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Multi Threading
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=> Multi Threading is used for parallel processing in projects.

=> Parallel processing means performing multiple tasks at a time.

=> Multi Threading is mainley used in batch jobs implementation.

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

Ex: 

1) Bank Accounts statements generation (Ex: SBI)

2) Credit-card-bill-generation

3) Post-paid-bill-generation

4) Sending-product-delivery-notifications to customers in ecomm


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

public class Bank{
	
	public void generateStatementsForAllAccs(){

	   List<Account> accList = dao.getAllAccounts(); // 50 cr

	   for(Account acc : accList){
	   	  List<AccTxInfo> txList = dao.getTxHistory(startDate, endDate, acc.getAccNumber());
	   	  preparePdfStmt(txList, acc.getAccNum());
	   }
	}

	public void preparePdfStmt(List<AccTxInfo> txList, Long accNum){
			// logic to generate pdf
			// store pdf into s3 bucket
			// update pdf url in db table
			sendEmail (filePath);
	}

	public void sendEmail(){
			// 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 second => 1 statement

1 minute => 60 stmts we can generate

1 hour ==> 60 * 60 ==> 3600 stmts

1 day ==> 24 * 3600 ==> 86, 400 stmts

1 Month ==> 30 * 86,400 ==> 25, 92, 000 stmts

Note: in SBI, crores of customers having accounts so with above logic to send account statements to all account holders it may take 2 to 3 years of time which is not accepted.


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

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

1 sec => 1 statement

1 minute => 60 * 1 => 60 statements

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

2 hours => 7,200 statements

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

30 days => 30 * 86, 400 => 25,92,000 statements

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

=> If we run our program with 10 threads then in 1 sec 10 statements will be processed.

1 sec => 10 statements

1 min => 60 * 10 => 600 stmts

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

2 hours => 2 * 36, 000 => 72, 000 statements

1 day => 24 * 36,000 => 8, 63, 000 statements

===================================
Run our program with 50 threads
===================================

=> If we run our program with 50 threads then in 1 sec 50 statements will be processed.

1 sec => 50 statements

1 min => 60 * 50 => 3000 statements

1 hour => 60 * 3000 => 1,80,000 statements

2 hours => 2 * 1,80,000 => 3, 60, 000 statements

1 day => 24 * 1,80,000 => 43,20,000 statements

=============================================
How to implement Multi Threading in project
=============================================

public class Bank{
	
	public void generateStatementsForAllAccs(){

	   List<Account> accList = dao.getAllAccounts(); // 50 cr

	   ExecutorService exServ = Executors.newFixedThreadPool(20); // threads creation

	   for(Account acc : accList){

	   	  exServ.submit(new Callable<Object>{
	   	  		public Object call(){
  					List<AccTxInfo> txList = dao.getTxHistory(startDate, endDate, acc.getAccNumber());
	   	  			preparePdfStmt(txList, acc.getAccNum());
	   	  		}
	   	  });	   	

	   }
	}

	public void preparePdfStmt(List<AccTxInfo> txList, Long accNum){
			// logic to generate pdf
			// store pdf into s3 bucket
			// update pdf url in db table
			sendEmail (filePath);
	}

	public void sendEmail(){
			// email sending logic
	}

}

=====================================
ECommerce Multi Threading Scenario
=====================================


public class DeliveryNotifications {
	
	public void sendDeliveryNotificationsToAll(){

		List<Order> orders = dao.getTodayDeliverables();

		ExecutorService ex = Executors.newFixedThreadPool(10);

		for(int i=0; i<=orders.size(); i++){
			ex.submit(new Callable<Object>(){
				public Object call(){
					sendNotification(orders.get(i));
				}
			});			
		}
	}	

 	public static void sendNotification(Order order) {
        // get order details
        // get customer details
        // get address details
        // send email
        // send wathsapp msg
    }
}