Revise: Override the findById method in BookRepository & Add Pessimis…

…tic Write lock

src/main/java/com/example/demo/repository/BookRepository.java

@@ -1,10 +1,27 @@
 package com.example.demo.repository;
 
 import com.example.demo.model.Book;
+import jakarta.annotation.Nonnull;
+import jakarta.persistence.LockModeType;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Lock;
 import org.springframework.stereotype.Repository;
 
+import java.util.Optional;
+
 @Repository
 public interface BookRepository extends JpaRepository<Book, String> {
 
+    /**
+     * <p>Retrieves a Book entity by its unique identifier while applying
+     * a pessimistic write lock on the database record to prevent concurrent updates.</p>
+     * <p>Note: A pessimistic write lock ensures that only one transaction can write to
+     * this specific database record at a time, preventing conflicts from concurrent updates.
+     * It helps maintain data integrity but may introduce slight performance overhead.</p>
+     * @param id The unique identifier of the Book.
+     * @return An Optional containing the Book entity if found, or an empty Optional if not found.
+     */
+    @Lock(LockModeType.PESSIMISTIC_WRITE)
+    @Nonnull // Added to suppress warning
+    Optional<Book> findById(@Nonnull String id);
 }