Java in Practice

      CompletableFuture

package com.studyskymate.dinesh.corejava.java8;

import java.util.Arrays;

import java.util.List;

import java.util.concurrent.CompletableFuture;

public class CompletableFutureEx {

      public static void main(String[] args) {

           List<Integer> list = Arrays.asList(5);

list.stream().map(

num -> CompletableFuture

.supplyAsync(() ->getNumber(num)))

.map(CompletableFuture -> CompletableFuture

.thenApply(n -> n * n)).map(t -> t.join())

.forEach(s -> System.out.println(s));

      }

      private static int getNumber(int a) {

           return a * a;

      }

}

  Transactions and Isolation Level

Isolation level in a transactional method

@Autowired
private TestDAO testDAO;

@Transactional(isolation=Isolation.READ_COMMITTED)
public void someTransactionalMethod(User user) {

  // Interact with testDAO

}

	dirty reads	non-repeatable reads	phantom reads
READ_UNCOMMITTED	yes	yes	yes
READ_COMMITTED	no	yes	yes
REPEATABLE_READ	no	no	yes
SERIALIZABLE	no	no	no

Example: Transaction A reads a range of records. Meanwhile, Transaction B inserts a new record in the same range that Transaction A initially fetched and commits. 

Later Transaction A reads the same range again and will also get the record that Transaction B just inserted.This is a phantom read: a transaction fetched a range of records multiple times from the database and obtained different result sets (containing phantom records).

Reference: https://www.byteslounge.com/tutorials/spring-transaction-isolation-tutorial

Microservices Design Pattern

1. Decomposition Patterns
   a. Decompose by Business Capability
   b. Decompose by Subdomain
   c. Strangler Pattern
2. Integration Patterns
   a. API Gateway Pattern
   b. Aggregator Pattern
   c. Client-Side UI Composition Pattern
3. Database Patterns
    a. Database per Service
    b. Shared Database per Service
    c. Command Query Responsibility Segregation (CQRS)
    d. Saga Pattern  credit limit
4. Observability Patterns
      a. Log Aggregation
      b. Performance Metrics
      c. Distributed Tracing
      d. Health Check
5.Cross-Cutting Concern Patterns
  a. External Configuration
  b. Service Discovery Pattern
  c. Circuit Breaker Pattern

  d. Blue-Green Deployment Pattern

How to avoid Diamond Problem With Default Methods in Java 8

package com.studyskymate.dinesh.corejava.java8;

public interface Poet {

      default void write() {

           System.out.println("Poet's default method");

      }

      static void write3() {

           System.out.println("poet Writer static default method");

      }

}

package com.studyskymate.dinesh.corejava.java8;

public interface Writer {

       default void write() {

              System.out.println("Writer default method");

          }

       

       default void write2() {

              System.out.println("Writer default method22");

          }

       

       static void write3() {

              System.out.println(" write Writer static default method");

          }

}

package com.studyskymate.dinesh.corejava.java8;

public class Multitalented implements Poet, Writer {

      public static void main(String[] args) {

           Multitalented m = new Multitalented();

           m.write();

      }

      //Need to override the ambiguous method otherwise there will be compile time error

      @Override

      public void write() {

           Writer.super.write();

           System.out.println("In Multitalented class");

      }

}