On Jan 9, 2012, at 11:09 AM, Denis Gervalle wrote: > On Mon, Jan 9, 2012 at 10:07, Vincent Massol <[email protected]> wrote: > >> +1 with the following caveats: >> >> * We need to guarantee that a migration cannot corrupt the DB. > > > The evolution of the migration was the first steps in that procedure, since > accessing a DB with an inappropriate XWiki core could have corrupt it. > > >> For example imagine that we change a document id but this id is also used >> in some other tables and the migration stops before it's changed in the >> other tables. The change needs to be done in transactions for each doc >> being changed across all tables. > > > That would be nice, but MySQL does not support transaction on ISAM table. > I use a single transaction for the whole migration process,
I think we should have one transaction per document update instead. We've had this problem in the past when upgrading very large systems. The migration was never going through in one go for some reason which I have forgotten so we had needed to use several tx so that the migrations could be restarted when it failed and so that it could complete. > so on systems > that support it (Oracle ?), there will be migration or not. But I could not > secure MySQL better that it is possible to. It should work fine on MySQL with InnoDB which recommend (see http://platform.xwiki.org/xwiki/bin/view/AdminGuide/InstallationMySQL). Thanks -Vincent >> Said differently the migrator should be allowed to be ctrl-c-ed at any >> time and you safely restart xwiki and the migrator will just carry on from >> where it was. >> > > The migrator will restart were it left-off, but the granularity is the > document. I proceed the updates by documents, updating all tables for each > one. If there is some issue during the migration let say on MySQL, and it > is restarted, it will start again skipping documents that have been > converted previously. So the corruption could be limited to a single > document. > > >> * OR we need to have a configuration parameter for deciding to run this >> migration or not so that users run it only when they decide thus ensuring >> that they've done the proper backups and saving of DBs. >> > > This is true using the new migration procedure, but not as flexible as you > seems to expect. Supporting two hashing algorithm is not a feature, but an > augmented risk of causing corruption for me. > Now, if you use a recent core, that use new id, and on the other side, you > have not activated migrations and access an old db, you will simply be > unable to access the database. You will receive a "db require migration" > exception. > > Anyway, migration are disable by default, and should be enabled by an > administrator in xwiki.cfg. The release notes will mention the needs to > proceed to it, and of course, to make a backup before. And you are always > supposed to have backup when you upgrade, or you are not a system admin ;) > > >> I prefer the first option but we need to guarantee it. >> > > We will never be able to guarantee it, but I have done my best to have it > the most secure. > > >> >> Thanks >> -Vincent >> >> On Jan 7, 2012, at 10:39 PM, Denis Gervalle wrote: >> >>> Now that the database migration mechanism has been improved, I would like >>> to go ahead with my patch to improve document ids. >>> >>> Currently, ids are simple string hashcode of a locally serialized >> document >>> reference, including the language for translated documents. The >> likelihood >>> of having duplicates with the string hashing algorithm of java is really >>> high. >>> >>> What I propose is: >>> >>> 1) use an MD5 hashing which is particularly good at distributing. >>> 2) truncate the hash to the first 64bits, since the XWD_ID column is a >>> 64bit long. >>> 3) use a better string representation as the source of hashing >>> >>> Based on previous discussion, point 1) and 2) has already been agreed, >> and >>> this vote is in particular about the string used for 3). >>> I propose it in 2 steps: >>> >>> 1) before locale are fully supported in document reference, use this >>> format: >>> >>> >> <lengthOfLastSpaceName>:<lastSpaceName><lengthOfDocumentName>:<documentName><lengthOfLanguage>:<language> >>> where language would be an empty string for the default document, so >> it >>> would look like 7:mySpace5:myDoc0: and its french translation could be >>> 7:mySpace5:myDoc2:fr >>> 2) when locale are included in reference, we will replace the >>> implementation by a reference serializer that would produce the same kind >>> of representation, but that will include all spaces (not only the last >>> one), to be prepared for the future. >>> >>> While doing so, I also propose to fix the cache key issue by using the >> same >>> reference, but prefixed by <lengthOfWikiName>:<wikiName>, so the previous >>> examples will have the following key in the document cache: >>> 5:xwiki7:mySpace5:myDoc0: and 5:xwiki7:mySpace5:myDoc2:fr >>> >>> Using such a key (compared to the usual serialization) has the following >>> advantages: >>> - ensure uniqueness of the reference without requiring a complex escaping >>> algorithm, which is unneeded here. >>> - potentially reversible >>> - faster than the usual serialization >>> - support language >>> - independent of the current serialization that may evolved >> independently, >>> so it will be stable over time which is really important when it is used >> as >>> a base for the hashing algorithm used for document ids stored in the >>> database. >>> >>> I would like to introduce this as early as possible, which means has soon >>> has we are confident with the migration mechanism recently introduced. >>> Since the migration of ids will convert 32bits hashes into 64bits ones, >> the >>> risk of collision is really low, and to be careful, I have written a >>> migration algorithm that would support such collision (unless it cause a >>> circular reference collision, but this is really unexpected). However, >>> changing ids again later, if we change our mind, will be really more >> risky >>> and the migration difficult to implements, so it is really important that >>> we agree on the way we compute these ids, once for all. >>> >>> Here is my +1, >>> >>> -- >>> Denis Gervalle >> _______________________________________________ >> devs mailing list >> [email protected] >> http://lists.xwiki.org/mailman/listinfo/devs >> > > > > -- > Denis Gervalle > SOFTEC sa - CEO > eGuilde sarl - CTO > _______________________________________________ > devs mailing list > [email protected] > http://lists.xwiki.org/mailman/listinfo/devs _______________________________________________ devs mailing list [email protected] http://lists.xwiki.org/mailman/listinfo/devs
