Hi Geert,
I've implemented this with a single document using eval:
declare function get-next-id() as xs:int
{
let $query :=
"xquery version '1.0-ml';
let $uri := '/sequence'
let $nextId :=
if (fn:doc-available($uri)) then
fn:data(fn:doc($uri)/nextId
else 1
let $insert := xdmp:document-insert($uri, <nextId>{$nextId + 1}</nextId>,
xdmp:default-permissions())
return nextId"
return xdmp:eval($query)
};
My expectation is that the /sequence document would be locked when reading the
nextId because the eval statement includes an update. Am I mistaken in this?
Bob
------------------------------------------------------------------------------------
The most simplest way would be to have two documents. Perform a dummy update on
the first, read the second doc after that, increment the number you got from
the second, and update the second doc with the new number. The dummy update
will cause a transaction long write lock on the first document, which causes
automatic synchronisation. You will need to do the update first, as reading the
second doc will not prevent it being updated or read by others..
Kind regards,
Geert
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