I'm sure you will find lots of mentions about this in markmail if you look for unique id, and random. MarkLogic was using that same method internally as well for creating its own objects. The idea is indeed that you run that if in update mode, in the same transaction in which you plan to do the insert. The 'lookahead' will create a read lock, which causes writes from other transactions to wait and retry if necessary..
Cheers, Geert -----Oorspronkelijk bericht----- Van: [email protected] [mailto:[email protected]] Namens Ron Hitchens Verzonden: donderdag 5 juni 2014 00:19 Aan: MarkLogic Developer Discussion Onderwerp: Re: [MarkLogic Dev General] New Feature Request: Unique Value Range Indexes Unless your unique-uri() function is running in a non-update query, in which case it runs lock free at a timestamp. If you're using the pattern of main code as a query and updates delegated to invoked/eval'ed transactions, you could get bit by this. It would work fine the vast majority of the time, but you wouldn't be protected from someone else's update happening between your check in the query and the execution of your invoked update. DOIs are a perfect example of what I'm talking about. Or account numbers, or patient record IDs, or aircraft tail numbers, etc. The impact of non-unique record identifiers can range from annoying all the way to legally/financially costly or even life-threatening if you're managing medication records, for example. --- Ron Hitchens {[email protected]} +44 7879 358212 On Jun 4, 2014, at 8:49 PM, "Whitby, Rob" <[email protected]> wrote: > I thought 2 simultaneous transactions would both get read locks on the uri, then one would get a write lock and the other would fail and retry. Maybe I'm missing something though. > > But anyway, I agree unique indexes would be a handy feature. e.g. our docs have a DOI element which *should* be unique but occasionally aren't, would be nice to enforce that rather than have to code defensively. > > Rob > ________________________________________ > From: [email protected] [[email protected]] on behalf of Ron Hitchens [[email protected]] > Sent: 04 June 2014 19:31 > To: MarkLogic Developer Discussion > Subject: Re: [MarkLogic Dev General] New Feature Request: Unique Value Range Indexes > > Rob, > > I believe there is a race condition here. A document may not exit as-of the timestamp when this request starts running, but some other request could create one while it's running. This request would then over-write that document. > > I'm actually more concerned about element values inside documents than generating unique document URIs. It's easy to generate document URIs with 64-bit random numbers that are very unlikely to collide. But I want to guarantee that some meaningful value inside a document is unique across all documents. > > In my case, the naming space is actually quite small because I want the IDs to be meaningful but unique. For example "images:cats:fluffy:XX.png", where XX can increment or be set randomly until the ID is unique. One way to check for uniqueness is to make the document URI from this ID, then test for an existing document. > > But this doesn't solve the general problem. I could conceivably have multiple elements in the document that I want to be unique. To check for unique element values it's necessary to run a cts query against the element(s). And I'm not sure if you can completely close the race window between checking for an existing instance and inserting a new one if the query comes back empty. > > Someone from ML pointed out privately that checking for uniqueness in the index would require cross-cluster communication. I'm sure that's true, but I'm also pretty sure that any user-level code solution is going to be far less efficient. I'd be happy to pay that ingestion time penalty for the guarantee that indexed element values are unique. At query time, such a unique value index should perform like any other range index. > > --- > Ron Hitchens {[email protected]} +44 7879 358212 > > On Jun 4, 2014, at 6:59 PM, "Whitby, Rob" <[email protected]> wrote: > >> How about something like this? >> >> declare function unique-uri() { >> let $uri := "/doc/" || xdmp:random() || ".xml" >> return if (fn:not(fn:doc-available($uri))) then $uri else unique-uri() >> }; >> >> I guess because indexes are distributed across forests, ensuring uniqueness is not that easy? >> >> Rob >> ________________________________________ >> From: [email protected] [[email protected]] on behalf of Ron Hitchens [[email protected]] >> Sent: 04 June 2014 18:01 >> To: MarkLogic Developer Discussion >> Subject: [MarkLogic Dev General] New Feature Request: Unique Value Range Indexes >> >> I'm working on a project, one aspect of which requires minting unique IDs and assuring that no two documents with the same ID wind up in the database. I know how to accomplish this using locks (I'm pretty sure) but any such implementation is awkward and prone to subtle edge case errors, and can be difficult to test. >> >> It seems to me that this is something that MarkLogic could do much more reliably and quickly than any user-level code. The thought that occurred to me is a variation on range indexes which only allow a single instance of any given value. >> >> Conventional range indexes work by creating term lists that look like this (see Jason Hunter's ML Architecture paper), where each term list contains an element (or attribute) value and a list of fragment IDs where that term exists. >> >> aardvark | 23, 135, 469, 611 >> ant | 23, 469, 558, 611, 750 >> baboon | 53, 97, 469, 621 >> etc... >> >> By making a range index like this but which only allows a single fragment ID in the list, that would ensure that no two documents in the database contain a given element with the same value. That is, attempting to add a second document with the same element or attribute value would cause an exception. And being a range index, it would provide a fast lexicon of all the current unique values in the DB. >> >> Such an index would look something like this: >> >> abc3vk34 | 17 >> bkx46lkd | 52 >> bz1d34nm | 37 >> etc... >> >> Usage could be something like this: >> >> declare function create-new-id-doc ($id-root as xs:string) as xs:string >> { >> try { >> let $id := $id-root || "-" || mylib:random-string(8) >> let $uri := "/idregistry/id-" || $id >> let $_ := >> xdmp:document-insert ($uri, >> <registered-id> >> <id>{ $id }</id> >> <created>{ fn:current-dateTime() }</created> >> </registered-id> >> return $id >> } catch (e) { >> create-new-id-doc ($id-root) >> } >> }; >> >> This doesn't require that I write any (possibly buggy) mutual exclusion code and I can be confident that once the xdmp:document-insert succeeds that the ID is unique in the database and that the type (as configured for the range index) is correct. >> >> Any love for Unique Value Range Indexes in the next version of MarkLogic? >> >> --- >> Ron Hitchens {[email protected]} +44 7879 358212 >> >> _______________________________________________ >> 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
