================
Multi Threading
================

=> Multi Threading is used for parallel processing

=> Multi Threading is mainley used in batch jobs.

=> Batch jobs are used for batch processing / bulk-operations

	Ex: account-statement-generation
	    credit-card-bill-generation
	    post-paid-bill-generation
	    sending-product-delivery-notifications
	    bulk-sms
	    bulk-email

========================================
Monthly Bank Account Statement Scenario
========================================

public class SbiBank{

	public void generateAndSendStatements(){
		List<Accounts> list = dao.getAccounts();
		for(Account acc : list){
			prepareStatementPdf(acc.number);
			sendStatement(file, acc.email);
		}
	}
	
	private void prepareStmtPdf(Long acctNum){
			// get acc tx info
			// generate pdf
			// store pdf file in s3
			// update pdf url in db
	}

	private void sendStatemtn(File f, String email){
			// email sending logic
	}
}

=> In the above program, we are generating account statements in sequential manner 
  (one after other)

Ex : for 1 customer statement generation it is taking 1 sec of time.

1 minute => 60*60 => 60 stmts

1 hour => 60 * 60 => 3,600 statements

1 day => 24 * 3,600 => 86, 400  statements

Note: in SBI, crores of customers having accounts so with above logic to send account statements to all account holders it will take 5 to 6 months of time which is not accepted.

=========================================================
E-Commerce application delivery notification scenario
=========================================================

=> Every data deliveries will be there in e-commerce application and application should send notification to customers regarding today's delivery in email and whatsapp.


public class DeliveryNotificationService {	

	public void sendDeliveryNotification(){
			List<Deliver> list = dao.getTodayDeliveryDetails();
			for(Deliver d : list){
				sendEmail();
				sendWhatsAppMsg();
			}
	}

	private sendEmail(){
		// logic to send email
	}

	private sendWhatsAppMsg(){
		// logic to send whatsapp msg
	}
}


=> In the above program, we are sending delivery notification in sequential manner 
   (one after other)

======================================
Running our program with 1 thread
====================================

Ex : for 1 customer notification it is taking 1 sec of time.

1 minute => 60*60 => 60 notifications

1 hour => 60 * 60 => 3,600 notifications

2 hours => 7,200 notifications

1 day => 24 * 3,600 => 86, 400  notifications

===================================
Run our program with 10 threads
===================================

1 sec => 10 notifications

1 min => 60 * 10 => 600 notifications

1 hour => 60 * 600 => 36,000 notifications

2 hours => 72, 000 notifications

===================================
Run our program with 20 threads
===================================

1 sec => 20 notifications

1 min => 60 * 20 => 1200 notifications

1 hour => 60 * 1200 => 72,000 notifications


===============================================
How to create Threads to run our program ?
================================================

=> We can create a thread in 3 ways

1) By Extending Thread class

2) By Implementing Runnable interface ( run() )

3) By Implementing Callable interface ( call() )


==================================================
Approach-1 : Extend Properties from Thread class
==================================================

public class Demo extends  Thread{

    public void run(){
        System.out.println("run - called...");
    }

    public static void main(String[] args){
        Demo d1 = new Demo();

        Thread t1  = new Thread(d1);
        t1.start();

        Thread t2 = new Thread(d1);
        t2.start();

        Thread t3 = new Thread(d1);
        t3.start();
    }
}

=> If we extend properties from thread class then we can't extend properties from any other class in future.

============================================
Approach-2 : Implement Runnable interface
============================================

=> If we implement Runnable interface then we have to override run () method which will not return anything.		

public class Demo2 implements Runnable{

    @Override
    public void run() {
        System.out.println("run()- method called");
    }
    public static void main(String[] args) {
        Demo2 d1 = new Demo2();
        Thread t1 = new Thread(d1);
        t1.start();
    }
}
------------------------------------------------
package in.ashokit;

public class Demo3 {
    public static void main(String[] args) {
        Runnable r = new Runnable() {
            @Override
            public void run() {
                System.out.println("run () - method called");
            }
        };
        Thread t = new Thread(r);
        t.start();
    }
}


============================================
Approach-3 : Implement Callable interface
============================================

=> Callable interface introduced in java 1.5v 

=> Using callable interface we can implement multi threading

=> We will use Executors framework in order to deal with Callable interface.


------------------------------------------

public class Demo4 {

    public static void main(String[] args) {
        Demo4 d = new Demo4();
        d.processNotifications();
    }

    public void processNotifications() {
        List<Orders> orders = dao.getOrders();
	      ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 1; i <= orders.size(); i++) {
            executorService.submit(new Callable<Object>() {
                @Override
                public Object call() throws Exception {
                    sendNotification(orders.get(i));
                    return null;
                }
            });
        }
    }

    public static void sendNotification(Order order) {
        //get order details
        // get customer details
        // send email
        // send wathsapp msg
        System.out.println("sending email ...");
        System.out.println("sending whatsapp msg...");
    }
}
----------------------------------------------------------------------------
Multi Threading Impl : https://youtu.be/tWxx-Sp9Rcg?si=mi-ogVDYqM-R1AuA
-----------------------------------------------------------------------------