On Fri, Feb 2, 2018 at 3:07 PM, Marc Sladek <[email protected]> wrote: > Introducing this certainly doesn't hurt, but 'm not sure how useful it is.
I never said it was the solution to all collisions but it will cover most of the (very rare and never reported) overwrites at 0 cost. > Firstly, it can show collisions only after a document is already > overwritten, thus the damage is already done. Again keep in mind that most of the code does: * getDocument(DocumentReference) * modify the XWikiDocument instance * saveDocument() so if getDocument() fail you are not going to overwrite anything. > 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. Not sure which use case you are referring to here. Are you talking about document deleted the a document with a different reference but same hash is saved ? > > 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 >> -- Thomas Mortagne
