[MarkLogic Dev General] xpath string construction

Geert Josten Geert.Josten at daidalos.nl
Tue Oct 14 11:18:28 PDT 2008


Fun to see how such a seemingly simple question can generate so much respons.. :-)

Eric,

One simple question: do you need to have unique id's per document, or over the whole database? I was assuming the first, thinking of your scaffolding application..

Kind regards,
Geert

>


Drs. G.P.H. Josten
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> From: general-bounces at developer.marklogic.com
> [mailto:general-bounces at developer.marklogic.com] On Behalf Of
> Wayne Feick
> Sent: dinsdag 14 oktober 2008 20:02
> To: General Mark Logic Developer Discussion
> Subject: Re: [MarkLogic Dev General] xpath string construction
>
> Hi Eric,
>
> A disadvantage of sequential ids is that you can end up read
> locking all of your documents in order to find the current
> max id. You can address this partially by moving the next id
> into a separate document, but that document can still become
> a bottleneck if you have a high insertion rate. You could
> also address this by creating a range index on the id and
> using cts:element-values() or cts:element-attribute-values()
> to find the max.
>
> By switching to random ids, you get better parallelism since
> our indexes can quickly determine if the id is already in use
> and will lock at most one document (or 0 if your existing id
> search is unfiltered). There is still a vanishingly small
> probability that two competing threads would allocate the
> same random id at the same moment in time, but that is
> improbable enough to be ignored.
>
> Wayne.
>
>
>
> On Tue, 2008-10-14 at 13:07 -0400, Eric Palmitesta wrote:
>
>
>       Wow, thanks for the reply, Michael.  I'll probably be
> using some
>       variation of one of your examples.
>
>       Michael Blakeley wrote:
>       > Many people ask about sequential ids. It is possible
> to model an id
>       > sequence as a database document. But as with RDBMS
> sequences, there are
>       > serialization penalties. I don't see the advantage of
> sequential ids, so
>       > I rarely, if ever, use this approach.
>
>       Assuming the recursive check isn't feasible (it doesn't
> scale well), the
>       advantage of sequential ids is being able to sleep at
> night knowing
>       collisions are simply impossible, and are not reliant
> on a 'good-enough'
>       random() function.  I'm nit-picking of course, I'm sure
> random() is
>       fine.  :)
>
>       Cheers,
>
>       Eric
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>



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