Wednesday, October 16, 2013

Scheduling Jobs In Play 2

In this tutorial, I'll show you how to schedule asynchronous jobs in the Play! Framework version 2.1.3.

The Play! Framework has moved away from using Job classes with Crontab-like annotations for application level task scheduling. In place of the old model we find heavy use of the Akka system, which operates a bit differently and can take some getting used to if you're accustomed to the CRON/Quartz way of doing things.

Let's get started!

After creating our application, we need to create a Global.java file in the app/ directory. Place the following in the file:

import play.Application;
import play.GlobalSettings;
import play.libs.Akka;
import scala.concurrent.duration.FiniteDuration;

import java.lang.reflect.Method;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class Global extends GlobalSettings {
    @Override
    public void onStart(Application app) {
      //Magic goes here
   }
       
}

The above is nothing too complex. It's a file that contains code that will be executed upon starting the application. The important piece is the onStart method that has the @Override annotation. That's where we're going to place our scheduling code.

Before continuing, you should probably take a look at what the Play devs have to say about the use of their Akka scheduler before I give my personal explanation.

Place the following in the onStart method of the Global.java file we just created.

FiniteDuration delay = FiniteDuration.create(0, TimeUnit.SECONDS);
FiniteDuration frequency = FiniteDuration.create(5, TimeUnit.SECONDS);

Runnable showTime = new Runnable() {
   @Override
   public void run() {
      System.out.println("Time is now: " + new Date());
   }
};

Akka.system().scheduler().schedule(delay, frequency, showTime, Akka.system().dispatcher());

The above simply allows us to log the current time to the console ever 5 seconds. Not very useful, but it's a start. Run the application and see for yourself.

schedule() Arguments Explained

Before we continue we should probably get a firm grip of the schedule(...) method signature.

delay - How long after the application starts should I wait before running my code?
frequency - After I've run my code the first time, how often should I repeat it?
showTime - A runnable containing the code that is to be run after the delay and at the defined frequency.
Akka.system.dispatcher() - Not entirely sure what this is, but it's needed as the last argument.

Neither of the first two arguments can be changed from within the Runnable body. The delay (and frequency) must be calculated on application start correctly and in such a way that the timing of the job is not affected by application restarts.

Defining a Proper Schedule

Now that we've covered the basics, let's move on to creating a useful schedule. The schedule that we'll create will run a task at 4PM every day.

The first step is to determine the delay i.e. how long the scheduler should wait to execute our task once the application has started. The challenge here is that the calculation of the delay must accommodate random application restarts. Say, for example, we start our application at 8AM, the job should wait roughly 6 hours before it starts, but if at 3PM the need arises for us to restart the application, the delay should be recalculated to 1 hour. We will calculate this delay in seconds for better precision.

I'll be using the Calendar class, but feel free to use JodaTime if you're more comfortable. So let's augment our previous code snippet to calculate this delay:
Long delayInSeconds;

Calendar c = Calendar.getInstance();
c.set(Calendar.HOUR_OF_DAY, 16);
c.set(Calendar.MINUTE, 0);
c.set(Calendar.SECOND, 0);
Date plannedStart = c.getTime();
Date now = new Date();
Date nextRun;
if(now.after(plannedStart)) {
   c.add(Calendar.DAY_OF_WEEK, 1);
   nextRun = c.getTime();
} else {
   nextRun = c.getTime();
  }
 delayinSeconds = (nextRun.getTime() - now.getTime()) / 1000; //To convert milliseconds to seconds.

Code explanation:
Calendar c = Calendar.getInstance();
c.set(Calendar.HOUR_OF_DAY, 16);
c.set(Calendar.MINUTE, 0);
c.set(Calendar.SECOND, 0);
Date plannedStart = c.getTime();

Use a Calendar object and set it's time to 4PM of the current day, then create a date object to store the current date and the intended start time based on the modified Calendar object.
Date now = new Date();
Date nextRun;
if(now.after(plannedStart)) {
   c.add(Calendar.DAY_OF_WEEK, 1);
   nextRun = c.getTime();
} else {
   nextRun = c.getTime();
  }

Find out the current date and time (now) and create a new Date object (nextRun) that will store the date and time of the next code execute.If the time now is after the time we planned to start the job, then we'll set the time that the job should execute to be tomorrow at 4PM. If not then we're on schedule and the nextRun will be today at 4PM.
delayInSeconds = (nextRun.getTime() - now.getTime()) / 1000; //To convert milliseconds to seconds.


Next we do some simple subtraction to find out how many seconds between now and the next time the code should run. This is our delay... In seconds, of course.

From here on it's gravy. Simply substitute the delayInSeconds value for the integer value in the FiniteDuration delay variable and change the frequency to 1 day as follows:


FiniteDuration delay = FiniteDuration.create(delayInSeconds, TimeUnit.SECONDS);
FiniteDuration frequency = FiniteDuration.create(1, TimeUnit.DAYS);
Runnable showTime ...
Altogether Now!

Long delayInSeconds;

Calendar c = Calendar.getInstance();
c.set(Calendar.HOUR_OF_DAY, 16);
c.set(Calendar.MINUTE, 0);
c.set(Calendar.SECOND, 0);
Date plannedStart = c.getTime();
Date now = new Date();
Date nextRun;
if(now.after(plannedStart)) {
   c.add(Calendar.DAY_OF_WEEK, 1);
   nextRun = c.getTime();
} else {
   nextRun = c.getTime();
  }
 delayInSeconds = (nextRun.getTime() - now.getTime()) / 1000; //To convert milliseconds to seconds.

FiniteDuration delay = FiniteDuration.create(delayInSeconds, TimeUnit.SECONDS);
FiniteDuration frequency = FiniteDuration.create(1, TimeUnit.DAYS);
Runnable showTime = new Runnable() {
            @Override
            public void run() {
                System.out.println("Time is now: " + new Date());
            }
        };

Akka.system().scheduler().schedule(delay, frequency, showTime, Akka.system().dispatcher());

Now every day at 4PM your application will remind you of the time. Awesome.

Thanks very much to +James Ward and this magnificent post for helping me wrap my head around this tricky concept. Thanks to the +Typesafe folks for all their hard work with the +Play Framework , and as always thank you for reading.








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