MediaWiki architecture document/text/revision 2

Introduction
From the start, MediaWiki was developed specifically to be Wikipedia's software. Developers have worked to facilitate reuse by third-party users, but Wikipedia's influence and bias have shaped MediaWiki's architecture throughout its history.

Wikipedia is one of the top ten websites in the world, currently getting about 400 million unique visitors a month. It gets over 100,000 hits per second. Wikipedia isn't commercially supported by ads; it is entirely supported by a non-profit organization, the Wikimedia Foundation, which relies on donations as its primary funding model. This means that MediaWiki must not only run a top-ten website, but also do so on a shoestring budget. To meet these demands, MediaWiki has a heavy bias towards performance, caching and optimization. Expensive features that can't be enabled on Wikipedia are either reverted or disabled through a configuration variable; there is an endless balance between performance and features.

The influence of Wikipedia on MediaWiki's architecture isn't limited to performance. Unlike generic CMSes, MediaWiki was originally written for a very specific purpose: supporting a community that creates and curates freely-reusable knowledge on an open platform. This means, for example, that MediaWiki doesn't include regular features found in corporate CMSes (like a publication workflow or ACLs), but does offer a variety of tools to handle spam and vandalism.

So, from the start, the needs and actions of a constantly evolving community of Wikipedia participants have affected MediaWiki's development, and vice versa. The architecture of MediaWiki has been driven many times by initiatives started or requested by the community, such as the creation of Wikimedia Commons, or the Flagged Revisions feature. Developers made major architectural changes, like MediaWiki 1.12's preprocessor, because the way that MediaWiki was used by Wikipedians made it necessary.

MediaWiki has also gained a solid external user base by being open-source software from the beginning. Third-party reusers know that, as long as such a high-profile website as Wikipedia uses MediaWiki, the software will be maintained and improved. MediaWiki used to be really focused on Wikimedia sites, but efforts have been made to make it more generic and better accommodate the needs of these third-party users. For example, MediaWiki ships with an excellent web-based installer, making the installation process much less painful than when everything had to be done via the command line, and the software contained hardcoded paths for Wikipedia.

Still, MediaWiki is and remains Wikipedia's software, and this shows throughout its history and architecture.

Phase I: UseModWiki
Wikipedia was launched in January 2001. At the time, it was mostly an experiment, to try to boost the production of content for Nupedia, a free-content, but peer-reviewed, encyclopedia created by Jimmy Wales. Because it was an experiment, Wikipedia was originally powered by UseModWiki, an existing GPL wiki engine written in Perl, using CamelCase and storing all pages in individual text files with no history of changes made.

It soon appeared that CamelCase wasn't really appropriate to name encyclopedia articles. In late January 2001, UseModWiki developer and Wikipedia participant Clifford Adams added a new feature to UseModWiki: free links, e.g. the ability to link to pages with a special syntax (double square brackets), instead of automatic CamelCase linking. A few weeks later, Wikipedia upgraded to the new version of UseModWiki supporting free links, and enabled them.

While this initial phase isn't about MediaWiki per se, it provides some context and shows that, even before MediaWiki was created, Wikipedia started to shape the features of the software that powered it. UseModWiki also influenced some of MediaWiki's features, for example its markup language. The Nostalgia Wikipedia (http://nostalgia.wikipedia.org) contains a complete copy of the Wikipedia database from December 2001, when Wikipedia still used UseModWiki.

Phase II: the PHP script
In 2001, Wikipedia was not yet a top 10 website; it was an obscure project sitting in a dark corner of the Interwebs, unknown to most search engines, and hosted on a single server. Still, performance was already an issue, notably because UseModWiki stored its content in a flat file database. At the time, Wikipedians were worried about being "inundated with traffic" following articles in the New York Times, Slashdot or Wired.

So, in summer 2001, Wikipedia participant Magnus Manske (then a university student) started to work on a dedicated Wikipedia wiki engine in his free time. He aimed to improve Wikipedia's performance using a database-driven app, and to be able to develop Wikipedia-specific features that couldn't be provided by a "generic" wiki engine. Written in PHP and MySQL-backed, the new engine was simply called the "PHP script", "PHP wiki", "Wikipedia software" or "phase II".

The "PHP script" was made available in August 2001, shared on SourceForge in September, and tested until late 2001. As Wikipedia suffered from recurring performance issues because of increasing traffic, the English language Wikipedia eventually switched from UseModWiki to the PHP script in January 2002. Other language versions also created in 2001 were slowly upgraded as well, although some of them would stay powered by UseModWiki until 2004. An automated program, called "User:Conversion script", converted the last version of the existing articles to the phase II format; Brion Vibber partly restored previous revisions of the UseModWiki history on the English Wikipedia in September 2002.

As PHP software using a MySQL database, the PHP script was the first iteration of what would later become MediaWiki. It also introduced many critical features still in use today, like namespaces to organize content (including talk pages), skins, and special pages (including maintenance reports, a contributions list and a user watchlist).

Phase III: MediaWiki
Despite the improvements from the PHP script and database back-end, the combination of increasing traffic, expensive features and limited hardware continued to cause performance issues on Wikipedia. In 2002, Lee Daniel Crocker rewrote the code again, calling the new software "Phase III". Because the site was experiencing frequent difficulties, Lee thought there "wasn't much time to sit down and properly architect and develop a solution", so he "just reorganized the existing architecture for better performance and hacked all the code". Profiling features were added to track down slow functions.

The Phase III software kept the same basic interface, and was designed to look and behave as much like the Phase II software as possible. A few new features were also added, like a new file upload system, side-by-side diffs of content changes, and interwiki links.

Other features were added over 2002, like new maintenance special pages, or the "edit on double click" option. Performance issues quickly reappeared, though. For example, in November 2002, administrators had to temporarily disable the "view count" and "site" statistics, which were causing two database writes on every page view. They would also occasionally switch the site to read-only mode to maintain the service for readers, and disable expensive maintenance pages during high-access times because of table locking problems.

In early 2003, developers discussed whether they should properly re-engineer and re-architect the software from scratch, before the fire-fighting became unmanageable, or continue to tweak and improve the existing code base. They chose the latter solution, mostly because most developers were sufficiently happy with the code base, and confident enough that further iterative improvements would be enough to keep up with the growth of the site.

In June 2003, administrators added a second server, the first database server separate from the web server. (The new machine was also the web server for non-English Wikipedia sites.) Load-balancing between the two servers would be set up later that year. Admins also enabled a new page caching system that used the file system to cache rendered, ready-to-output pages for anonymous users.

June 2003 is also when Jimmy Wales created the Wikimedia Foundation, a nonprofit to support Wikipedia and manage its infrastructure and day-to-day operations. The "Wikipedia software" was officially named "MediaWiki" in July, as wordplay on the Wikimedia Foundation's name. What was thought at the time to be a clever pun would confuse generations of users and developers.

New features were added in July, like the automatically-generated table of contents, and the ability to edit page sections, both still in use today. The first release under the name "MediaWiki" happened in August 2003, concluding the long genesis of an application whose overall structure would remain fairly stable from there on.

PHP
PHP was chosen as the framework for Wikipedia's "Phase II" software in 2001; MediaWiki has grown organically since then, and is still evolving. Most MediaWiki developers are volunteers contributing in their free time, and they were very few in the early years. Some software design decisions or omissions may seem wrong in retrospect, but it's hard to criticize the founders for not implementing some abstraction which is now found to be critical, when the initial code base was so small, and the time taken to develop it so short.

For example, MediaWiki uses unprefixed class names, which can cause conflicts when PHP core and PECL developers add new classes: MediaWiki  class had to be renamed to   to be compatible with PHP 5.3. Consistently using a prefix for all classes (e.g. " ") would have made it easier to embed MediaWiki inside another application or library.

Relying on PHP was probably not the best choice for performance, since it has not benefited from improvements that some other dynamic languages have seen. Using Java would have been much better for performance, and simplified execution scaling for back-end maintenance tasks. On the other hand, PHP is very popular, which facilitates recruiting new developers.

Even if MediaWiki still contains "ugly" legacy code, major improvements have been made over the years, and new architectural elements have been introduced to MediaWiki throughout its history. They include the,  , and   classes, the   class and the   class hierarchy, ResourceLoader, and the   hierarchy. MediaWiki started without any of these things, but all of them support features that have been around since the beginning. Many developers are interested primarily in feature development, and architecture is often left behind, only to catch up later, as the cost of working within an inadequate architecture becomes apparent.

Security
Because MediaWiki is the platform for high-profile sites such as Wikipedia, core developers and code reviewers have enforced strict security rules. To make it easier to write secure code, MediaWiki gives developers wrappers around HTML output and database queries to handle escaping. To sanitize user input, one uses the  class, which analyzes data passed in the URL or via a POSTed form. It removes "magic quotes" slashes, strips illegal input characters and normalizes Unicode sequences. Cross-site request forgery (CSRF) is avoided by using tokens, and cross-site scripting (XSS) by validating inputs and escaping outputs, usually with PHP's  function. MediaWiki also provides (and uses) an XHTML sanitizer with the  class, and database functions that prevent SQL injection.

Configuration
MediaWiki offers hundreds of configuration settings, stored in global PHP variables. Their default value is set in, and the system administrator can override them by editing.

MediaWiki used to over-depend on global variables, including for configuration and context processing. Globals cause serious security implications with PHP's  function (which MediaWiki hasn't needed since version 1.2). This system also limits potential abstractions for configuration, and makes it more difficult to optimize the start-up process. Moreover, the configuration namespace is shared with variables used for registration and object context, leading to potential conflicts. From a user perspective, global configuration variables have also made MediaWiki seem difficult to configure and maintain. MediaWiki development has been a story of slowly moving context out of global variables and into objects. Storing processing context in object member variables allows those objects to be reused in a much more flexible way.

Database and text storage


MediaWiki has been using a relational database back-end since the Phase II software. The default (and best-supported) DBMS for MediaWiki is MySQL, which is the one that all Wikimedia sites use, but other DBMSes (such as PostgreSQL, Oracle, and SQLite) have community-supported implementations. A sysadmin can choose a DBMS while installing MediaWiki, and MediaWiki provides both a database abstraction and a query abstraction layer that simplify database access for developers.

The current layout contains dozens of tables. Many are about the wiki's content (e.g.,  ,  , and  ). Other tables include data about users, media files , caching and internal tools (  for the job queue), among others. Indices and summary tables are used extensively in MediaWiki, since SQL queries that scan huge numbers of rows can be very expensive, particularly on Wikimedia sites. Unindexed queries are usually discouraged.

The database went through dozens of schema changes over the years, the most notable being the decoupling of text storage and revision tracking in MediaWiki 1.5.



In the 1.4 model, the content was stored in two important tables,  (containing the text and metadata of the current revision of the page) and   (containing previous revisions); deleted pages were kept in. When an edit was made, the previously current revision was copied to the  table, and the new edit was saved to. When a page was renamed, the page title had to be updated in the metadata of all the  revisions, which could be a long operation. When a page was deleted, its entries in both the  and   tables had to be copied to the   table before being deleted; this meant moving the text of all revisions, which could be very large and thus take time.

In the 1.5 model, revision metadata and revision text were split: the  and   tables were replaced with   (pages metadata),   (metadata for all revisions, old or current) and   (text of all revisions, old, current or deleted). Now, when an edit is made, revision metadata don't need to be copied around tables: inserting a new entry and updating the  pointer is enough. Also, the revision metadata don't include the page title anymore, only its ID: this removes the need for renaming all revisions when a page is renamed

The  table stores metadata for each revision, but not their text; instead, they contain a text ID pointing to the   table, which contains the actual text. When a page is deleted, the text of all revisions of the page stays there and doesn't need to be moved to another table. The  table is composed of a mapping of IDs to text blobs; a flags field indicates if the text blob is gzipped (for space savings) or if the text blob is only a pointer an external text storage. Wikimedia sites use a MySQL-backed external storage cluster with blobs of a few dozen revisions. The first revision of the blob is stored in full, and following revisions to the same page are stored as diffs relative to the previous revision; the blobs are then gzipped. Because the revisions are grouped per page, they tend to be similar, so the diffs are relatively small and gzip works well. The compression ratio achieved on Wikimedia sites nears 98%.

MediaWiki also has built-in support for load balancing, added as early as 2004 in MediaWiki 1.2 (when Wikipedia got its second server — a big deal at the time). The load balancer (MediaWiki's PHP code that decides which server to connect to) is now a critical part of Wikimedia's infrastructure, which explains its influence on some algorithm decisions in the code. The system administrator can specify in MediaWiki's configuration that there is one master database server, and any number of slave database servers; a weight can be assigned to each server. The load balancer will send all writes to the master, and will balance reads according to the weights. It also keeps track of the replication lag of each slave. If a slave's replication lag exceeds 30 seconds, it will not receive any read queries to allow it to catch up; if all slaves are lagged more than 30 seconds, MediaWiki will automatically put itself in read-only mode.

MediaWiki's "chronology protector" ensures that replication lag never causes a user to see a page that claims an action they've just performed hasn't happened yet. This is done by storing the master's position in the user's session if a request they made resulted in a write query. The next time the user makes a read request, the load balancer read this position from the session, and tries to select a slave that has caught up to that replication position to serve the request. If none is available, it will wait until one is. It may appear to other users as though the action hasn't happened yet, but the chronology remains consistent for each user.

Execution workflow of a web request
is the main entry point for MediaWiki, and handles most requests processed by the application servers (i.e. requests that were not served by the caching infrastructure; see below). The code executed from  performs security checks, loads default configuration settings from , guesses configuration with   and then applies site settings contained in. It then instantiates a  object, and creates a   object  depending on the title and action parameters from the request.

can take a variety of action parameters in the URL request; the default action is, which shows the regular view of an article's content. For example, the request  displays the content of the article "Apple" on the English Wikipedia. Other frequent actions include  (to open an article for editing),   (to preview or save an article),   (to show an article's history) and   (to add an article to the user's watchlist). Administrative actions include  (to delete an article) and   (to prevent edits to an article).

is then called to handle most of the URL request. It checks for bad titles, read restrictions, local interwiki redirects, and redirect loops, and determines whether the request is for a normal or a special page.

Normal page requests are handed over to, to create an   object for the page , and then to  , which handles "standard" actions. Once the action has been completed,  finalizes the request by committing DB transactions, outputting the HTML and launching deferred updates through the job queue. commits the deferred updates and closes the task gracefully.

If the page requested is a Special page (i.e., not a regular wiki content page, but a special software-related page such as ),   is called instead of  ; the corresponding PHP script is then called. Special pages can do all sorts of magical things, and each has a specific purpose, usually independent of any one article or its content. Special pages include various kinds of reports (recent changes, logs, uncategorized pages) and wiki administration tools (user blocks, user rights changes), among others. Their execution workflow depends on their function.

Many functions contain profiling code, which makes it possible to follow the execution workflow for debugging, if profiling is enabled. Profiling is done by calling the  and   functions to respectively start and stop profiling a function; both functions take the function's name as a parameter. On Wikimedia sites, profiling is done for a percentage of all requests, to preserve performance. MediaWiki sends UDP packets to a central server that collects them and produces profiling data.

Caching
MediaWiki itself is improved for performance because it plays a central role on Wikimedia sites, but it is also part of a larger operational ecosystem that has influenced its architecture. Wikimedia's caching infrastructure has imposed limitations in MediaWiki; developers worked around the issues, not by trying to shape Wikimedia's extensively optimized caching infrastructure around MediaWiki, but rather by making MediaWiki more flexible, so it could work within that infrastructure, without compromising on performance and caching needs. For example, by default, MediaWiki displays the user's IP in the top-right corner of the interface (for left-to-right languages), as a reminder that it's how they're known to the software when they're not logged in. The  configuration variable allows the system administrator to disable this feature, thus making the page content independent of the user: all anonymous visitors can then be served the exact same version of each page.

On Wikimedia sites, most requests are handled by reverse caching proxies (Squids), and never even make it to the MediaWiki application servers. Squids contain static versions of entire rendered pages, served for simple reads to users who aren't logged in to the site. MediaWiki natively supports Squid and Varnish, and integrates with this caching layer by, for example, notifying them to purge a page from the cache when it has been changed. For logged-in users, and other requests that can't be served by Squids, Squid forwards the requests to the web server (Apache).

The second level of caching happens when MediaWiki renders and assembles the page from multiple objects, many of which can be cached to minimize future calls. Such objects include the page's interface (sidebar, menus, UI text) and the content proper, parsed from wikitext. The in-memory object cache has been available in MediaWiki since the early 1.1 version (2003), and is particularly important to avoid re-parsing long and complex pages.

Login session data can also be stored in memcached, which lets sessions work transparently on multiple front-end web servers in a load-balancing setup (Wikimedia heavily relies on load balancing, using LVS with PyBal).

Since version 1.16, MediaWiki uses a dedicated object cache for localized UI text; this was added after noticing that a large part of the objects cached in memcached consisted of UI messages localized into the user's language. The system is based on fast fetches of individual messages from constant databases (CDB), i.e. files with key-value pairs. CDBs minimize memory overhead and start-up time in the typical case; they're also used for the interwiki cache.

The last caching layer consists of the PHP opcode cache, commonly enabled to speed up PHP applications. Compilation can be a lengthy process; to avoid compiling PHP scripts into opcode every time they're invoked, a PHP accelerator can be used to store the compiled opcode and execute it directly without compilation. MediaWiki will "just work" with many accelerators such as APC, PHP accelerator and eAccelerator.

Because of its Wikimedia bias, MediaWiki is optimized for this complete, multi-layer, distributed caching infrastructure. Nonetheless, it also natively supports alternate setups for smaller sites. For example, it offers an optional simplistic file caching system that stores the output of fully rendered pages, like Squid does. Also, MediaWiki's abstract object caching layer lets it store the cached objects in several places, including the file system, the database, or the opcode cache.

ResourceLoader
Like in many web applications, MediaWiki's interface has become more interactive and responsive over the years, mostly through the use of JavaScript. Usability efforts initiated in 2008, as well as advanced media handling (e.g. online editing of video files), called for dedicated front-end performance improvements.

To optimize the delivery of JavaScript and CSS assets, the ResourceLoader module was developed to optimize delivery of JS and CSS. Started in 2009, it was completed in 2011 and has been a core feature of MediaWiki since version 1.17. ResourceLoader works by loading JS and CSS assets on demand, thus reducing loading and parsing time for unused features, for example in older browsers. It also minifies the code, groups resources to save requests, and can embed images as data URIs.

Context and rationale
A central part of effectively contributing and disseminating free knowledge to all is to provide it in as many languages as possible. Wikipedia is available in more than 280 languages, and encyclopedia articles in English represent less than 20 % of all articles. Because Wikipedia and its sister sites exist in so many languages, it is important not only to provide the content in the readers' native language, but also to provide a localized interface, and effective input and conversion tools, so that participants can contribute content.

For this reason, localization and internationalization (l10n & i18n) are a central component of MediaWiki. The i18n system is pervasive, and impacts many parts of the software; it's also one of the most flexible and feature-rich. Translator convenience is usually preferred to developer convenience, but this is believed to be an acceptable cost.

MediaWiki is currently localized in more than 350 languages, including non-latin and right-to-left (RTL) languages, with varying levels of completion. The interface and content can be in different languages, and have mixed directionality.

Content language
MediaWiki originally used per-language encoding, which led to a lot of issues; for example, foreign scripts could not be used in page titles. UTF-8 was adopted instead. Support for character sets other than UTF-8 was dropped in 2005, along with the major database schema change in MediaWiki 1.5; content must now be encoded in UTF-8.

Characters not available on the editors keyboard can be customized and inserted via MediaWiki's, an interface message that appears below the edit window; its JavaScript version automatically inserts the character clicked into the edit window. The WikiEditor extension for MediaWiki, developed as part of a usability effort, merges special characters with the edit toolbar. Another extension, called "Narayam", provides additional input methods and key mapping features for non-ASCII characters.

Interface language
Interface messages have been stored in PHP arrays of key-values pairs since the Phase III software was created. Each message is identified by a unique key, which is assigned different values across languages. Keys are determined by developers, who are encouraged to use prefixes for extensions; for example, message keys for the UploadWizard extension will start with, where   stands for MediaWiki extension.

MediaWiki messages can embed parameters provided by the software, which will often influence the grammar of the message. In order to support virtually any possible language, MediaWiki's localization system has been improved and complexified over time to accommodate their specific traits and exceptions, often considered oddities by English speakers.

For example, adjectives are invariable words in English, but languages like French require adjective agreement with nouns. If the user specified their gender in their preferences, the  files, where   is the ISO-639 code of the language (e.g.   for French); default messages are in English and stored in. MediaWiki extensions use a similar system, or host all localized messages in an  file. Along with translations, Message files also include language-dependent information such as date formats.

Contributing translations used to be done by submitting PHP patches for the  files. In December 2003, MediaWiki 1.1 introduced "database messages", a subset of wiki pages in the MediaWiki namespace containing interface messages. The content of the wiki page  is the message's text, and overrides its value in the PHP file. Localized versions of the message are at, e.g..

This feature has allowed power users to translate (and customize) interface messages locally on their wiki, but the process doesn't update i18n files shipping with MediaWiki. In 2006, Niklas Laxström created a special, heavily hacked MediaWiki website (now hosted at ) where translators can easily localize interface messages in all languages, simply by editing a wiki page. The  files are then updated in the MediaWiki code repository, where they can be automatically fetched by any wiki, and updated using the LocalisationUpdate extension. On Wikimedia sites, database messages are now only used for customization, and not for localization any more. MediaWiki extensions and some related programs, such as bots, are also localized at translatewiki.net.

To help translators understand the context and meaning of an interface message, it is considered a good practice in MediaWiki to provide documentation for every message. This documentation is stored is a special Message file, with the  language code, which doesn't correspond to a real language. The documentation for each message is then displayed in the translation interface on translatewiki.net. Another helpful tool is the  language code: when used with the   parameter to display a wiki page (e.g.  ), MediaWiki will display the message keys instead of their values in the user interface; this is very useful to identify which message to translate or change.

Registered users can set their own interface language in their preferences, in which case it overrides the site's default interface language. MediaWiki also supports fallback languages: if a message isn't available in the chosen language, it will be displayed in the closest possible language, and not necessarily in English. For example, the fallback language for Breton is French.

Users
Users are represented in the code using instances from the  class, which encapsulates all of the user-specific settings (user id, name, rights, password, email address, etc.). Client classes use accessors to access these fields; they do all the work of determining whether the user is logged in, and whether the requested option can be satisfied from cookies or whether a database query is needed. Most of the settings needed for rendering normal pages are set in the cookie to minimize use of the database.

MediaWiki provides a very granular permissions system, with basically a user permission for every possible action. For example, to perform the "Rollback" action (i.e. to "quickly rollback the edits of the last user who edited a particular page"), a user needs the  permission, included by default in MediaWiki's   user group. But it can also be added to other user groups, or have a dedicated user group only providing this permission (this is the case on the English Wikipedia, with the  array in  ; for instance,   allows all registered users to rename files. A user can belong to several groups, and inherits the highest rights associated with each of them.

However, MediaWiki's user permissions system was really designed with Wikipedia in mind, i.e. a site whose content is accessible to all, and only certain actions are restricted to some users. MediaWiki lacks a unified, pervasive permissions concept; it doesn't provide traditional CMS features like restricting read or write access by namespace, category, etc. A few MediaWiki extensions provide such features to some extent.

Content structure
The concept of namespaces was used in the UseModWiki era of Wikipedia, where talk pages where at the title " /Talk". Namespaces were formally introduced in Magnus Manske's first "PHP script". They were reimplemented a few times over the years, but have kept the same function: to separate different kinds of content. They consist of a prefix, separated from the page title by a colon (e.g.  or   and  ); the main content namespace has no prefix. Wikipedia users quickly adopted them, and they provided the community with different spaces to evolve. Namespaces have proven to be an important feature of MediaWiki, as they create the necessary preconditions for a wiki's community and set up meta-level discussions, community processes, portals, user profiles, etc.

The default configuration for MediaWiki's main content namespace is to be flat (no subpages), because it's how Wikipedia works, but it is trivial to enable them. They are enabled in other namespaces (e.g., where people can for instance work on draft articles) and display breadcrumbs.

Namespaces separate content by type; within a same namespace, pages can be organized by topic using categories, a pseudo-hierarchical organization scheme introduced in MediaWiki 1.3.

Content processing: MediaWiki markup language & Parser
The user-generated content stored by MediaWiki isn't in HTML, but in a markup language specific to MediaWiki, sometimes called "wikitext". It allows users to make formatting changes (e.g. bold, italic using quotes), add links (using square brackets), include templates, insert context-dependent content (like a date or signature), and make an incredible number of other magical things happen.

To display a page, this content needs to be parsed, assembled from all the external or dynamic pieces it calls, and converted to proper HTML. The parser is one of the most essential parts of MediaWiki, which also makes it difficult to change or improve. Because hundreds of millions of wiki pages worldwide depend on the parser to continue outputting HTML the way it always has, it has to remain extremely stable.

The markup language wasn't formally spec'd from the beginning; it started based on UseModWiki's markup, then morphed and evolved as needs have demanded. In the absence of a formal specification, the MediaWiki markup language has become a complex and idiosyncratic language, basically only compatible with MediaWiki's parser; it can't be represented as a formal grammar using BNF, EBNF or ANTLR syntaxes. The current parser's specification is jokingly referred to as "whatever the parser spits out from wikitext, plus a few hundred test cases".

There have been many attempts at alternative parsers, but none has succeeded so far. In 2004, an experimental tokenizer was written by Jens Frank to parse wikitext, and enabled on Wikipedia; it had to be disabled three days later, because of the poor performance of PHP array memory allocations. Since then, most of the parsing has been done with a huge pile of regular expressions, and a ton of helper functions. The wiki markup, and all the special cases the parser needs to support, have also become considerably more complex, making future attempts even more difficult.

A notable improvement was Tim Starling's preprocessor rewrite in MediaWiki 1.12, whose main motivation was to improve the parsing performance on pages with complex templates. The preprocessor converts wikitext to an XML DOM tree representing parts of the document (template invocations, parser functions, tag hooks, section headings, and a few other structures), but can skip "dead branches" in template expansion, such as unfollowed  cases and unused defaults for template arguments. The parser then iterates through the DOM structure and converts its content to HTML.

Recent work on a visual editor for MediaWiki has made it necessary to improve the parsing process (and make it faster), so work has resumed on the parser and intermediate layers between MediaWiki markup and final HTML (see Future, below).

Magic words and templates
MediaWiki offers "Magic words" that modify the general behavior of the page or include dynamic content into it. They consist of: behavior switches like  (to hide the automatic table of content) or   (to tell search engines not to index the page); variables like   or  ; and parser functions, i.e. magic words that can take parameters, like   (to output   in lowercase). Constructs like,   and  , used to localize the UI, are parser functions.

The most common way to include content from other pages in a MediaWiki page is to use templates. Templates were really intended to be used to include the same content on different pages, e.g. navigation panels or maintenance banners on Wikipedia articles; having the ability to create partial page layouts and reuse them in thousands of articles with central maintenance made a huge impact on sites like Wikipedia.

However, templates have also been used (and abused) by users for a completely different purpose. MediaWiki 1.3 made it possible for templates to take parameters that change their output; the ability to add a default parameter (introduced in MediaWiki 1.6) enabled the construction of a functional programming language implemented on top of PHP, which was ultimately one of the most costly features in terms of performance.

Tim Starling then developed additional parser functions (the ParserFunctions extension), as a stopgap measure against insane constructs created by Wikipedia users with templates. This set of functions included logical structures like  and , and other functions like   (to evaluate mathematical expressions) and   (for time formatting).

Soon enough, Wikipedia users started to create even more complex templates using the new functions, which considerably degraded the parsing performance on template-heavy pages. The new preprocessor introduced in MediaWiki 1.12 (a major architectural change) was implemented to partly remedy this issue. Recently, MediaWiki developers have discussed the possibility of using an actual scripting language, perhaps Lua, to improve performance.

Media files
Users upload files through the  page; administrators can configure the allowed file types through an extension whitelist. Once uploaded, files are stored in a folder on the file system, and thumbnails in a dedicated  directory.

Because of Wikimedia's educational mission, MediaWiki supports file types that may be uncommon in other web applications or CMSes, like SVG vector images, and multipage PDFs & DjVus. They are rendered as PNG files, and can be thumbnailed and displayed inline, as are more common image files like GIFs, JPGs and PNGs.

When a file is uploaded, it is assigned a  page containing information entered by the uploader; this is free text, which usually includes copyright information (author, license) and items describing or classifying the content of the file (description, location, date, categories, etc.). While private wikis may not care much about this information, on media libraries like Wikimedia Commons they are critical to organise the collection and ensure the legality of sharing these files. It has been argued that most of these metadata should, in fact, be stored in a queryable structure like a database table. This would considerably facilitate search, but also attribution and reuse by third parties — for example, through the API.

Most Wikimedia sites also allow "local" uploads to each wiki, but the community tries to store freely-licensed media files in Wikimedia's free media library, Wikimedia Commons. Any Wikimedia site can display a file hosted on Commons as if it were hosted locally. This custom avoids having to upload a file to every wiki to use it there.

As a consequence, MediaWiki natively supports foreign media repositories, i.e., the ability to access media files hosted on another wiki through its API and the  system. Since version 1.16, any MediaWiki website can easily use files from Wikimedia Commons through the  feature. When using a foreign repository, thumbnails are stored locally to save bandwidth. However, it is not (yet) possible to upload to a foreign media repository from another wiki.

Levels
MediaWiki's architecture provides different ways to customize and extend the software. This can be done at different levels of access: External programs can also communicate with MediaWiki through its machine API, if it's enabled, basically making any feature and data accessible to the user.
 * System administrators can install extensions and skins, and configure the wiki's separate helper programs (e.g. for image thumbnailing and TeX rendering) and global settings (see Configuration above).
 * Wiki sysops (sometimes called "administrators" too) can edit site-wide gadgets, JavaScript and CSS settings.
 * Any registered user can customize their own experience and interface using their preferences (for existing settings, skins and gadgets) or make their own modifications (using their personal JS and CSS pages).

JavaScript and CSS
MediaWiki can read and apply site-wide or skin-wide JavaScript and CSS using custom wiki pages; these pages are in the  namespace, and thus can only be edited by sysops; for example, JavaScript modifications from   apply to all skins, CSS from   applies to all skins, but   only applies to users with the Vector skin.

Users can do the same types of changes, which will only apply to their own interface, by editing subpages of their user page (e.g.  for JavaScript on all skins,   for CSS on all skins, or   for CSS modifications that only apply to the Vector skin).

If the Gadgets extension is installed, sysops can also edit gadgets, i.e. snippets of JavaScript code providing features that can be turned on and off by users in their preferences. Upcoming developments on gadgets will make it possible to share gadgets across wikis, thus avoiding duplication.

This set of tools has had a huge impact and greatly increased the democratization of MediaWiki's software development. Individual users are empowered to add features for themselves; power users can share them with others, both informally and through globally-configurable sysop-controlled systems. This framework is ideal for small, self-contained modifications, and presents a lower barrier of entry than heavier code modifications done through hooks and extensions.

Extensions and skins
When JavaScript and CSS modifications are not enough, MediaWiki provides a system of hooks that let third-party developers run custom PHP code before, after, or instead of MediaWiki code for particular events. MediaWiki extensions use hooks to plug into the code.

Before hooks existed in MediaWiki, adding custom PHP code meant modifying the core code, which was neither easy nor recommended. The first hooks were proposed and added in 2004 by Evan Prodromou; many more have been added over the years when needed. Using hooks, it is even possible to extend MediaWiki's wiki markup with additional capabilities, using tag extensions.

The extension system isn't perfect: extension registration is based on code execution at startup, rather than cacheable data, which limits abstraction and optimization and hurts MediaWiki's performance. But overall, the extension architecture is now a fairly flexible infrastructure that has helped make specialized code more modular, keeping the core software from expanding (too) much, and making it easier for third-party users to build custom functionality on top of MediaWiki.

Conversely, it's very difficult to write a new skin for MediaWiki without reinventing the wheel. In MediaWiki, skins are PHP classes each extending the parent  class; they contain functions that gather the information needed to generate the HTML. The long-lived "MonoBook" skin was difficult to customize because it contained a lot of browser-specific CSS to support old browsers; editing the template or CSS required many subsequent changes to reflect the change for all browsers and platforms.

API
The other main entry point for MediaWiki, besides, is  , used to access its machine-readable query API (Application Programming Interface).

Wikipedia users originally created "bots" that worked by screen scraping the HTML content served by MediaWiki; this method was very unreliable and broke many times. To improve this situation, developers introduced a read-only interface (located at ), which then evolved into a full-fledged read and write machine API providing direct, high-level access to the data contained in the MediaWiki database.

Client programs can use the API to login, get data, and post changes. The API supports thin web-based JavaScript clients and end-user applications. Almost anything that can be done via the web interface can basically be done through the API. Client libraries implementing the MediaWiki API are available in many languages, including Python and .NET.

Future
What started as a summer project done by a single volunteer PHP developer has grown into MediaWiki, a mature, stable wiki engine powering a top-ten website with a ridiculously small operational infrastructure. This has been made possible by constant optimization for performance, iterative architectural changes and a team of awesome developers.

The evolution of web technologies, and the growth of Wikipedia, call for ongoing improvements and new features, some of which require major changes to MediaWiki's architecture. This is, for example, the case for the ongoing visual editor project, which has prompted renewed work on the parser and on the wiki markup language, the DOM and final HTML conversion.

MediaWiki is a tool used for very different purposes. Within Wikimedia projects, for instance, it's used to create and curate an encyclopedia (Wikipedia), to power a huge media library (Wikimedia Commons) or to transcribe scanned reference texts (Wikisource); and so on. In other contexts, MediaWiki is used as a corporate CMS, or as a data repository, sometimes combined with a semantic framework. These specialized uses that weren't planned for will probably continue to drive constant adjustments to the software's internal structure. As such, MediaWiki's architecture is very much alive, just like the immense community of users it supports.

Notes and references

 * Automatically-generated MediaWiki documentation:
 * Domas Mituzas, Wikipedia: site internals, configuration, code examples and management issues, MySQL Users conference, 2007. Full text available at