Best practices for extensions

This page lists best practices for MediaWiki extensions to follow. Each item has a rating of how important it is. Ideally every extension should strive for the "gold standard".

This list was originally started at the 2017 Wikimania Hackathon.


 * REQUIRED - Meeting these criteria likely means your extension will work, but it may not be sustainable or maintainable.
 * SHOULD - Meeting these criteria likely means your extension works well and will continue to work well in the future
 * GOLD - Meeting these criteria means your extension is the ideal standard we all aspire to be, and should be used as an example to other developers

Meta

 * REQUIRED: Use the normal MediaWiki bug tracker / code review systems
 * REQUIRED: The extension should not be Wikimedia or organization specific
 * REQUIRED: For deployment on Wikimedia Wikis - Performance & Security review
 * SHOULD: Have co-maintainers! Doesn't matter if it's a WMF employee, but there should be 2 minimum.
 * SHOULD: Create a MediaWiki-Vagrant role for your extension.

Architecture

 * SHOULD: Provides all the same functionality over API and web (index.php)
 * GOLD: The same functionality should be implemented in a way without any duplication
 * SHOULD: Use dependency injection, avoid static calls for other than utility methods or hook entry points
 * GOLD: Don't overuse private visibility in services
 * SHOULD: Avoid global state (TODO: expand)
 * SHOULD: Be easy to debug
 * SHOULD: Use structured logging, with meaningful levels
 * SHOULD: Throw exceptions only for situations it should not handle and need to bubble up
 * SHOULD: Don't hardcode wikicode / templates or stuff, especially in a way that's not configurable for other websites
 * Code should be readable by someone who is familiar in that area
 * SHOULD: Don't call out to static functions in an unrelated class because it wasn't refactored or thought out well
 * SHOULD: Has a clear separation of concerns between what it actually does, and how its presented to the user
 * SHOULD: Think twice before adding new wikitext syntax functionality or something that will have a stable API for a very long time
 * SHOULD: skin and extension functionality should not be tightly integrated.
 * SHOULD: Don't add new user preferences, unless you really have a good reason to do so
 * GOLD: Expose your Javascript methods so 1) user scripts can access them, 2) it is easy to debug them. Do not make everything private.

Documentation

 * REQUIRED: Have an Extension:Foo page on mediawiki.org
 * REQUIRED: Add it to the appropriate extension categories
 * REQUIRED: Quick explanation of what it does
 * REQUIRED: Complete list of any dependencies and how to install them, configure, and uninstall* (TODO: clarify uninstall)
 * SHOULD: Clear description of exactly which MW versions it works with or declare the compatibility policy of your extension.
 * SHOULD: All configuration settings described in one place, from most-used to most-obscure
 * GOLD: Compare and contrast with similar extensions
 * GOLD: Document hooks used in the extension infobox, it's a nice method of exposing examples so that other developers can learn (they get automagically categorised).
 * GOLD: Instructions how to/information on limitations regarding uninstallation
 * REQUIRED: Having a Help:Extension:Foo page on mediawiki.org
 * GOLD: Documentation should have screenshots in multiple languages, ideally one in RTL
 * GOLD: Documentation should discuss some edge cases that were tested - to prove due diligence of not just testing on simple/generic articles
 * SHOULD: Consistently use &#x3C;code>, &#x3C;source>, &#x3C;kbd>, &#x3C;pre> in documentation
 * GOLD: Add/update the extension's page on WikiApiary (if required).

TODO: Update Documentation/Style guide

File structure
Overall, the extension's file layout should be organized: consistent naming, directory structure that is logical and not messy.
 * REQUIRED: Using the standard directory layout of,  ,  , etc.
 * or : Contains all (and only) PHP code
 * SHOULD: One class per file.
 * SHOULD: Classes in  namespace.
 * GOLD: Use PSR-4 structure for classes and files.
 * : Contains JS and CSS for ResourceLoader.
 * : Contains localized messages in JSON files.
 * : Contains PHPUnit test cases
 * : Contains parser test files
 * or : Contains full copy of the relevant license the code is released under
 * REQUIRED: Not having a giant root level directory
 * SHOULD: Not having dozens of nested directories that all only contain one or two things
 * SHOULD: Not having giant files/many separate tiny files (but follow one class per file pattern - many tiny classes may be a sign of something else wrong)
 * SHOULD: A README file that summarizes the docs and gives detailed installation directions
 * Prominent version-number indication in the README and main PHP files (ideally all files) (TODO: ???)
 * Use Semantic Versioning guidelines for version numbers.
 * Use Semantic Versioning guidelines for version numbers.

Database

 * REQUIRED: If adding database tables, it should use the LoadExtensionSchemaUpdates hook to ensure update.php works.
 * REQUIRED: Database access should always use database abstraction layer
 * Doing so will avoid a lot of easy SQL injection attack vectors
 * SHOULD: It works well in a distributed environment (concurrency, multiple databases, clustering).
 * Uninstallation maintenance script (dropping tables, removing added columns, deleting log entries, deleting page properties). (TODO: ???)
 * SHOULD: If it needs persistence, it creates nice SQL (primary keys, indexes where needed) and uses some caching mechanism where/if necessary

Coding conventions
Overall, follow the MediaWiki coding conventions for PHP, JavaScript, CSS, and any other languages that could be used.
 * SHOULD: Run MediaWiki-CodeSniffer for PHP coding conventions
 * SHOULD: Run eslint for JS coding conventions
 * SHOULD: Run stylelint for CSS coding conventions
 * SHOULD: Don't dump everything into one function (in JavaScript especially). If you're indented off the screen, you're doing it wrong
 * GOLD: Use code comments to document why you do things, not what you do. In long blocks of code, adding comments stating what each paragraph does is nice for easy parsing, but generally, comments should focus on the questions that can't be answered by just reading the code.

Testing

 * SHOULD: Have and run PHPUnit and/or QUnit tests
 * SHOULD: If there are parser functions or tags, have and run parser tests
 * GOLD: Have and run browser tests
 * GOLD: Test against RTL languages! (how to verify?)
 * GOLD: Test against language converter languages! (how to verify?)

i18n & Accessibility
Overall, your extension should be fully usable and compatible with non-English and non-left-to-right languages.
 * REQUIRED: Don't have hardcoded non-translatable strings in your code, use the proper localization functions (wfMessage)
 * REQUIRED: Using i18n and translate wiki, and have a clear prefix to ease search of messages to translate.
 * REQUIRED: Use HTML elements for their intended purpose. Aka use a &#x3C;button> and not a &#x3C;div> with a click handler for accessibility.
 * SHOULD: id's should only be for things you can navigate to. Everything else should be a class. Even if you want to only use it once, just use a unique class
 * SHOULD: i18n escape as close to output as possible. document whether functions take/except wikitext vs HTML
 * GOLD: Add qqq message strings for all messages that exist in en.json, and verify them using grunt-banana-checker

Security

 * REQUIRED: Don't touch HTML after it has been sanitized (common pattern is to use regex, but that's bad)
 * REQUIRED: Shelling out should escape arguments
 * REQUIRED: All write actions must be protected against CSRF
 * SHOULD: Use the normal MediaWiki csrf token system
 * SHOULD: Don't load external resources for privacy and performance
 * REQUIRED: Make sure privacy related issues (checkuser, oversight) are still covered when refactoring or writing new code

Don't reinvent / abuse MediaWiki
Overall, don't re-implement functionality or code that MediaWiki provides.
 * REQUIRED: Use MediaWiki functionality/wrappers for things like WebRequest vs $_GET. etc.
 * REQUIRED: Use hooks as opposed to hacks/reimplementing things
 * REQUIRED Use MediaWiki's validation/sanitization methods e.g. Html class, htmlspecialchars, etc.
 * REQUIRED: Don't disable parser cache unless you have a really good reason
 * REQUIRED: Use composer for 3rd party PHP library management.
 * SHOULD: Don't reimplement libraries that exist, don't reinvent the wheel
 * SHOULD: Don't disable OutputPage
 * SHOULD: If an abstraction exists (e.g. ContentHandler), use that instead of hooks
 * SHOULD: Don't make things harder than yourself - use standard functionality like extension.json's tests/phpunit auto discovery stuff
 * SHOULD: Use global MediaWiki configuration such as read-only mode

Uncategorized

 * Can I read the code with reasonable effort? Is it convoluted / unnecessarily complex?
 * Know when you should use ParserOutput methods vs. same methods in OutputPage
 * It's okay to file duplicate bugs (it's better than not filing it at all!)
 * Should extensions be adding user groups in the default extension?
 * Adding user rights are easy in code, but can be politically controversial and should be discussed with the community about who grants the rights and who has them by default

TODO: Check on Extension:BoilerPlate -- T97105 ("BoilerPlate extension should show latest coding practices")