Grails use Promises to answer AJAX calls - ajax

Question

I'm trying to leverage multithreading in Grails to handle AJAX calls. The webpage fires an AJAX call, the controller allocates a new thread doing that job, and when it finishes, the result is returned and rendered. Here's my attempt. It failed. Seemingly the second AJAX call was not fired at all.
In the javascript in gsp: Two AJAX calls. The 1st one triggers the 2nd when complete.
function asynchroCrawl(){
var jsonData = $.ajax(
{
url : "${createLink(controller:'environment', action:'asynchroCrawl')}",
dataType : "json",
async : true
}).done(function(jsonData) {
console.log("Crawler completed");
crawlFinished=true;
asynchroWordCloud();
});
}
function asynchroWordCloud() {
var jsonData = $.ajax(
{
url : "${createLink(controller:'environment', action:'asynchroKeywords')}",
dataType : "json",
async : false
}).done(function(jsonData) {
keywordFinished = true;
});
}
In the controller: The other acton function is omitted.
def asynchroCrawl={
User u=session.getAttribute("user");
FrameworkController.crawlStarted=true;
println "Crawling task started.";
def p=task{
NetworkGenerator.formNetwork(u);
}
p.onError { Throwable err -> println "An error occured \n${err.message}" }
p.onComplete { result ->
println "User crawl complete.";
FrameworkController.crawlComplete=true;
render u as JSON;
return;
}
}
NetworkGenerator is just a normal class that runs some job and updates the User object with User.withTransaction{u.merge();}
My understanding is that a Promise is created and handles my job, and when it is complete, the data should be returned to the webpage answering to the AJAX call. So the .done() function should also be fired, leading the flow to another AJAX call. However, the done() is never triggered. I see no "Crawler completed" printed in my browser console.
In my IDE console I do see "User crawl complete.", indicating the promise has completed. But an exception follows, saying:
2015-12-10 21:05:47,540 [Actor Thread 7] ERROR gpars.LoggingPoolFactory - Async execution error: null
Message: null
Line | Method
->> 1547 | notifyAttributeAssigned in org.apache.catalina.connector.Request
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| 1538 | setAttribute in ''
| 541 | setAttribute . . . . . in org.apache.catalina.connector.RequestFacade
| 288 | setAttribute in org.apache.catalina.core.ApplicationHttpRequest
| 431 | storeGrailsWebRequest . in org.codehaus.groovy.grails.web.util.WebUtils
| 61 | doCall in org.codehaus.groovy.grails.plugins.web.async.WebRequestPromsiseDecorator$_decorate_closure1
| -1 | call . . . . . . . . . in ''
| 61 | doCall in org.grails.async.factory.gpars.GparsPromise$_onComplete_closure1
| -1 | call . . . . . . . . . in ''
| 62 | run in groovyx.gpars.dataflow.DataCallback$1
Please could anybody tell me what I'm doing wrong here. Really appreciate your help!

Answer 1

Each servlet request is already serving within a thread, so there's no need to spawn another thread. You can spawn a new thread in an action - either with new Thread().start() or with GPars - doesn't really matter.
What matters is, that you have to wait (with thread.join() or alike) till the thread completes, so that your action can deliver the results your JS code expects