CODEWITHSUNDEEP
javamultithreadinggoogle-app-engine
user1768830
user1768830

Reputation:

How to impose Google App Engine's DeadlineExceededException?

I'd like to take the attitude that if my server-side is too slow, just throw an exception and send back a meaningful error message to the client. I am arbitrarily setting this time limit to 5 seconds and will tweak it as needed.

I see that App Engine has ApiProxy.getCurrentEnvironment().getRemainingMillis() which gives us the number of milliseconds we have left before it throws a DeadlineExceedException and times us out.

I'd like to do something similar, but imposing my own 5-second restriction over GAE's 60-second restriction (again, for right now doesn't matter if I pick 5-seconds or 40-seconds, as long as they are stricter than what GAE imposes). I'd like to write a server-side handler/servlet that starts a 5-second timer. If the request does not return inside that amount of time it throws an exception and returns to the client:

public class MyTimedServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws IOException {
        // Start 5-second timer
        Timer timer = new Timer(5000);

        try {
            timer.start();
            response = handleRequest(request);
        }
        catch(TimeException timerException) {
            response = ResponseFactory.newTimeExceededResponse();
        }
    }
}

Unfortunately, I don't see anything in the GAE API that resembles the Timer object that I'm looking for.

I'm already planning on making use of multihthreading every request via ThreadManager.currentRequestThreadFactory().newRequestThread(...), so I don't want to waste one of these (I only get 10 at a time!) threads to create a background "Timer Thread" unless I absolutely have to.

So I ask: does GAE have something like this out of the box, and if I have to roll my own, how do I go about doing so? Thanks in advance!

Upvotes: 2

Views: 457

Answers (1)

Ma Jerez
Ma Jerez

Reputation: 6005

Any asynchronous task (that don't follow the normal flow) in your code involves a new thread execution, in this case this task is to count the time.

However you can use the classes Timer & TimerTask both of them included in the App-Engine JRE white list.

Example code: not tested just a guide. 

public class YourServlet extends HttpServlet {

    Timer timer;
    HttpServletResponse response;   

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)...{
        int seconds = 5;
        timer = new Timer();
        timer.schedule(new DeadLine(), seconds * 1000);     

        /* the rest of your code here */
    }

    class DeadLine extends TimerTask {
        public void run() {      
            response.getWriter().print("Error: deadline reached.");     
        }
      }
}

Read this article about job scheduling in Java, for more info.

Upvotes: 1

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