[xquery-talk] Finding a XML-Database to fit our needs

[John Snelson]

Michael's right. As it happens Berkeley DB XML has page level locking 
with a user configurable page size. The internal storage format breaks 
XML documents up and stores them with a record for every element, where 
some number of these records will be stored on an individual page. This 
means it is quite feasible to update part of a large document and not 
end up locking the entire document.

John

Michael Kay wrote:
>  
>> Ok, point taken, but please explain how you can achieve data 
>> consistency without granular locking without having to chunk 
>> your large data sets that conform to a unified schema into 
>> multiple pieces that facilitate such concurrent access?
> 
> The database literature is full of alternative techniques. I haven't
> followed it in detail for the last 20 years or so, but even then at least
> three techniques were recognized: record-level locking (which I guess
> equates to node-level in an XML database), page-level locking, and predicate
> locks. Fine-granularity locking looks good on paper but the benefits can
> often be lost because you spend too much time doing lock acquisition/release
> and deadlock detection; in real workloads you can often achieve higher
> throughput by reducing the concurrency and the lock granularity, though if
> you take this to extremes (for example by queuing the transactions to run in
> a single thread) then of course response times will suffer. The best way to
> reduce contention is not to use finer-grained locks, but to reduce the
> length of time for which they are held. A common error in the client-server
> days was to run with a far higher concurrency than is appropriate for
> optimum throughput, and to leave locks in place across user interactions. If
> you can avoid those mistakes, you may well find that document-level locking
> is actually quite good enough.
> 
> Michael Kay
> http://www.saxonica.com/
> 
> _______________________________________________
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-- 
John Snelson, Oracle Corporation
Berkeley DB XML:        http://www.oracle.com/database/berkeley-db/xml
XQilla:                                  http://xqilla.sourceforge.net