I say: the value of $assetUri is undefined. But if $assetUri is simply whatever comes back from choose-uri(), then yes. That's how updates work: any reads in an update take read-locks, and writes will upgrade those locks as needed. http://docs.marklogic.com/5.0doc/docapp.xqy#display.xqy?fname=http://pubs/5.0doc/xml/dev_guide/transactions.xml tries to explain this, especially http://docs.marklogic.com/5.0doc/docapp.xqy#display.xqy?fname=http://pubs/5.0doc/xml/dev_guide/transactions.xml%2340227
BTW consider writing choose-uri to use something like 'asset/' + random. That way you can use 'asset/' as a directory prefix with xdmp:directory() or cts:directory-query(). Along those lines, http://blakeley.com/blogofile/2012/03/19/directory-assistance/ might interest you too. -- Mike On 30 Aug 2012, at 08:15 , Danny Sinang wrote: > Hi Mike, > > When I call addAsset (see code below), will the read lock that happens inside > xutils:choose-uri() be upgraded to a write lock by the time > xdmp:document-insert() is called inside updateAsset() ? > > Regards, > Danny > > declare function xutils:choose-uri() as xs:string > { > let $uri := xutils:buildUri(xdmp:random(), "asset") > return > if (fn:exists(fn:doc($uri))) then > choose-uri() > else > $uri > }; > > declare function xutils:assets-uuid() { > let $uri := xutils:choose-uri() > let $assetId := > fn:replace(fn:replace( > fn:base-uri($uri), > "/assets/","") > ,"\.xml","") > return $assetId > }; > > declare function addAsset($doc as element(asset), $user as xs:string) { > let $assetId := xutils:assets-uuid() > return > updateAsset($assetId, $doc, $user) > }; > > declare function updateAsset($assetId as xs:string, $doc as element(asset), > $user as xs:string) { > (: cut out some code for brevity :) > > let $insert := xdmp:document-insert($assetUri, $assetDoc, > xdmp:default-permissions(), vars:getCollections("assets"), 0, > vars:forest-ids("assets")) > return $assetId > }; > > > On Thu, Aug 30, 2012 at 11:05 AM, Michael Blakeley <[email protected]> wrote: > These are URI locks, not fragment locks. The URI doesn't have to exist in > order to create the lock. The point is to guarantee read-consistency for the > update, so that the if-then-else expression operates reliably. > > The case where the URI does exists would be vanishingly rare, since > xdmp:random() returns a 64-bit pseudo-random unsigned long. You could test > the cost by using a smaller random space, if you were interested. But you > can't simply drop the read locks without sacrificing the guarantee of > uniqueness. So if you do end up taking extra read locks, they are quite > necessary. > > To put it more simply: how are you going to guarantee the uniqueness of the > URI, if not by checking to see if it exists? > > -- Mike > > On 29 Aug 2012, at 22:25 , Geert Josten wrote: > > > Hi Mike, > > > > Not quite sure, but the conflict occurs when the uri doesn't exist yet, so > > there would be nothing to lock. Does that still create a read-lock? > > > > And in case the uri does exist, wouldn't this create potentially a lot of > > unnecessary read-locks (in case it takes a lot of attempts to find an > > unused uri)? > > > > Kind regards, > > Geert > > > > -----Oorspronkelijk bericht----- > > Van: [email protected] > > [mailto:[email protected]] Namens Michael Blakeley > > Verzonden: woensdag 29 augustus 2012 21:35 > > Aan: MarkLogic Developer Discussion > > Onderwerp: Re: [MarkLogic Dev General] Performance of > > fn:exists(fn:doc($uri)) > > > > No, you can't do that safely because cts:uris-match won't take a > > read-lock. You are opening yourself up to a race condition. And in some > > circumstances it will be slower than the recommended technique. There > > seems to be a popular idea that cts:uris-match() is always fastest, but > > that is not always true. > > > > The recommended technique is probably the fastest way to guarantee a new, > > unique URI. If you are going through the process of inserting a new > > document, this technique adds very little extra work. The document-insert > > itself always has to look for an existing document, because it might be > > replacing an existing document or it might be inserting a new document. It > > always has to write-lock the URI. So the extra exists() call merely > > repeats the URI lookup, which is cheap because it will be cached for the > > xdmp:document-insert call, and also gets a read-lock before > > xdmp:document-insert gets the write lock. In the vanishingly rare event > > that xdmp:random() produces an existing URI, this extra work is repeated - > > but is still quite cheap. > > > > -- Mike > > > > On 29 Aug 2012, at 12:29 , William Merritt Sawyer wrote: > > > >> If you have the uri-lexicon turned on you can use > > cts:uri-match(fn:concat("/document-", xdmp:random(), ".xml")) > >> > >> From: [email protected] > > [mailto:[email protected]] On Behalf OfDanny Sinang > >> Sent: Wednesday, August 29, 2012 12:33 PM > >> To: MarkLogic Developer Discussion > >> Subject: Re: [MarkLogic Dev General] Performance of > > fn:exists(fn:doc($uri)) > >> > >> Thanks Geert. > >> > >> I did try fn:exists(fn:doc($uri)) on CQ before your response came in > > and found it to be fast. > >> > >> The locking / prevention of duplicate id's is discussed in > > http://markmail.org/message/mm5vtacpdzwfy44j . > >> > >> Regards, > >> Danny > >> > >> On Wed, Aug 29, 2012 at 2:23 PM, Geert Josten <[email protected]> > > wrote: > >> Hi Danny, > >> > >> Performance should be easy to measure. Call the function from within > > QConsole x number of time and request profile output. Do the same while > > using xdmp:exists instead of fn:exists. That function works only on > > (partially) searchable expression, because it doesn't retrieve the actual > > content. It won't create a read-lock either, but I'm not sure why you want > > one. It won't prevent duplicate id's from being generated in concurrent > > requests.. > >> > >> Kind regards, > >> Geert > >> > >> Van: [email protected] > > [mailto:[email protected]] Namens Danny Sinang > >> Verzonden: woensdag 29 augustus 2012 19:11 > >> Aan: general > >> Onderwerp: [MarkLogic Dev General] Performance of > > fn:exists(fn:doc($uri)) > >> > >> 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 > >> > >> > >> > >> _______________________________________________ > >> General mailing list > >> [email protected] > >> http://developer.marklogic.com/mailman/listinfo/general > >> > >> > >> _______________________________________________ > >> General mailing list > >> [email protected] > >> http://developer.marklogic.com/mailman/listinfo/general > > > > _______________________________________________ > > General mailing list > > [email protected] > > http://developer.marklogic.com/mailman/listinfo/general > > _______________________________________________ > > General mailing list > > [email protected] > > http://developer.marklogic.com/mailman/listinfo/general > > > > _______________________________________________ > General mailing list > [email protected] > http://developer.marklogic.com/mailman/listinfo/general > > _______________________________________________ > General mailing list > [email protected] > http://developer.marklogic.com/mailman/listinfo/general _______________________________________________ General mailing list [email protected] http://developer.marklogic.com/mailman/listinfo/general
