As MediaWiki 1.19 is getting ready, I'd like to offer information on how MediaWiki 1.18.0 was the most difficult MW upgrade I've ever been through. Some background: my team administers an internal wiki at a major company with ~2000 users, over 100 extensions (many of them custom/unreleased), and 100K articles. I've been upgrading MW regularly since 1.11 - every release and patch - and have never had this much trouble before, mainly because of extensions that broke in 1.18. The typical MW upgrade takes me a day or two including regression-testing our extensions. But 1.18 has taken me weeks and I'm still not done. This message is meant to be constructive & helpful, not blameful: it's quite possible that every issue was "our fault" for not keeping up on exactly which functions & globals were being deprecated, etc. I'd just like to describe what kinds of things broke for a reasonably active wiki run by well-meaning people, and to document how we fixed them. So, here's the list of what we had trouble with, and what we did. I welcome any improvements to our fixes! 1. The global variable $action disappeared, breaking a bunch of our extensions. I switched to $wgRequest->getVal('action'). 2. The removal of Xml::hidden() caused one of our extensions to break. I switched to Xml::input(..., array('type', 'hidden')) 3. A few of our older extensions were not ported to ResourceLoader yet and were adding JS and CSS via $wgOut->add... calls. They worked in 1.17 and all broke in 1.18. I ported them to use ResourceLoader, but this is not a good solution yet because of bug 31676 (the 32-stylesheet limit of IE, https://bugzilla.wikimedia.org/show_bug.cgi?id=31676) which IMHO is a very serious time-bomb waiting to explode. I hope it makes it into "1.19wmf deployment" as planned. 4. Some of our parser tag extensions had a bug, in that they didn't return a value in the tag callback. (These tags had no visual display.) This didn't cause problems in 1.17 and earlier, but in 1.18.0 it caused a UNIQ.....QINU string to render on the page. I fixed our extensions to return the empty string, and the problem went away. 5. The removal of $wgMessageCache->addMessage() broke many extensions, some ours and some from mediawiki.org like SimpleForms. Some fixes just required use of the i18n file. Our more difficult issue was that we were injecting system messages into articles to add tracking categories. On advice from this list (thanks!), we used code patterned after Parser::addTrackingCategory() to inject categories and it works fine, actually much better than what we had. 6. The removal of ts_makeSortable() from wikibits.js threw off a bunch of our JavaScript: we were using the function to sort on a different column than the first one on render, and in extensions that create tables within dialogs. We left the problem unfixed until I can understand the new jQuery UI way of doing things (jquery.ui.sortable.js). 7. Nearly 100% of our customizations to WikiEditor 1.17 broke in 1.18. We had followed the documented rules on mediawiki.org, using extensions, ResourceLoader, etc., and everything worked in 1.17. Nevertheless in 1.18, toolbars and menus disappeared in IE. Menus appeared multiple times instead of once in Firefox. JavaScript objects in one module became undefined in others, even with proper dependencies. Some of these issues are still not worked out, but most were fixed by a variety of changes. 8. Our MediaWiki:common.js stopped running on the login page. I realize this was a security fix; it just took me by surprise. Fixed by writing a custom extension using the hook UserLoginForm to inject the few lines of JS we needed, and I'm evaluating other non-JS solutions for more security. 9. The addHandler() function in JavaScript does not seem to work in IE8 anymore. We worked around this by using jQuery's "bind" function. At this point, our test wiki is stable and I am not anticipating any further large issues, so we should roll out in the next two weeks or so. Thanks for reading, and I hope this helps someone, DanB