Hi!
Over the last year, I have been using the Wikipedia XML dumps extensively. I used it to conduct the Editor Trends Study [0] and me and the Summer Research Fellows [1] have used it in the last three months during the Summer of Research. I am proposing some changes to the current XML schema based on those experiences.
The current XML schema presents a number of challenges for both the people who are creating dump files as the people who are consuming the dump files. Challenges include:
1) The embedded structure of the schema, a single <page> tag with multiple <revision> tags makes it very hard to develop an incremental dump utility 2) A lot of post processing is required. 3) By storing the entire text for each revision, the dump files are getting so large that they become unmanageable for most people.
1. Denormalization of the schema Instead of having a <page> tag with multiple <revision> tags, I propose to just have <revision> tags. Each <revision> tag would include a <page_id>, <page_title>, <page_namespace> and <page_redirect> tag. This denormalization would make it much easier to build an incremental dump utility. You only need to keep track of the final revision of each article at the moment of dump creation and then you can create a new incremental dump continueing from the last dump. It would also easier to restore a dump process that crashed. Finally, tools like Hadoop would have a way easier time handling this XML schema than the current one.
2. Post-processing of data Currently, a significant amount of time is required for post-processing the data. Some examples include: * The title includes the namespace and so to exclude pages from a particular namespace requires generating a separate namespace variable. Particularly, focusing on the main namespace is tricky because that can only be done by checking whether a page does not belong to any other namespace (see bug https://bugzilla.wikimedia.org/show_bug.cgi?id=27775). * The <redirect> tag currently is either True or False, more useful would be the article_id of the page to which a page is redirected. * Revisions within a <page> are sorted by revision_id, but they should be sorted by timestamp. The current ordering makes it even harder to generate diffs between two revisions (see bug https://bugzilla.wikimedia.org/show_bug.cgi?id=27112) * Some useful variables in the MySQL database are not yet exposed in the XML files. Examples include: - Length of revision (part of Mediawiki 1.17) - Namespace of article
3. Smaller dump sizes The dump files continue to grow as the text of each revision is stored in the XML file. Currently, the uncompressed XML dump files of the English Wikipedia are about 5.5Tb in size and this will only continue to grow. An alternative would be to replace the <text> tag with a <text_added> and <text_removed> tags. A page can still be reconstructed by patching multiple <text_added> and <text_removed> tags. We can provide a simple script / tool that would reconstruct the full text of an article up to a particular date / revision id. This has two advantages: 1) The dump files will be significantly smaller 2) It will be easier and faster to analyze the types of edits. Who is adding a template, who is wikifying an edit, who is fixing spelling and grammar mistakes.
4. Downsides This suggestion is obviously not backwards compatible and it might break some tools out there. I think that the upsides (incremental backups, Hadoop-ready and smaller sizes) outweigh the downside of being backwards incompatible. The current way of dump generation cannot continue forever.
[0] http://strategy.wikimedia.org/wiki/Editor_Trends_Study, http://strategy.wikimedia.org/wiki/March_2011_Update [1] http://blog.wikimedia.org/2011/06/01/summerofresearchannouncement/
I would love to hear your thoughts and comments!
Best, Diederik