[fix] 팔로잉 동시성 이슈 문제 1차 해결시도

src/main/java/konkuk/thip/user/adapter/out/persistence/UserCommandPersistenceAdapter.java

@@ -21,7 +21,6 @@
 public class UserCommandPersistenceAdapter implements UserCommandPort {
 
     private final UserJpaRepository userJpaRepository;
-
     private final UserMapper userMapper;
 
     @Override
@@ -38,6 +37,14 @@ public User findById(Long userId) {
         return userMapper.toDomainEntity(userJpaEntity);
     }
 
+    @Override
+    public User findByIdWithLock(Long userId) {
+        UserJpaEntity userJpaEntity = userJpaRepository.findByUserIdWithLock(userId).orElseThrow(
+                () -> new EntityNotFoundException(USER_NOT_FOUND));
+
+        return userMapper.toDomainEntity(userJpaEntity);
+    }
+
     @Override
     public Map<Long, User> findByIds(List<Long> userIds) {
         List<UserJpaEntity> entities = userJpaRepository.findAllById(userIds);  // 내부 구현 메서드가 jpql 기반이므로 필터 적용 대상임을 확인함

src/main/java/konkuk/thip/user/adapter/out/persistence/repository/UserJpaRepository.java

@@ -1,8 +1,12 @@
 package konkuk.thip.user.adapter.out.persistence.repository;
 
+import jakarta.persistence.LockModeType;
+import jakarta.persistence.QueryHint;
 import konkuk.thip.user.adapter.out.jpa.UserJpaEntity;
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Lock;
 import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.jpa.repository.QueryHints;
 import org.springframework.data.repository.query.Param;
 
 import java.util.List;
@@ -15,6 +19,13 @@ public interface UserJpaRepository extends JpaRepository<UserJpaEntity, Long>, U
      */
     Optional<UserJpaEntity> findByUserId(Long userId);
 
+    @Lock(LockModeType.PESSIMISTIC_WRITE)
+    @QueryHints({
+        @QueryHint(name = "jakarta.persistence.lock.timeout", value = "5000") // 5초
+    })
+    @Query("select u from UserJpaEntity u where u.userId = :userId")
+    Optional<UserJpaEntity> findByUserIdWithLock(@Param("userId") Long userId);
+
     Optional<UserJpaEntity> findByOauth2Id(String oauth2Id);
 
     boolean existsByNickname(String nickname);

src/main/java/konkuk/thip/user/application/port/out/UserCommandPort.java

@@ -9,6 +9,7 @@ public interface UserCommandPort {
 
     Long save(User user);
     User findById(Long userId);
+    User findByIdWithLock(Long userId);
     Map<Long, User> findByIds(List<Long> userIds);
     void update(User user);
     void delete(User user);

src/main/java/konkuk/thip/user/application/service/following/UserFollowService.java

@@ -1,6 +1,8 @@
 package konkuk.thip.user.application.service.following;
 
 import konkuk.thip.common.exception.BusinessException;
+import konkuk.thip.common.exception.InvalidStateException;
+import konkuk.thip.common.exception.code.ErrorCode;
 import konkuk.thip.notification.application.port.in.FeedNotificationOrchestrator;
 import konkuk.thip.user.application.port.in.UserFollowUsecase;
 import konkuk.thip.user.application.port.in.dto.UserFollowCommand;
@@ -9,12 +11,15 @@
 import konkuk.thip.user.domain.Following;
 import konkuk.thip.user.domain.User;
 import lombok.RequiredArgsConstructor;
+import org.springframework.retry.annotation.Backoff;
+import org.springframework.retry.annotation.Recover;
+import org.springframework.retry.annotation.Retryable;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
 import java.util.Optional;
 
-import static konkuk.thip.common.exception.code.ErrorCode.*;
+import static konkuk.thip.common.exception.code.ErrorCode.USER_CANNOT_FOLLOW_SELF;
 
 @Service
 @RequiredArgsConstructor
@@ -27,6 +32,18 @@ public class UserFollowService implements UserFollowUsecase {
 
     @Override
     @Transactional
+    @Retryable(
+            notRecoverable = {
+                    BusinessException.class,
+                    InvalidStateException.class
+            },
+            noRetryFor = {
+                    BusinessException.class,
+                    InvalidStateException.class
+            },
+            maxAttempts = 3,
+            backoff = @Backoff(delay = 100, maxDelay = 500, multiplier = 2)
+    )
     public Boolean changeFollowingState(UserFollowCommand followCommand) {
         Long userId = followCommand.userId();
         Long targetUserId = followCommand.targetUserId();
@@ -35,7 +52,7 @@ public Boolean changeFollowingState(UserFollowCommand followCommand) {
         validateParams(userId, targetUserId);
 
         Optional<Following> optionalFollowing = followingCommandPort.findByUserIdAndTargetUserId(userId, targetUserId);
-        User targetUser = userCommandPort.findById(targetUserId);
+        User targetUser = userCommandPort.findByIdWithLock(targetUserId);
 
         boolean isFollowRequest = Following.validateFollowingState(optionalFollowing.isPresent(), type);
 
@@ -53,6 +70,11 @@ public Boolean changeFollowingState(UserFollowCommand followCommand) {
         }
     }
 
+    @Recover
+    public Boolean recoverChangeFollowingState(Exception e, UserFollowCommand followCommand) {
+        throw new BusinessException(ErrorCode.RESOURCE_LOCKED);
+    }
+
     private void sendNotifications(Long userId, Long targetUserId) {
         User actorUser = userCommandPort.findById(userId);
         feedNotificationOrchestrator.notifyFollowed(targetUserId, actorUser.getId(), actorUser.getNickname());

src/main/resources/db/migration/V251120__Add_following_unique_constraint.sql

@@ -1,3 +1,4 @@
+-- 팔로잉 테이블에 사용자와 타겟 사용자 간의 유니크 제약 조건 추가
 ALTER TABLE followings
     ADD CONSTRAINT uq_followings_user_target
         UNIQUE (user_id, following_user_id);

src/test/java/konkuk/thip/user/application/service/UserFollowServiceTest.java

@@ -71,7 +71,7 @@ void follow_newRelation() {
                     .thenReturn(Optional.empty());
 
             User user = createUserWithFollowerCount(0);
-            when(userCommandPort.findById(targetUserId)).thenReturn(user);
+            when(userCommandPort.findByIdWithLock(targetUserId)).thenReturn(user);
             when(userCommandPort.findById(userId)).thenReturn(user); // 알림 전송용
 
             UserFollowCommand command = new UserFollowCommand(userId, targetUserId, true);
@@ -106,7 +106,7 @@ void unfollow_existingRelation() {
 
             when(followingCommandPort.findByUserIdAndTargetUserId(userId, targetUserId))
                     .thenReturn(Optional.of(existing));
-            when(userCommandPort.findById(targetUserId)).thenReturn(user);
+            when(userCommandPort.findByIdWithLock(targetUserId)).thenReturn(user);
 
             UserFollowCommand command = new UserFollowCommand(userId, targetUserId, false);