Hi Devs, Since I fall on this well-know XWiki caveat recently, I would like to improve this.
Currently, XWikiDocument.getId() is almost equivalent to String.hashcode(fullName:lang). Since this is a really poor hashing method for small changes, the risk that two documents with similar names of same length are colliding is really high. This Id is used by the Hibernate store for document unicity and really needs to be unique, and at most a 64bit-numeric at the same time. Currently we use only 32bits since java hashcode are limited to 32bits integers. The ideal would be not to have this link between ids and documents fullName:lang, but converting the current implementation is not really easy. This is probably why XWIKI-4396 has never been fixed. Therefore, my current goal is to reduce the likeliness of a collision by choosing a better hashing method and taking into account the fact that document fullname are short string and the number of unique ids required are very limited (since the unicity is only expected for a given XWiki database) compared to the 64bits integrer range. So we need to choose a better algorithm, and here are IMHO the potential options: A) use a simple but more efficient non-cryptographic hashing function that runs on 64bits, I was thinking about using the algorithm produced by Professor Daniel J. Bernstein (DJB) since it is well-know, wildly used, easy to implement algorithm with a good distribution on small strings. Pro: no dependency; fast; 64bits better than hashcode Cons: probably more risk of collision compare to MD5 or SHA, but far less than now; require db migration of all document keys B) use an MD5 or even stronger SHA1 or SHA256 algorithm from JCA, truncating to the lower 64bits. Note that oldcore already use MDA5 for hashcoding a whole XWikiDocument to provide the API with a getVersionHashcode(), and for the validation hash used by the persistent login manager. The first use Object.hashcode() as a fallback, which is really bad and defeat the purpose. The second does not provide any fallback and may fail unexpectedly. For our case, if we really want a fallback, we needs to store the hashing algorithm used in the database at creation time, and anyway, fail when it was not available. Pro: already used in oldcore, probably less collision; with fallback, really flexible since it would be possible to choose the algorithm at creation time and does not require full migration for existing database. Cons: require at least a DB schema change to add the hashing algorithm, probably as a column of xwikidbversion; if this config value is broken, the whole DB is broken C) use our own MD5 implementation when the JCA provider is missing it. I was thinking about integrating something like http://twmacinta.com/myjava/fast_md5.php (non-native version) which is LGPL. This will ensure availability of the hashing algorythm while having a rather strong one. Pro: no dependency, could also provide MD5 to getVersionHashcode and the persistent manager Cons: require db migration of all document keys A) is really quick to implement, simple, and the less risky, but some may found it insufficient. Caleb ? Obviously, B) with fallback is really nice, but I wonder if it is not overkill ? I am worried about the B) without fallback, but maybe I want it too flawless C) is rather solid, while staying simple, but maybe overkill too. I am really undecided. WDYT ? -- Denis Gervalle SOFTEC sa - CEO eGuilde sarl - CTO _______________________________________________ devs mailing list [email protected] http://lists.xwiki.org/mailman/listinfo/devs
