#27 stock: stock decrease - race condition

stock/src/main/java/com/example/stock/domain/Stock.java

@@ -0,0 +1,37 @@
+package com.example.stock.domain;
+
+import lombok.AllArgsConstructor;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+
+@Entity
+@NoArgsConstructor
+public class Stock {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    private Long productId;
+
+    public Stock(Long productId, Long quantity) {
+        this.productId = productId;
+        this.quantity = quantity;
+    }
+
+    @Getter
+    private Long quantity;
+
+    public void decrease(Long quantity) {
+        if (this.quantity - quantity < 0) {
+            throw new RuntimeException("out of stock");
+        }
+
+        this.quantity = this.quantity - quantity;
+    }
+}

stock/src/main/java/com/example/stock/repository/StockRepository.java

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+package com.example.stock.repository;
+
+import com.example.stock.domain.Stock;
+import org.springframework.data.jpa.repository.JpaRepository;
+
+public interface StockRepository extends JpaRepository<Stock, Long> {
+}

stock/src/main/java/com/example/stock/service/StockService.java

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+package com.example.stock.service;
+
+import com.example.stock.domain.Stock;
+import com.example.stock.repository.StockRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+@Service
+@RequiredArgsConstructor
+public class StockService {
+
+    private final StockRepository stockRepository;
+
+    @Transactional
+    public void decrease(Long id, Long quantity) {
+        // 재고 조회
+        Stock stock = stockRepository.findById(id).orElseThrow();
+
+        // 재고 감소
+        stock.decrease(quantity);
+
+        // 저장
+        stockRepository.saveAndFlush(stock);
+    }
+}

stock/src/test/java/com/example/stock/service/StockServiceTest.java

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+package com.example.stock.service;
+
+import com.example.stock.domain.Stock;
+import com.example.stock.repository.StockRepository;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@SpringBootTest
+class StockServiceTest {
+
+    @Autowired
+    private StockService stockService;
+
+    @Autowired
+    private StockRepository stockRepository;
+
+    @BeforeEach
+    public void before() {
+        Stock stock = new Stock(1L, 100L);
+
+        stockRepository.saveAndFlush(stock);
+    }
+
+    @AfterEach
+    public void after() {
+        stockRepository.deleteAll();
+    }
+
+    @Test
+    public void stock_decrease() {
+        stockService.decrease(1L, 1L);
+
+        // 100 - 1 = 99
+        Stock stock = stockRepository.findById(1L).orElseThrow();
+
+        assertEquals(99, stock.getQuantity());
+    }
+    
+    @Test
+    public void 동시에_100개의_요청() throws InterruptedException {
+        int threadCount = 100;
+        ExecutorService executorService = Executors.newFixedThreadPool(32);
+
+        CountDownLatch latch = new CountDownLatch(threadCount);
+
+        for (int i = 0; i < threadCount; i++) {
+            executorService.submit(() -> {
+                try {
+                    stockService.decrease(1L, 1L);
+                } finally {
+                    latch.countDown();
+                }
+            });
+        }
+
+        latch.await();
+
+        Stock stock = stockRepository.findById(1L).orElseThrow();
+
+        // 100 - (1 * 100) = 0
+        assertEquals(0L, stock.getQuantity());
+    }
+}