This is a rough sketch to seek input from other developers.
Enable MediaWiki to respond to page views in a fairly performant manner for all users (logged-in or not).
- A site admin can enable static caching (eg. wgUseFileCache) and the cache will be used for both logged-out and logged-in users by having the minimal run-time still apply the skin after FileCache. (It is currently applied before FileCache, and anon-only.)
- Routing logic will know which urls are page views and which not. (Currently special pages can disable OutputPage mid-way.)
- The Skin is given (cacheable) data about the skin, the page and the user. It will produce its HTML output without performing additional queries. Therefore, any additional DB queries skins currently make for page-related data must instead happen in a way that is triggered earlier when the page is being cached (e.g. skin hooks for Parser, ParserCache or FileCache?)
Small but impactful changes
- MediaWiki will have a router that reliably know how to construct an output handler (e.g. no unpredictable OutputPage::disable() half-way through the request, such as in SpecialExport, RawAction and elsewhere).
- FileCache no longer caches entire HTML response. Instead it just stores the core page content and relevant data properties needed by the skin (e.g. display title, last modified date, categories, permissions, etc.).
- Skin Template must be programming-language neutral (e.g. Mustache template).
- Data flow in one direction only. No more two-way communication between Skin and OutputPage. OutputPage injects all relevant data (maybe some of it lazy-computed) into the Skin class. Skin class may combine it with User info and transform properties (renaming, deriving, etc.), and then invokes the Skin template with it.
- Skin hooks must change from being "callables with global context returning uncacheable raw html" to instead be "callables that return cacheable HTML or template partials". They can still vary by user and page (e.g. "What links here", "My contributions"), but the decision must be made in Mustache syntax using only the available user/page information. This way it can be rendered in a different server process, or even client-side, and still work and have all the information (T106099). It also means that on page views, all available meta data is naturally already fetched and cacheable. If extension hooks require additional information, they can do so, but must use hook behind the cache, not in front of it. E.g. to do their query when the page cache blob is generated. Not on-demand for every page view.
- As things unfold, it might make sense to obsolete FileCache in favour of a better ParserOutput cache (if it has all the needed data and gains support for storing on-disk instead of in SQL). Given that the run-time overhead of taking a ParserOutput object and applying the Skin template would be cheap enough that it would be negligible. Alternatively, we may change FileCache to be more like a companion to ParserCache that caches only the extra data needed by skins. – Or store them in regular ObjectCache/mainCache instead.
- Deprecate OutputPage::disable().
- Provide a router. This will map paths and query parameters to a handler, similar to MediaWiki::parseTitle() and other methods in MediaWiki.php do now. The decision to not use a skin or OutputPage for a response must be made here. The router registry should be in a static programming-language neutral format that can be exposed over the API for potential use by a different server process.
- Make data flow in one direction from OutputPage to Skin.
- Convert Skin hooks.
- [ x] Convert one Skin (e.g. Vector) to become a template, as example. (Note, there is no pressure to remove support for PHP-based skins, the only requirement is that it can handle responses in front of the page cache instead of behind it, e.g. using data properties instead of run-time database queries and WikiPage methods calls).
What this task is not
This task does not change the architecture of MediaWiki or the skin system. It also does not change how MediaWiki is used or deployed at WMF.
This work covered by this task resolves long-standing technical debt in MediaWiki for the purposes of improved rendering performance and reduced overall complexity and maintenance cost for the skin system – whilst remaining fully backwards compatible.
The intended result is improved performance of rendering page views in MediaWiki core, both out-of-the-box and at WMF.
The works covered by this task should in my opinion be considered a prerequisite for the following proposals. Some of the below might be possible without this task, although I believe doing so would incur significant amounts of maintenance overhead and technical debt that can and should be avoided (through this task).
- Add a way for MediaWiki to render wiki page, actions, and special pages without a skin. – T114596
- Re-implement MediaWiki skin rendering for WMF-deployed skins in a micro-service that runs near the CDN. – T111588
- Re-implement MediaWiki skin rendering for WMF-deployed skins in a way that can be run offline and client-side using service workes. – T106099.