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
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
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.


Marc Sladek
synventis gmbh

On 30 November 2017 at 15:21, Denis Gervalle <d...@softec.lu> 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
> On 30 Nov 2017, 14:14 +0100, Marc Sladek <marc.sla...@synventis.com>,
> 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

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