Deadlock with queue representation

deadlock-queue.xqy

(: The basic problem I'm trying to solve is that of a queue that can also updated 
   for priority overrides. I thought about using map:map, but this approach seemed 
   easier for a first cut. :)

(: If I call queue-lib:retrieve-queue() first, everything works fine. If I try to 
   call queue-lib:retrieve-queue() from within queue-lib:enqueue(), I'm creating 
   an update within an update condition, as I understand it, and I end up with a 
   deadlock condition. I haven't resolved this yet, so any suggestions are 
   welcome! :)

xquery version "1.0-ml";
module namespace queue-lib = "http://me.com/queue-lib";
declare namespace queue = "http://me.com/queue";

declare private variable $QUEUE-URI := "/queue/queue.xml";

(: Retrieves the entire queue. :)
declare function queue-lib:retrieve-queue() as element(queue:queue) {
    let $queue-document := queue-lib:initialize()
    
    return $queue-document/queue:queue
};

(: Adds a new queue:item if it does not already exist, which is checked by id. :)
declare function queue-lib:enqueue($item as element(queue:item)) {
    (: REVIEW: I'm not clear why re-using retrieve-queue() is a transactional problem when no queue exists ahead of time. :)
    (: But if I can't do this, I *have* to initialize the queue first, then call this second. :)
    (:let $queue-document := queue-lib:retrieve-queue():)

    (: This won't work if I don't call queue-lib:retrieve-queue() beforehand, outside of this function. :)
    let $queue-document := fn:doc($QUEUE-URI)/element() 
    let $id := $item/queue:id/text()
    
    return 
        if( fn:exists($queue-document/descendant::queue:item[queue:id = $id]) ) then
            fn:error( xs:QName("ERROR"), fn:concat($id, " already exists in the queue. ") )        
        else 
            xdmp:node-insert-child( $queue-document, $item )
};

(: If a queue document does not already exist, create one. Return the existing or newly created document. :)
declare private function queue-lib:initialize() {
    (: Transactions + functions http://developer.marklogic.com/pipermail/general/2010-July/005852.html :)
    if(fn:not(fn:exists(fn:doc($QUEUE-URI)))) then 
        xdmp:eval(
            fn:concat(
                (: REVIEW: Seems like I can't pass an element in as a variable because of concat(). :)
                'xdmp:document-insert("' , $QUEUE-URI , '", <queue xmlns="http://me.com/queue"/>, (), "queue")'
            ),
            (),
            <options xmlns="xdmp:eval">
                <isolation>different-transaction</isolation>
                <prevent-deadlocks>true</prevent-deadlocks>
            </options>            
        ) 
    else ()
    ,
    xdmp:eval(
        'fn:doc("/queue/queue.xml")',
        (),
        <options xmlns="xdmp:eval">
            <isolation>different-transaction</isolation>
            <prevent-deadlocks>true</prevent-deadlocks>
        </options>
    )
};

Ah, I wish I had thought of directories. If my thinking is correct, I can even use the search API. Excellent. Thanks so much for the suggestion!