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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