diff --git a/src/main/kotlin/org/gitanimals/render/domain/AbstractTime.kt b/src/main/kotlin/org/gitanimals/render/domain/AbstractTime.kt
index 0b86180..85f6ec8 100644
--- a/src/main/kotlin/org/gitanimals/render/domain/AbstractTime.kt
+++ b/src/main/kotlin/org/gitanimals/render/domain/AbstractTime.kt
@@ -2,6 +2,7 @@ package org.gitanimals.render.domain
 
 import jakarta.persistence.Column
 import jakarta.persistence.MappedSuperclass
+import jakarta.persistence.PrePersist
 import org.springframework.data.annotation.CreatedDate
 import org.springframework.data.annotation.LastModifiedDate
 import java.time.Instant
@@ -14,5 +15,14 @@ abstract class AbstractTime(
 
     @LastModifiedDate
     @Column(name = "modified_at")
-    val modifiedAt: Instant = createdAt,
-)
+    var modifiedAt: Instant? = null,
+) {
+
+    @PrePersist
+    fun prePersist() {
+        modifiedAt = when (modifiedAt == null) {
+            true -> createdAt
+            false -> return
+        }
+    }
+}
diff --git a/src/main/kotlin/org/gitanimals/render/domain/Persona.kt b/src/main/kotlin/org/gitanimals/render/domain/Persona.kt
index e22ccb3..da8f6a1 100644
--- a/src/main/kotlin/org/gitanimals/render/domain/Persona.kt
+++ b/src/main/kotlin/org/gitanimals/render/domain/Persona.kt
@@ -26,6 +26,10 @@ class Persona(
     @JoinColumn(name = "user_id")
     @ManyToOne(fetch = FetchType.LAZY, optional = false)
     var user: User? = null,
+
+    @Version
+    @Column(name = "version", nullable = false)
+    var version: Long? = null,
 ) : AbstractTime() {
 
     constructor(
diff --git a/src/main/kotlin/org/gitanimals/render/domain/UserService.kt b/src/main/kotlin/org/gitanimals/render/domain/UserService.kt
index f55ee71..0264459 100644
--- a/src/main/kotlin/org/gitanimals/render/domain/UserService.kt
+++ b/src/main/kotlin/org/gitanimals/render/domain/UserService.kt
@@ -49,6 +49,7 @@ class UserService(
     fun createNewUser(name: String, contributions: Map<Int, Int>): User =
         userRepository.save(User.newUser(name, contributions))
 
+    @Retryable(retryFor = [ObjectOptimisticLockingFailureException::class], maxAttempts = 100)
     @Transactional
     fun giveBonusPersona(id: Long, persona: String) {
         requireIdempotency("$id:bonus")
@@ -58,6 +59,7 @@ class UserService(
         user.giveBonusPersona(persona)
     }
 
+    @Retryable(retryFor = [ObjectOptimisticLockingFailureException::class], maxAttempts = 100)
     @Transactional
     fun changePersona(id: Long, personChangeRequest: PersonaChangeRequest) {
         val user = getUserById(id)