[MarkLogic Dev General] Performance of fn:exists(fn:doc($uri))

Michael Blakeley mike at blakeley.com
Wed Aug 29 12:44:25 PDT 2012

If md5 were a perfect hash, you would have its entire 128-bit space. But it is not: see http://merlot.usc.edu/csac-f06/papers/Wang05a.pdf for some background. Flaws in the design make collisions more likely than they ought to be.

Even if md5 were perfect, I would still recommend implementing a hash-collision strategy of some kind. For example, you might treat the md5-based URI as the directory prefix, and put multiple documents underneath it based on an additional naming scheme. Most directories would contain only one document, but the occasional hash-collision would be taken care of automatically. Sometimes folks will suggest concatenating multiple hashes... I have always been suspicious of that approach, but I don't have any firm arguments against it.

For most purposes I think random numbers are more straightforward. If you want to be able to track down documents that hash to a particular md5, it may be just as effective to create an md5 element or add a property.

-- Mike

On 29 Aug 2012, at 12:32 , Tim Finney wrote:

> Hi All,
> On a slightly related topic, what's the chance of a collision if one
> uses something like this to generate a (hopefully) unique URI?
> fn:concat("/foo/", xdmp:md5(xdmp:get-original-url()), ".bar")
> Best,
> Tim
> On Wed, 2012-08-29 at 12:00 -0700, Danny Sinang wrote:
>> Hi,
>> ML support suggested we do this to generate a unique ID for our
>> documents :
>> declare function choose-uri() as xs:string
>>    {
>>       let $uri := fn:concat("/document-", xdmp:random(), ".xml")
>>       return if (fn:exists(fn:doc($uri))) then choose-uri() else $uri
>>    };
>> My question is, will the call to fn:exists(fn:doc($uri)) be fast,
>> considering that we now have 8 million documents ?
>> The fn:exists(fn:doc($uri)) call is needed to obtain a read lock,
>> which
>> will be upgraded to a write lock when xdmp:document-insert is called.
>> Regards,
>> Danny
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