service/src/main/kotlin/com/sangdol/roomescape/reservation/business/ReservationService.kt

@@ -154,7 +154,7 @@ class ReservationService(
     }
 
     private fun validateCanCreate(request: PendingReservationCreateRequest) {
-        val schedule = scheduleService.findSummaryById(request.scheduleId)
+        val schedule = scheduleService.findSummaryWithLock(request.scheduleId)
         val theme = themeService.findInfoById(schedule.themeId)
 
         reservationValidator.validateCanCreate(schedule, theme, request)

service/src/main/kotlin/com/sangdol/roomescape/schedule/business/ScheduleService.kt

@@ -174,10 +174,16 @@ class ScheduleService(
     // Other-Service (API 없이 다른 서비스에서 호출)
     // ========================================
     @Transactional(readOnly = true)
-    fun findSummaryById(id: Long): ScheduleSummaryResponse {
+    fun findSummaryWithLock(id: Long): ScheduleSummaryResponse {
         log.info { "[ScheduleService.findDateTimeById] 일정 개요 조회 시작 : id=$id" }
 
-        return findOrThrow(id).toSummaryResponse()
+        val schedule: ScheduleEntity = scheduleRepository.findByIdForUpdate(id)
+            ?: run {
+                log.warn { "[ScheduleService.updateSchedule] 일정 조회 실패. id=$id" }
+                throw ScheduleException(ScheduleErrorCode.SCHEDULE_NOT_FOUND)
+            }
+
+        return schedule.toSummaryResponse()
             .also {
                 log.info { "[ScheduleService.findDateTimeById] 일정 개요 조회 완료: id=$id" }
             }

service/src/main/kotlin/com/sangdol/roomescape/schedule/infrastructure/persistence/ScheduleRepository.kt

@@ -1,7 +1,9 @@
 package com.sangdol.roomescape.schedule.infrastructure.persistence
 
 import com.sangdol.roomescape.schedule.business.domain.ScheduleOverview
+import jakarta.persistence.LockModeType
 import org.springframework.data.jpa.repository.JpaRepository
+import org.springframework.data.jpa.repository.Lock
 import org.springframework.data.jpa.repository.Modifying
 import org.springframework.data.jpa.repository.Query
 import org.springframework.data.repository.query.Param
@@ -11,6 +13,17 @@ import java.time.LocalTime
 
 interface ScheduleRepository : JpaRepository<ScheduleEntity, Long> {
 
+    @Lock(value = LockModeType.PESSIMISTIC_WRITE)
+    @Query("""
+        SELECT 
+            s
+        FROM 
+            ScheduleEntity s
+        WHERE 
+            s._id = :id
+    """)
+    fun findByIdForUpdate(id: Long): ScheduleEntity?
+
     @Query(
         """
         SELECT