In GAE we can add cron job manually by editing the cron.xml file

<?xml version="1.0" encoding="UTF-8"?>
<cronentries>
  <cron>
    <url>/cron/addCount/1</url>
    <description>Add count + 1 every 1 minutes</description>
    <schedule>every 1 minutes</schedule>
  </cron>
</cronentries>

Is there any way that I can make a user interface and create/update/delete as many cron jobs on the fly.That means after I deploy to GAE , I should be able to update the cron.xml ? Or is there a work around so that I can create cron jobs on the fly ?

No this is not possible.

You should just save this schedule data to datastore, then run cron every minute and check if there is something you need to do. Basically you'd be doing your own simple scheduler.

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