Introducing this certainly doesn't hurt, but 'm not sure how useful it is. Firstly, it can show collisions only after a document is already overwritten, thus the damage is already done. Secondly, loadXWikiDoc has to be called for the document which doesn't exist anymore, I guess this doesn't happen so often since the system won't list it anymore.
On 2 February 2018 at 13:36, Thomas Mortagne <[email protected]> wrote: > For document what could help a lot already without any performance > penalty is to compare the loaded document reference and the passed one > in XWikiHibernateStore#loadXWikiDoc. That's because most of the code > in XWiki apply the following logic: getDocument(), modify it, > saveDocument(). > > On Fri, Feb 2, 2018 at 12:19 PM, Marc Sladek <[email protected]> > wrote: > > Hi Denis, > > > > Thanks a lot for your answer. I know it's been a while, but I'd still > like > > to follow up on it since it's quite the fundamental issue. > > > >> Therefore, by improving the hash algorithm, the size of the ids, and the > > quality of the hashed key, we have considered ourselves to be saved > enough > > for a normal usage. > > > > Still, with enough bad luck, documents and objects may be overwritten > > without a trace. This is not a stable implementation. And even worse, if > on > > any XWiki installation hash collisions will happen in the future (or have > > already happened since 4.x), they probably won't be easily associated > with > > this issue because it's nearly impossible to debug. > > > > While I do now understand the motivation to stick with hashes, I'm still > > not sure why a collision detection would be difficult to introduce and > why > > it's even "impossible for some API". Let me briefly outline an idea: > > > > In XWikiHibernateStore#saveXWikiDoc on L615 > > <https://github.com/xwiki/xwiki-platform/blob/stable-9.11.x/ > xwiki-platform-core/xwiki-platform-oldcore/src/main/java/ > com/xpn/xwiki/store/XWikiHibernateStore.java#L615> > > an exists check on the doc id is already performed. If now > > xwikidoc.fullName is also selected in the HQL, a comparison to > > doc.getDocumentReference() can expose an imminent collision before data > is > > overwritten. At least an XWikiException should be thrown in this case. A > > similar thing could be done before saving BaseObjects on L1203 > > <https://github.com/xwiki/xwiki-platform/blob/stable-9.11.x/ > xwiki-platform-core/xwiki-platform-oldcore/src/main/java/ > com/xpn/xwiki/store/XWikiHibernateStore.java#L1203> > > to avoid collisions on Object IDs. > > > > I don't think a change like this would be difficult to implement, I could > > provide a PR of that sort. The performance penalty has to be tested for > > your systems though, since the full name isn't indexed afaik. > > > > Regards > > > > Marc Sladek > > synventis gmbh > > > > On 30 November 2017 at 15:21, Denis Gervalle <[email protected]> wrote: > > > >> Hi Marc, > >> > >> Here are some answers: > >> > >> 1) MD5 was already a dependency of our oldcore and using SHA1 would have > >> added a dependency without bringing much benefit. Since we only used 64 > >> bits of the MD5 anyway, I doubt using SHA1 would have provided a better > >> distribution. > >> > >> 2) Such a collision detection is difficult to be introduced in the > >> existing code base, for some API it is even impossible. What you > experience > >> with the 32-bit ids had been my motivation to the changes in 4.x and I > >> could say, based on my long XWiki experience, that even with the poor > >> 32 bit ids, very few users had been affected. Therefore, by improving > the > >> hash algorithm, the size of the ids, and the quality of the hashed key, > we > >> have considered ourselves to be saved enough for a normal usage. > >> > >> 3) That’s the worst point. I cannot answer about the first decision, I > >> wasn’t yet involve, but regarding the changes introduced in 4.0, a > change > >> had been considered. The ids are only there to satisfy Hibernate and its > >> loading mechanism. If we had used a counter, we had to manage a > conversion > >> table between ids and entity references with all the additional > complexity > >> (consistency issues, caching, ...). This is so because we use entity > >> reference to point directly to document (or even objects) everywhere in > >> XWiki. This would have been a huge work to introduce that behaviour and > at > >> the same time keeping all the existing API unchanged. It would probably > >> have introduced a performance penalty as well. This is why we resigned > and > >> go for an improved hash solution. IMO, if we had to make such a change, > we > >> are even better rewriting the storage service completely, and even stop > >> using Hibernate, which, to be honest, does not bring much benefit to > >> XWiki with its ORM aspects. > >> > >> But if you really want the complete answers, you can look at those > threads: > >> http://xwiki.markmail.org/thread/fuprtrnupz2uy37f > >> http://xwiki.markmail.org/thread/fsd25bvft74xwgcx > >> > >> Regards, > >> > >> -- > >> Denis Gervalle > >> SOFTEC sa - CEO > >> > >> On 30 Nov 2017, 14:14 +0100, Marc Sladek <[email protected]>, > >> wrote: > >> > 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/x > >> wiki-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/x > >> wiki-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 > >> > > > > -- > Thomas Mortagne >
