cancelWorkflow in SharePoint hosted app

Hi,

I am developing a SharePoint hosted app for office 365 site.I have a list in the app web on which a workflow is running on item added event.On click of a Cancel link in the list item, I am trying to cancel the workflow using following code

   // connect to the instance service
     var workflowInstanceService = workflowServicesManager.getWorkflowInstanceService();
     var workflowInstances = workflowInstanceService.enumerateInstancesForListItem(ListId, itemID)

      context.load(workflowInstances);
      context.executeQueryAsync(function (sender, args) {
                var instancesEnum = workflowInstances.getEnumerator();
                while (instancesEnum.moveNext()) {
                       var instance = instancesEnum.get_current();
       workflowInstanceService.cancelWorkflow(instance);
                       context.executeQueryAsync(function (sender, args) {},errfunc);

Everything works fine when I login to the app, as site collection administrator and cancel the workflow.With any other user I get Permission denied, UnAuthorizedAccess exception on cancelWorkflow method.Please guide how can I achieve this.

If I need to call a webservice from javascript, please help me understand the steps to host the webservice for it to be accessible from sharepoint online.Thanks!

regards,

Sharepoint lead