Dear XWiki devs We are using the XWiki platform for our applications but sadly are still stuck with 2.7.2. Lately we ran into issues on a large database and noticed "disappearing" BaseObjects. We were able to link it to XWIKI-6990 <http://jira.xwiki.org/browse/XWIKI-6990>, where hibernate IDs collided (hash collisions) and overwrote other objects without any trace - neither visible in the history nor in a log file.
We analysed your implemented solution from 4.0+ in XWikiDocument <https://github.com/xwiki/xwiki-platform/blob/stable-8.4.x/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/doc/XWikiDocument.java#L841> and BaseElement <https://github.com/xwiki/xwiki-platform/blob/stable-8.4.x/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/objects/BaseElement.java#L237> and noticed that you changed the 32bit String#hashCode to 64bit MD5, which makes a collision less likely. I have a few questions regarding your solution: 1) Is there any specific reason why you have chosen MD5 over SHA-1 or 2? 2) Collisions are still possible and would be extremely hard to notice since they are completely silent. Have you considered to implement a collision detection to at least log occurring collisions - or even better reserve 1-2bits of the 64bit to be used as collision counter in the case of it happening? 3) To question the concept of generating a hash for an ID in general: Wouldn't a database defined "auto increment" be a much more robust solution for the hibernate IDs? A collision would be impossible and clustering/scalability is still possible with e.g. the InnoDB “interleaved” autoincrement lock mode. Why have you chosen a hash based solution in the first place? I'm sorry if these questions were already answered in the dev mailing list or on issues, please link me to them since I couldn't find any concrete answers. Thanks for your time and regards Marc Sladek synventis gmbh
