Re: [Wikitech-l] Changing XML Wikipedia Schema to Enable Smaller Incremental Dumps that are Hadoop ready
On Thu, Aug 18, 2011 at 10:30 AM, Diederik van Liere dvanli...@gmail.comwrote: 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
Re: [Wikitech-l] Changing XML Wikipedia Schema to Enable Smaller Incremental Dumps that are Hadoop ready
On Tue, Aug 23, 2011 at 5:35 PM, Brion Vibber br...@pobox.com wrote: snip 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. :) I've found that diff representations of the full history can knock off about 95% of the uncompressed size. When stacked with generic compressors such as bz2 and 7z, an intelligent differencing scheme can still see improvement such that .diff.7z is about 10-50% smaller than .xml.7z while representing the same content. As you note though, the trade-off is that you have to look at many diffs to reconstruct the page's content. Given that hard disks are cheap, the biggest advantage is probably really for people who want to study diffs as their main object of study. -Robert Rohde ___ Wikitech-l mailing list Wikitech-l@lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikitech-l
Re: [Wikitech-l] Changing XML Wikipedia Schema to Enable Smaller Incremental Dumps that are Hadoop ready
Hi, (I don’t post often here and I’m not a MW developer but I try to follow, correct me if I’m wrong.) I see a couple of things which must be done carefully and willingly about page titlesref. Currently there is a difference between page_id and page title, since the page_id is conserved when the title of the page changes (during a move), so there is currently no canonical page title associated to a revision, only a page_id, or in other words I think it is theoretically non possible to retrieve the original page title of a given past revision (this could be discussed on another thread) and I have some doubts also about retrieving the original page_id of a revision in very rare cases (with a succession of deletion-undeletion of some revisions-moves) but I’m not sure of that. So introduce a page_title in the revisions (your §1.) is a new interesting information if your consider this as the title as of date of saving of the revision, and then page_id-title and page_title can be different, the same for the namespace. But this information is not currently available in the database. This would pose the problem of definition of existing revisions in the dumps: use the current page title associated to the current page_id? If you put the current page_title associated to the current page_id of the revision this means the page_title will change accross dumps every time a move is done, I don’t find it is semantically correct, but at least it should be clearly explained. This is the current behaviour but since the page_title is outside of a revision you implicitly aggree this behaviour which is semantically correct. In the §2. there is a similar thing for the redirect: currently the redirect points to a title, not a page_id (if you move the pointed page, the redirect will point to the new page). ref: I tried to work two years ago about an extension to restore ideally pixel-per-pixel an old revision, but I think it’s not (currently) possible mainly because of this problem of page titles. There are other problems but this is the main problem. Others include retrieving of an old version of the templates (related to the problem on the title), color of links and categories, version of an image, external ressources like site CSS/JS, status about deleted revisions (display or not), and finer things like user preferences and rights, ultimately differences due to changes of MW configuration or MW version, etc. (I don’t consider a change of version of the user browser :) I didn’t publish it then (Sumana was not here to say me to publish it ;) but I retrieved it on my computer, I try to publish it and explain on mw.org. Sébastien Thu, 18 Aug 2011 13:30:18 -0400, Diederik van Liere dvanli...@gmail.com wrote: 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
Re: [Wikitech-l] Changing XML Wikipedia Schema to Enable Smaller Incremental Dumps that are Hadoop ready
Sounds all very reasonable. Some thoughts: * Having revisions not wrapped into page means that for reconstructing the history of a page, the entire dump has to be scanned, unless there is an index of all revisions * Such an index should probably accompany the XML file, ideally if the XML is in a seekable zip container (bgzip etc.) * I suggest that the current article version at the time of dump is stored in full, and not as a diff; if you want to do history, you'll probably calculate all diffs anyway, but the current version should be accessible right away Magnus On Thu, Aug 18, 2011 at 6:30 PM, Diederik van Liere dvanli...@gmail.com wrote: 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 ___ Wikitech-l mailing list Wikitech-l@lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikitech-l ___ Wikitech-l mailing list Wikitech-l@lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikitech-l
