Question

I am using LockService to avoid duplicated actions, however I cannot make tryLock to fail during my testing.

Supposedly this code should write an error in ScriptProperties when running more than one time almost simultaneously, but it does not so far.

A second App instance should fail after tryLock for 1 second, while the first instance is sleeping for 15 seconds, right?

Any suggestions?

function doGet() {
  testingLockService(1000, 15000);
  return;
}
function testingLockService(trying, sleeping) {
  var lock = LockService.getPrivateLock();
  var hasMutex = lock.tryLock(trying);
  if (hasMutex == false) { ScriptProperties.setProperty("LockService",new Date().toString()+" tryLock failed"); return; }
  Utilities.sleep(sleeping);
  lock.releaseLock();
  return;
}

Answer 1

Interesting question. After having a little play with this I think the locking is working, it just appears it isn’t because Google Apps Script does not seem to allow concurrent get requests, but rather queues them up. By moving your lock test to the server side it then works.

This is much easier to debug if you have your get request return something to the user rather than put it in a script property.

The following code will demonstrate the get requests being queued up. To test: make two concurrent requests, and look at the timestamps coming back, interesting you’ll notice the second request will not have a start timestamp before the end timestamp of the first request, no matter how close together you make them. So the second request can perfectly validly get the lock. Here’s the code:

function doGet() {
  var app = UiApp.createApplication();

  var tS = new Date();
  var gotLock = testingLockService(0, 5000);
  var tF = new Date();

  var label = app.createLabel(gotLock ? 'Got the lock, and slept' : "Didn't get the lock");
  app.add(label);

  var label = app.createLabel('tS ' + tS.getTime());
  app.add(label);
  var label = app.createLabel('tF ' + tF.getTime());
  app.add(label);
  var label = app.createLabel('t delta ' + (tF - tS));
  app.add(label);

  return app;
}

function testingLockService(trying, sleeping) {
  var lock = LockService.getPrivateLock();
  var hasMutex = lock.tryLock(trying);
  if (!hasMutex) { return false; }
  Utilities.sleep(sleeping);
  lock.releaseLock();
  return true;
}

Now, to prove the locking does work, simply move the locking code to the server side. Again, to test, have two browser windows open and and click on both buttons. This time you will see the second request fail to get the lock and return immediately.

function doGet() {
  var app = UiApp.createApplication();
  var serverHandler = app.createServerHandler('doClick');
  var button = app.createButton().setText("click me").addClickHandler(serverHandler);
  app.add(button);
  return app;
}

function doClick() {
  var app = UiApp.getActiveApplication();
  // code from here on is identical to previous example
  var tS = new Date();
  var gotLock = testingLockService(0, 5000);
  var tF = new Date();

  var label = app.createLabel(gotLock ? 'Got the lock, and slept' : "Didn't get the lock");
  app.add(label);

  var label = app.createLabel('tS ' + tS.getTime());
  app.add(label);
  var label = app.createLabel('tF ' + tF.getTime());
  app.add(label);
  var label = app.createLabel('t delta ' + (tF - tS));
  app.add(label);

  return app;
}

function testingLockService(trying, sleeping) {
  var lock = LockService.getPrivateLock();
  var hasMutex = lock.tryLock(trying);
  if (!hasMutex) { return false; }
  Utilities.sleep(sleeping);
  lock.releaseLock();
  return true;
}

Hopefully that has answered your question on the locking. Though it raises questions in my mind about the get request queueing. Is it only requests from the same user? I would love to hear from someone else if they have any more info on that, although, maybe that belongs in a question on its own.