Effie Mouzeli

Wikimedia Foundation

According to Wikipedia, “Wikipedia is a multilingual, web-based, free-content encyclopedia project supported by the Wikimedia Foundation and based on a model of openly editable content.” Serving billions of page views per month, Wikipedia is one of highest-traffic websites in the world. Let me explain what happens when you are visiting Wikipedia to read about Saint Helena or llamas.

First, these are the three most important building blocks of our infrastructure:

• The CDN (content delivery network), which is our caching layer

• The application layer

• Open-source software

When you request a page, the magic of our geographic DNS and internet routing sends this request to the nearest Wikimedia data center, based on your location, while with the wizardry of TLS, ATS (Apache Traffic Server) encrypts your connection. Each data center has two caching layers: in-memory (Varnish) and on disk (ATS). Most requests terminate here, because the hottest URLs are always cached. In case of cache misses, the request will be forwarded to the application layer, which might be very near if this is a primary data center, or a bit farther away if this is a caching point.

Our application layer has MediaWiki at its core, supported by a number of microservices and databases. MediaWiki is an Apache, PHP, MySQL open-source application, developed for Wikipedia. MediaWiki will look for a rendered version of the article initially on Memcached and, if not found, then on a MariaDB database cluster called Parser Cache.

If MediaWiki gets misses from Memcached and Parser Cache, it will pull the article’s Wikitext and render it. Articles are stored in two database clusters: the Wikitext cluster, where Wikitext is stored in blobs, and the metadata cluster, which tells MediaWiki where an article is located in the Wikitext cluster. After an article is rendered, it is stored in turn in all aforementioned caches and, of course, is served back to you.

Things are slightly simpler when the request is a media file rather than a page. On a cache miss in the caching layer, ATS will directly fetch the file from Swift, a scalable object storage system by OpenStack.

As you can see, MediaWiki is surrounded by a very thick caching layer, and the reason is simple: rendering pages is costly. Furthermore, when a page is edited, it needs to be invalidated from all these caches and then populated again. When very famous people die, our infrastructure experiences a phenomenon called celebrity death spikes (or the Michael Jackson effect ¹). During this event, everyone links to Wikipedia to read about them while editors are spiking the edit rate by constantly updating the person’s article. Eventually, this could cause noticeable load as heavy read traffic focuses on an article that’s constantly being invalidated from caches.

The final building block is our use of open-source software. Everything we run in our infrastructure is open source, including in-house developed applications and tools. The community around the Wikimedia movement is not only limited to caring for the content in the various projects, its contribution extends to the software and systems serving it. Open source made it possible for members of the community to contribute; it is an integral part of Wikipedia and one of the driving forces behind our technical choices. Wikipedia obeys Conway’s law in a way: a website that promotes access to free knowledge runs on free software.

It might sound surprising that one of the most popular websites is run using only open-source software and without an army of engineers—but this is Wikipedia; openness is part of its existence.