On Thu, Aug 18, 2011 at 10:30 AM, Diederik van Liere <dvanli...@gmail.com>wrote:
> 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. page title/namespace and redirect-ness are not fixed to a revision, and may change over time. This means that simply knowing the last revision you left off at doesn't give you enough information for a continuation point; you'd have to go back and see if any revisions have been deleted or had their pages' title, redirectness, or other properties have changed. I think it may be better to abandon the "single XML stream" data model and allow for structure and random-access. A directory tree with separate files for various pages/revisions may be a lot easier to produce & update "in-place", and could be downloaded & resynced with standard tools like rsync or a custom tool that optimizes what files it looks for. There's basically a couple different problems to solve: 1) Building a complete data set and getting that out to people 2) Updating an existing data set with new data 3) Processing a data set in some useful way Generating the initial dump today is super expensive -- because it's a single compressed XML stream, we have to copy and re-copy most of the same data over, and over, and over. And today there's no good way to just "apply" an incremental dump on top of your existing download. 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. > > Broadly speaking some sort of diff storage makes a lot of sense; especially if it doesn't require reproducing those diffs all the time. :) But be warned that there are different needs and different ways of processing data; diffs again interfere with random access, as you need to be able to fetch adjacent items to reproduce the text. If you're just trundling along through the entire dump and applying diffs as you go to reconstruct the text, then you're basically doing what you already do when doing on-the-fly decompression of the .xml.bz2 or .xml.7z -- it may, or may not, actually save you anything for this case. Of course if all you really wanted was the diff, then obviously that's going to help you. :) -- brion _______________________________________________ Wikitech-l mailing list Wikitech-l@lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikitech-l
