Using Marklogic 8 triggers for managed triples

I have a use case where I want to send a notification every time triples are added or deleted from MarkLogic. The notification should contain those triples and should say whether they were added or deleted.

I didn't find any mention in the MarkLogic triggers guide regarding how it might work with (managed) triples. Is there a way to write a trigger module so that for a modified document (containing the managed triples), compare the new version with the old version to work out what's added and deleted and send a HTTP request containing these changes?

I understand that doc($trgr:uri) will give me the latest state of the document in question - but is there a way to retrieve the previous version, before the change? I'm fairly new to MarkLogic and Xquery so some guidance is much appreciated. Thanks!

Solution

Many thanks to @grtjn for providing the way to access the pre-change document. For determining the difference between documents I found a way inspired by this blog post. The solution that I found to be working looks like this:

xquery version '1.0-ml';
import module namespace trgr='http://marklogic.com/xdmp/triggers' at '/MarkLogic/triggers.xqy';

declare function local:diff($seq1 as item()*, $seq2 as item()*) as item()* {
    let $map1 := map:new($seq1 ! map:entry(fn:string(.), .))
    let $map2 := map:new($seq2 ! map:entry(fn:string(.), .))
    return map:keys($map1 - $map2) ! map:get($map1,.)
};

declare variable $trgr:uri as xs:string external;
declare variable $after := doc($trgr:uri)/sem:triples/sem:triple;
declare variable $before := xdmp:eval('doc("'||$trgr:uri||'")', (),
    <options xmlns="xdmp:eval"><isolation>different-transaction</isolation></options>)/sem:triples/sem:triple;

declare variable $added_triples := local:diff($after, $before);
declare variable $added_graph := xdmp:document-get-collections($trgr:uri);

declare variable $deleted_triples := local:diff($before, $after);
declare variable $deleted_graph := xdmp:eval('xdmp:document-get-collections("'||$trgr:uri||'")', (),
    <options xmlns="xdmp:eval"><isolation>different-transaction</isolation></options>);


xdmp:log(fn:concat('***** Trigger processing:  ', $trgr:uri, '*****')),
xdmp:log('***** added triples *****'),
xdmp:log($added_graph),
xdmp:log($added_triples),
xdmp:log('***** deleted triples *****'),
xdmp:log($deleted_graph),
xdmp:log($deleted_triples)

I created 3 pre-commit triggers, one for each of the trgr:document-content options: create, modify and delete, all invoking the above module. A SPARQL Update query will cause the above module to trigger one or more times, printing the lists of triples which were added and deleted.

Couple of observations:

A single SPARQL Update statement can create, modify and delete multiple documents, so will trigger the module multiple times.
INSERT statements seem to always create new documents, so you'll will never get added triples and deleted triples in the same invocation.
The code assumes there's only one collection for a document, which is the named graph for managed triples. It will need extra work if there are to be multiple collections per document.