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
>
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