ResourceLoader/Architecture

This page goes in-depth on the features of ResourceLoader. Together they create the environment that is ResourceLoader, which is known to make developers, servers and users alike, happy!

Check Presentations for videos and slides about these features.

Modules
ResourceLoader works with a concept of modules. A module is a group of resources identified by a symbolic name. They can contain any of the following types of resources:
 * Scripts
 * Styles
 * Messages

Aside from that a module may have several properties:
 * Dependencies
 * Group

All in all, this makes it possible to get a module by just using its name (instead of listing out all the resources and/or dependencies etc.).

Modules are delivered to the client as a bundle with all their containing resources in a single request. More about this follows in the resource sections. The package is unboxed by the Client.

Wrapping
When one or more module packages are sent to the client, the script components are wrapped in a special closure. They are not immediately executed when the browser parses the response. Instead, the closure is passed to the ResourceLoader client. This allows it to control the order in which the closures execute independently from the order in which they arrive from the server, with regards to the dependency tree (which the client has in memory). See also the Client section for more about the loading procedure and a walkthrough of a simple and more advanced example scenario.

Minification
All scripts are minified before being put in the package. For this we use the JavaScriptMinifier library. In case of a cache-miss, the minification is done on-the-fly on the web server. See also the Caching section for more about the performance of packaging and the caching infrastructure around it.

Conditions
Scripts can be conditionally included in a module based on the environment of the requesting client. This keeps responses as small as possible by only including relevant components while also allowing progressive enhancement and conditional inclusion of features based on the environment.

Examples:
 * A grammar parser that includes different script files based on the user language.
 * The "mediawiki.log" module is only accompanied with the framework in debug mode. It overwrites the empty placeholder function "mw.log" with one that passes it on to the native console (if there is one), or it builds one in the DOM for older browsers and inserts the messages in there.
 * The Vector skin in MediaWiki has a custom jQuery UI theme. When a jQuery UI module is loaded, depending on the environment it loads the default jQuery UI theme or the Vector theme.
 * jQuery UI Datepicker has regional definitions for 62 different languages. Only one of the regional definition files will be included. Fallback is handled by MediaWiki's localization framework.

Embedding


In order to reduce the number of HTTP requests for images used in the interface, ResourceLoader makes use of Data URI embedding. When enabled, images will be automatically base64-encoded and embedded into the stylesheet. While it will make the stylesheet larger (due to base64 inflation), it improves performance by removing the overhead associated with requesting all those additional files. The actual server response (which contains the minified result of all concatenated stylesheets and all embedded images in a single request) uses gzip compression. This enables the response to function a bit like a "super sprite" (more on this later). Regardless of the expansion caused by base64 encoding, the gzipped ResourceLoader response is still smaller than the sum of the individual CSS and image binary files.

To enable embedding for an image, use the " " annotation in a CSS comment over the relevant css declaration. For example:

Using this technique makes traditional sprites obsolete. The motivation of a sprite is good:
 * It reduces HTTP requests by combining multiple images into one.
 * It allows for better compression by providing more sample data in a single file.

But sprites come with a few caveats: If an image needs to be updated, one has to regenerate the entire sprite, update the background positions, etc. Unrelated stylesheets would be referencing the same image with arbitrary background positions, it isn't obvious which image is being referred to, like:
 * Maintenance.
 * Produces overly complex CSS.

compared to the much clearer: . No freedom in background-repeat, -size or -position as that would "leak" other parts of the sprite. Therefore in various layouts, images cannot be converted into a sprite.
 * Imposes restrictions on usage of the image.

These caveats aren't the end of the world (sprites are in wide use, clearly they do work). Many resource delivery systems out there use sprites, some even in a semi-automated fashion (making some of the above caveats less painful). ResourceLoader's automated embedding technique, however, provides the best of both worlds – without any of the caveats.

The advantages of sprites still hold up:
 * Reduced number of HTTP requests.
 * Improved compression by combining images in one file.

In addition to that, it is even better than sprites:
 * No maintenance.
 * Clean CSS.
 * No restrictions or sprite "leakage" bugs (with regards to background-position).
 * Even smaller number of requests. The CSS and the images are now in the same request.
 * No download delay. Once the stylesheet is there, all the images are there as well. This greatly improves on how responsive the site feels to the user. Because of how CSS works (browsers don't download files referenced in CSS rules for selectors that don't (yet) match any elements, i.e. ), this means there will be no "blink" when the user first hovers a button, as the image is instantly available from the embedded data URI.
 * The Gzip compression will be even better. Completely unrelated modules can be combined and compressed together, without having to worry about anything - a "super sprite", if you will. Also, PNG headers will be compressed even stronger.

Remapping
ResourceLoader applies path remapping for images not being embedded (e.g. images that exceed the size limit or for browsers that don't support embedding). When the stylesheets are combined during the packaging process, these image urls would break as they would lose the context of a parent directory. Remapping corrects these urls to be relative to the loader location instead.

Flipping

 * See also Directionality support for more information about directionality support in MediaWiki.



With the Flipping functionality it is no longer necessary to manually maintain a copy of the stylesheet for right-to-left languages. ResourceLoader automatically changes direction-sensitive CSS declarations (and more). Internally, the CSSJanus library provides that smart "flipping" logic.

Aside from flipping direction-oriented values, it also converts property names and shorthand values. And it converts references to filenames ending in  into filenames ending in , thereby loading direction-specific iconography,

Consider the following example:

When loaded by ResourceLoader, without any additional changes or configuration, it is automatically turned into the following for users with a right-to-left interface language set:

Sometimes you may want to exclude a rule from being flipped. For that one can use the  annotation. This instructs CSSJanus to skip the next CSS declaration. Or, when used in the selector part, it skips the entire following CSS ruleset.

For example:

Output will be:

Bundling
As mentioned, all resources are combined in a single package. The loader response from the server bundles both scripts and styles from the requested module(s) in the same request. The Client receives this and loads the stylesheet in the DOM at the right time, so they are in memory when the relevant scripts that use these CSS classes, execute.

This means that neither the JS nor the CSS will run if JS is disabled. However, if you need the CSS to still run, you can add one or more CSS-only module with.

Minification
All stylesheets are minified before being put in the package. For this we use the CSSMin library, which was especially developed for ResourceLoader.

Conditions

 * See the Conditions section under Scripts for more information.

Similar to scripts, style bundling also features the ability to compose the module dynamically based on the environment.

Resource: Messages
Messages are exported as a JSON blob, mapping the message keys to the correct translation. They're fetched on the server from MediaWiki's localization framework (including its language fallback logic). Only message keys used by the module are included in the package.

Bundling
Again, all resources are bundled in the same request. The Client then takes the messages and registers them in the localization system on the client side, before the javascript body is executed.

Conditions
As with the other two resource types, the messages component is also optimized to load only what is necessary for the requesting environment. This is especially important considering that MediaWiki is localized in over 300 languages. Only 1 unique set of messages is delivered to the client.

Front-end
So, how does all this play out in the front-end? Lets walkthrough a typical page view in MediaWiki, focussing on the ResourceLoader Client.

Startup Module


The startup module is the first and only hardlinked script being loaded on every page from a tag. It is a lightweight module that does 3 things:


 * Sanity check It starts by performing a quick sanity check that bails out if the current browser is not supported. This saves bandwidth as well as preventing broken interfaces, basically leaving the user with an untouched page with the natural non-javascript fallback behavior. Browsers such as Internet Explorer 5 and early versions of Mozilla fall in this category. For those the startup module is the first and last script to be loaded.
 * Module manifest It exports the module manifest. Containing all the dependency information of all modules, cache group (if any) and the last-modified timestamp of each module.
 * Configuration A subset of the server-side configuration of MediaWiki is made available to client-side scripts.

After that it makes a loader request for jQuery + the main ResourceLoader Client.

Client
The ResourceLoader Client is loader software written in JavaScript that is instantiated given the module manifest (from the startup module) and from there everything continues.

Loader
The loader can be given a list of module names to load. It automatically handles dependency resolution using the internal dependency map. Then it takes the list of modules that still need to be loaded and requests them in a batch from a server. The loading process is fully asynchronous.

Execution

 * This section is incomplete


 * Execution separated from loading/parsing
 * Direct or delayed execution as appropriate based on module dependencies
 * Insert messages and styles into memory before script execution

Back-end

 * This section is incomplete

Environment
Modules are composed based on the following environmental variables:
 * mode (debug or production)
 * skin
 * user language

The result of the generated module package is cached (see also Caching).

The load responder supports various methods of encouraging client-side caching, such as responding with an empty  to   headers if possible.

Response
GET /load.php?modules=foo|bar|quux&lang=en&skin=vector

Debug mode

 * This section is incomplete

To make development easier, there is a debug mode in ResourceLoader.

Differences:
 * Script resources: No longer minified, concatenated and loaded from load.php. Instead load.php instructs the client loader to request the files directly. As a result they are also executed in the global scope (since browsers execute scripts like that by default when not otherwise modified).
 * Style resources: No longer minified, concatenated and loaded from load.php. Instead load.php instructs the client loader to request the files directly. As a result data URI embedding and RTL-flipping no longer applies, because it will load the raw file as-is from disk.

Toggle mode
The mode can be toggled in several ways. In order of precedence:


 * Query parameter  (string): Set to "true" to enable, to "false" to disable. When absent, falls back to next step.  http://example.org/wiki/Main_Page?debug=true
 * Cookie  (string): Set to "true" or "false". When absent, falls back to next step.


 * There is a user script available to simplify toggling this. Add:

// (workaround for [[bugzilla:33355]])


 * to Special:MyPage/common.js on the wiki where you're debugging. A link to enable or disable debug mode will be added to the toolbox.  In some browsers, you may need to hard refresh before the change takes effect.


 * (boolean): The default mode is determined by this configuration setting. Unless overridden in LocalSettings, this will be set to . A production wiki should never set this to true as debug mode will then be served to everybody. Thus being inefficient and likely introducing bugs due to the nature of debug mode.

Balance

 * This section is incomplete


 * Batching
 * Alphabetical order
 * Module timestamps in url (to allow static reverse-proxy caching)

Groups
Groups are used as fine-tune mechanism to balance cache fragmentation.

Imagine two unrelated front-end applications using jQuery UI. One on Special:Foo and one on Special:Bar. Without groups, visiting Foo would result in a batch request for. Visiting Bar would result in a request for. Bar's module request would not be a cache hit as it has a different url. Re-downloading all of jQuery UI again would be unfortunate. It may also consume additional space in the browser cache to store both sets (e.g. on mobile).

Cache groups can help us out here by forcing jQuery UI to load in its own HTTP request. Then, Foo would, at the expense of one extra HTTP roundtrip, request  and   separately. Visiting Bar would request  and hit the local cache for.

Use groups sparingly as it comes at the cost of an extra HTTP roundtrip. It should be reserved for fine-tuning after deployment if loading of a certain module is a measurable performance problem (or with high confidence beforehand). A preferred solution for performance is to defer loading to later point in time (not at page load). E.g. based on user interaction. That way the event would naturally perform its own dedicated (cacheable) request, without needing to hardcode any module groups.

Groups can have freeform strings as their name.

There is one special group, " ", which is reserved for modules that are not allowed to be loaded from the public  endpoint. Instead, these are embedded in page output (to prevent CSRF).

On-demand package generation
ResourceLoader features on-demand generation of the module packages. The on-demand generation is very important in MediaWiki because cache invalidation can come from many places. Here's a few examples:
 * Core
 * Extensions Core and extensions generally only change when a wiki is upgraded. But especially on large sites such as Wikipedia, deployments happen many times a day (even updates to core).
 * Users Wiki users granted certain user rights (administrators by default) have the ability to modify the "site" module (which is empty by default and will be loaded for everybody when non-empty). This is all without servers-side access, these scripts/styles are stored as wiki pages in the database. On top of that, each user also has its own module space that is only loaded for that user.
 * Translators The interface messages are shipped with MediaWiki core and are generally considered part of core (and naturally update when upgrading/deploying core). However wikis can customize their interface by using the MediaWiki message namespace to modify interface messages (or create new ones to use in their own modules).

Cache invalidation
Every module has an automatically generated Last-Modified timestamp. This timestamp is based on a number of factors. All together, used to ensure proper cache busting when needed while also avoiding unnecessary re-generation of the package when it isn't needed.
 * JavaScript files and CSS files Last modified timestamps from the files on disk.
 * Files referenced in CSS Extracted from the stylesheets (e.g., see also Remapping). Last modified timestamps from the files on disk.
 * Localization Last time any of the messages has been updated in the localization storage.
 * Dependencies It works recursively for any module that this module depends on.

Because of all these different origins and cache invalidation factors, it is not desirable to have to manually (or scheduled) perform a "build" of some kind that would generate a huge package with everything in it.
 * Not only would it waste resources re-building many modules that haven't changed,
 * it would also be impractical to have one big build with everything in it because there are over 350 modules on an average Wikipedia site. Many of these come from extensions and wiki-users and these modules are likely only needed/loaded on certain pages or in certain states of the interface. For more on that see client side loader.
 * And it would not make efficient use of browser cache, since it would have to invalidate the entire "build" if even the slightest change is made (the solution that ResourceLoader has for this is explained under Balance).

Whenever a module is requested from the server, the above is evaluated. If needed, the cache will be re-generated. All phases of the packaging process have been optimized to be able to run on-demand on the web server. No build scripts, no periodic cron tasks.

Conclusion


In conclusion we'd like to think of ResourceLoader as creating a development environment that is optimized for:


 * Happy developers Easy to work with modules without worrying about optimization, maintenance, building and what not.
 * Happy servers The application itself is lightweight, scales well and is optimized to run on-demand.
 * Happy users Faster pages!

JavaScriptMinifier
Although the re-generation of a module package should be relatively rare (since cache is very well controlled), when it does happen it has to perform well from a web server.

For that reason it doesn't use the famous JSMin.php library (based on Douglas Crockford's JSMin) because it is too slow to run on-demand during a request response. Although JSMin.php only takes about 1 second for (which is okay if you're on the command-line), when working on-demand in a web server response (with potentially dozens of large files needing to be minified) waiting this long is unacceptable. Especially if potentially thousands of requests could come in at the same time, all finding out that the cache isn't up to date (to avoid a cache stampede).

Instead ResourceLoader uses an implementation of Paul Copperman's JavaScriptMinifier, which is up to 4X faster than JSMin. In addition to the speed, time has told that JavaScriptMinifier interprets the JavaScript syntax more correctly and succeeds in situations where JSMin outputs invalid JavaScript. The output size of JavaScriptMinifier is slightly larger than JSMin (about 0.5%; based on a comparison by minifying jquery.js, for which the difference was 0.8KB). The reason this is not considered a loss is because it is put in the bigger picture. ResourceLoader doesn't aim to compress as small as can be no matter the cost. Instead it aims for balance, get large gains in a wide range of areas while also featuring instant cache invalidation, fast module generation, a transparent "build"-free environment for the developer, etc. The fact that it could be a little bit smaller then becomes an acceptable trade off.

CSSMin
Features:
 * Minification
 * Remapping
 * Data URI Embedding