javascript - Concurrent beforeSave calls allowing duplicates -

in effort prevent objects being created, set conditional in type of object's beforesave cloud function.

however, when 2 objects created simultaneously, conditional not work accordingly.

here code:

parse.cloud.beforesave("entry", function(request, response) {     var thecontest = request.object.get("contest");     thecontest.fetch().then(function(contest){       if (contest.get("isfilled") == true) {           response.error('this contest full.');       } else {           response.success();     }); }); 

basically, don't want entry object created if contest full. however, if there 1 spot in contest remaining , 2 entries saved simultaneously, both added.

i know edge-case, legitimate concern.

parse using mongodb nosql database designed scalable , therefore provides limited synchronisation features. need here mutual exclusion unfortunately not supported on boolean field. parse provides atomicity counters , array fields can use enforce control.

see http://blog.parse.com/announcements/new-atomic-operations-for-arrays/ , https://parse.com/docs/js/guide#objects-updating-objects