Sunday, January 8, 2012

Google Apps Script

  Here’s the scenario: you create a form, you have a script that triggers onFormSubmit and all is well... until it gets popular. Occasionally you start having interlacing modifications from separate invocations of your script to the spreadsheet. Clearly, this kind of interlacing is not what you intended for the script to do. Up until now, there was no good solution to this problem -- except to remain unpopular or just be lucky. Neither are great solutions. Now, my friend, you are in luck! We’ve just launched the LockService to deal with exactly this problem. The LockService allows you to have only one invocation of the script or portions thereof run at a time. Others that would’ve run at the same time can now be made to wait nicely in line for their turn. Just like the line at the checkout counter. The LockService can provide two different kinds of locks-- one that locks for any invocation of your script, called a public lock, and another that locks only invocations by the same user on your script, called a private lock. If you’re not sure, using a public lock is the safest bet. For example, in the scenario in the previous paragraph you would want something like this:
function onFormSubmit() {
  // we want a public lock, one that locks for all invocations
  var lock = LockService.getPublicLock();
  lock.waitLock(30000);  // wait 30 seconds before conceding defeat.
  // got the lock, you may now proceed
  ...whatever it used to do here....
It’s best to release the lock at the end, but if you don’t, any locks you hold will be released at the end of script execution. How long should you wait? It depends on two things mainly: how long the thing you’re going to do while holding the lock takes, and how many concurrent executions you expect. Multiply those two and you’ll get your timeout. A number like 30 seconds should handle a good number of cases. Another way to pick the number is frankly to take an educated guess and if you guess too short, the script will occasionally fail. If you want to avoid total failure if you can’t get the lock, you also have the option trying to get the lock and doing something else in the event of not being able to get it:
function someFunction() {
  var lock = LockService.getPublicLock();
  if (lock.tryLock(30000))  {
    // I got the lock!  Wo000t!!!11 Do whatever I was going to do!
  } else {
    // I couldn’t get the lock, now for plan B :(
    GmailApp.sendEmail(“”, “epic fail”,
        “lock acquisition fail!”);
So now your scripts can be as popular as they can get with no worries about messing up shared resources due to concurrent edits! Check out the LockService documentation for more information.

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