Triggering a Java program based on database updates and time interval

问题

I want a mechanism that will start a java program ( quite a big one ) depending on 2 conditions:

N new inserts in a MySQL table
Every 5 minutes interval.

I know that I can do this through crontab or using Timer or using Stored Procedure etc.

My plan is to write a Java class ( I am most familiar with ), Listener having two threads in parallel - Database Listener and Time listener threads each of them monitoring one of these conditions. If one says, yes, the parent class will start a new thread to run the Program.

I feel that it will be a heavy weight program. Is there some other option that I am overlooking at?

回答1:

Write a single job. Have it execute regularly.

Effectively, you'll be doing some something of the nature of:

SELECT count(*) FROM table WHERE new = 1;

(or whatever)

Run that every second, 5 seconds, 10 seconds, whatever seems reasonable based on your activity.

When count == N, run your process. When "time since last run" == 5 minutes, run your process.

The process is the same, you just check it more often with the two criteria.

This offers an advantage that you won't get rogue race condition where the job fires TWICE (because Job A found the insert count that just-so-happens to have been 5 minutes from when the last job ran). Rare, yes, but race conditions always seem to actively seek "rare" events that "never happen".

As for scheduling, a crontab is easy because you don't have to maintain your process, keep it alive, daemonize, etc. etc.

If you're already running in a long running container (app server, tomcat, etc.) then that problem is already solved and you can just leverage that.

Downside of cron is it's granularity, it only runs at most every minute. If that too long, it won't work for you. But if it's ok, then there's real value in having a simple process that just lights up, does it's check, and quits. Of course, it will have to persist it's state somehow (it could look in a job log to see when the last job ran, for example).

Within java, there are lots of options: raw threads, sleeping, Timers, ScheduledExecutorService, something like Quartz, EJB Timer beans (if you're running a Java EE container).

But, I'm a KISS fan. If a cron job can do it, let it, and do it once.

回答2:

It is actually not that big using a ScheduledExecutorService:

private static final Runnable PROGRAM_RUNNABLE =  new Runnable() {
    @Override
    public void run() {
        // run the program
    }
}

private ScheduledExecutorService ses = Executors.newScheduledThreadPool(2);

public static void main(String[] args) {
    // database based
    ses.scheduleAtFixedRate(new Runnable() {
        @Override
        public void run() {
            boolean inserted = checkDatabase(); // check the insert in the db
            if(inserted) {
                PROGRAM_RUNNABLE.run();
            }
        }
    }, 0, 1, TimeUnit.MINUTES);

    // time based
    ses.scheduleAtFixedRate(PROGRAM_RUNNABLE, 5, 5, TimeUnit.MINUTES);
}

来源：https://stackoverflow.com/questions/26042526/triggering-a-java-program-based-on-database-updates-and-time-interval

标签

java

mysql

multithreading

crontab

scheduling