synchronize block of code in multiple JVM instances

[vittal mareddy]

I have a servlet code, synchronized the code using synchrozie block, it is working if i use single instace of web server(tomcat), all threads accessing same code are synchronized, whereas same code if i deploye in load balanced environement 2 application servers(each 2 jvms ) i.e total 4 JVMs, synchronization is failing.

Load balanced environement(websphere application server version 7.0.0.13).

here is sample code

protected String instantiateObject(
synchronized(Sample.class){
//logic insert data to be synchrozied
}
}

I see from other supporting forums, synchronization across multiple jvs can be achieved using

file object then filechannel and lock on it. please let me know.


public synchronized void mymethod() throws Exception {
RandomAccessFile raf = new RandomAccessFile("mylock","rw");
FileChannel fc = raf.getChannel();
fc.lock(); // this will block if the lock can't be obtained

// login for insert information
fc.close(); // end the block and unlock raf.close();
}



I am not sure this approach will work, or anything more i have to do.

[Michael Ernest]

Hello Vittal -

In order to synchronize threads in different JVMs, there has to be some external object to act as a guard for the resource you want to protect from unsynchronized access. All participating JVMs have to agree on this mechanism and have to use it properly. A disk-based object like a file is a typical choice, but there are other possibilities: a database record, or a serializable token that is passed among the JVMs, etc. But in each case, there has to be some thing the JVMs can use to arbitrate access to shared data.

[vittal mareddy]

I was trying, as below to synchronize execution in multiple jvms and multi thread invocation, i can see inconsistance, do i need to change anything or any suggestions please help me.

String instantiateObjectCreate()
{
	RandomAccessFile raf = null;
	FileChannel fc = null;

	synchronized(SampleTest.class) {
		raf = new RandomAccessFile("\\\\10.20.30.40\\d$\\abc\\mylock.txt","rw");   
		fc = raf.getChannel();   
		fc.lock(); 
		s0 = runServerMethod(clientId, "CLIENT_EXIST");
		
		if (StringUtils.hasText(s0)) {
			return s0;
		}
			
		s0 = instantiateObjectCreate(svcCreate);
		return s0;	
			
		} catch(FileNotFoundException fne) {
			fne.printStackTrace();	
		} catch(IOException ioe) {
			ioe.printStackTrace();
		}
		finally {
			try {
				if(raf != null){
					raf.close();
			}
			if(fc != null) {
				fc.close();
			}
		} catch(Exception eot) {
			eot.printStackTrace();
		}
	}
}
}	

[Maarten Bodewes]

Don't try and create locks on remote files, use something else, e.g. a database for synchronization (or try and work around your sync options).

[Chris Hurst]

Other technologies to do this do exist but it would depend very much on your circumstance ...

As an example you could run your JVM's on terracotta in which case the code as you initially described can work. terracotta

Other good shared cache technologies do exist ;-) , again it depends on your circumstance and/or what shared resources are available, a shared database is another common solution.