callbacks, which are accessed internally via an API.This design allows third-party contributed modules and themes to extend or override Drupal's default behaviors without changing Drupal core's code.

Drupal isolates core files from contributed modules and themes. This increases flexibility and security and allows administrators to cleanly upgrade to new releases without overwriting their site's customizations.The Drupal community has the saying, "Never hack core," a strong recommendation that site developers do not change core files.

Modules
Contributed modules offer such additional or alternate features as image galleries, custom content types and content listings, WYSIWYG editors, private messaging, third-party integration tools,integrating with BPM portals,and more. As of December 2019 the Drupal website lists more than 44,000 free modules.

Some of the most commonly used contributed modules include:

• Content Construction Kit (CCK): allows site administrators to dynamically create content types by extending the database schema. "Content type" describes the kind of information. Content types include, but are not limited to, events, invitations, reviews, articles, and products. The CCK Fields API is in Drupal core in Drupal 7.
• Views: facilitates the retrieval and presentation, through a database abstraction system, of content to site visitors. Basic views functionality has been added to core in Drupal 8.
• Panels: drag and drop layout manager that allows site administrators to visually design their site.
• Features: enables the capture and management of features (entities, views, fields, configuration, etc.) into custom modules.
• Context: allows definition of sections of site where Drupal features can be conditionally activated
• Media: makes photo uploading and media management easier
•     Services: provides an API for Drupal.
•     Organic Groups Mailing List

Themes

As of December 2019, there are more than 2,800 free community-contributed themes. Themes adapt or replace a Drupal site's default look and feel.

Drupal themes use standardized formats that may be generated by common third-party theme design engines. Many are written in the PHPTemplate engine or, to a lesser extent, the XTemplate engine.Some templates use hard-coded PHP. Drupal 8 will integrate the Twig templating engine.

The inclusion of the PHPTemplate and XTemplate engines in Drupal addressed user concerns about flexibility and complexity.The Drupal theming system utilizes a template engine to further separate HTML/CSS from PHP. A popular Drupal contributed module called 'Devel' provides GUI information to developers and themers about the page build.

Community-contributed themes at the Drupal website are released under a free GPL license,and the most installed Drupal themes are listed on this page.

Distributions
In the past, those wanting a fully customized installation of Drupal had to download a pre-tailored version separately from the official Drupal core. Today, however, a distribution defines a packaged version of Drupal that upon installation, provides a website or application built for a specific purpose.

The distributions offer the benefit of a new Drupal site without having to manually seek out and install third-party contributed modules or adjust configuration settings.They are collections of modules, themes, and associated configuration settings that prepare Drupal for custom operation. For example, a distribution could configure Drupal as a "brochure" site rather than a news site or online store.

Architecture
Drupal is based on the Presentation Abstraction Control architecture, or PAC.

The menu system acts as the Controller. It accepts input via a single source (HTTP GET and POST), routes requests to the appropriate helper functions, pulls data out of the Abstraction (nodes and, from Drupal 5 onwards, forms), and then pushes it through a filter to get a Presentation of it (the theme system).

It even has multiple, parallel PAC agents in the form of blocks that push data out to a common canvas (page.tpl.php

Specifications

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