commit 0f5d513b06efca2520bea754022d8443a51ae435
Author: woozu.shin <woozu.shin@kakaoent.com>
Date:   Fri Jan 23 11:45:18 2026 +0900

    Initialize projecet

diff --git a/.claude/settings.local.json b/.claude/settings.local.json
new file mode 100644
index 0000000..2c5cf78
--- /dev/null
+++ b/.claude/settings.local.json
@@ -0,0 +1,7 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(./gradlew compileKotlin:*)"
+    ]
+  }
+}
diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..8af972c
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,3 @@
+/gradlew text eol=lf
+*.bat text eol=crlf
+*.jar binary
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..5a979af
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,40 @@
+HELP.md
+.gradle
+build/
+!gradle/wrapper/gradle-wrapper.jar
+!**/src/main/**/build/
+!**/src/test/**/build/
+
+### STS ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/main/**/bin/
+!**/src/test/**/bin/
+
+### IntelliJ IDEA ###
+.idea
+*.iws
+*.iml
+*.ipr
+out/
+!**/src/main/**/out/
+!**/src/test/**/out/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/
+
+### Kotlin ###
+.kotlin
diff --git a/build.gradle.kts b/build.gradle.kts
new file mode 100644
index 0000000..8405e32
--- /dev/null
+++ b/build.gradle.kts
@@ -0,0 +1,52 @@
+plugins {
+	kotlin("jvm") version "2.2.21"
+	kotlin("plugin.spring") version "2.2.21"
+	id("org.springframework.boot") version "4.0.1"
+	id("io.spring.dependency-management") version "1.1.7"
+}
+
+group = "com.myoauniverse.engineering"
+version = "0.0.1-SNAPSHOT"
+description = "Demo project for Spring Boot"
+
+java {
+	toolchain {
+		languageVersion = JavaLanguageVersion.of(24)
+	}
+}
+
+configurations {
+	compileOnly {
+		extendsFrom(configurations.annotationProcessor.get())
+	}
+}
+
+repositories {
+	mavenCentral()
+}
+
+dependencies {
+	implementation("org.springframework.boot:spring-boot-starter-data-mongodb")
+	implementation("org.springframework.boot:spring-boot-starter-mongodb")
+	implementation("org.springframework.boot:spring-boot-starter-webmvc")
+	implementation("org.jetbrains.kotlin:kotlin-reflect")
+	implementation("tools.jackson.module:jackson-module-kotlin")
+	implementation("io.github.oshai:kotlin-logging-jvm:7.0.3")
+	developmentOnly("org.springframework.boot:spring-boot-devtools")
+	annotationProcessor("org.springframework.boot:spring-boot-configuration-processor")
+	testImplementation("org.springframework.boot:spring-boot-starter-data-mongodb-test")
+	testImplementation("org.springframework.boot:spring-boot-starter-mongodb-test")
+	testImplementation("org.springframework.boot:spring-boot-starter-webmvc-test")
+	testImplementation("org.jetbrains.kotlin:kotlin-test-junit5")
+	testRuntimeOnly("org.junit.platform:junit-platform-launcher")
+}
+
+kotlin {
+	compilerOptions {
+		freeCompilerArgs.addAll("-Xjsr305=strict", "-Xannotation-default-target=param-property")
+	}
+}
+
+tasks.withType<Test> {
+	useJUnitPlatform()
+}
diff --git a/docs/20260121_195722_mongodb_nanosecond_precision_poc.md b/docs/20260121_195722_mongodb_nanosecond_precision_poc.md
new file mode 100644
index 0000000..15deb09
--- /dev/null
+++ b/docs/20260121_195722_mongodb_nanosecond_precision_poc.md
@@ -0,0 +1,228 @@
+# MongoDB Nanosecond Precision POC
+
+> 작성일: 2026-01-21
+
+## 개요
+
+MongoDB BSON Date는 millisecond 정밀도만 지원하므로, nanosecond 정밀도를 보존하기 위한 POC 구현.
+
+## 최종 저장 형식
+
+```javascript
+// MongoDB Document
+{
+  "_id": "...",
+  "eventId": "event-001",
+  "userId": "user-001",
+  "participatedAt": ISODate("2024-01-21T06:30:45.123Z"),  // BSON Date (ms)
+  "nanoAdjustment": 456789,                               // Int (0-999999)
+  "metadata": { ... }
+}
+```
+
+| 필드 | 타입 | 설명 |
+|------|------|------|
+| `participatedAt` | BSON Date | 밀리초 정밀도, UTC 저장 |
+| `nanoAdjustment` | Int | 0-999999 (서브밀리초 나노) |
+
+## 설계 결정 사항
+
+### 1. Offset 저장 안함
+- Client가 i18n parameter (country code)를 요청 시 전달
+- Server에서 응답 시 해당 timezone으로 변환하여 반환
+- 별도 offset 저장 불필요
+
+### 2. BSON Date 사용 이유
+- Native MongoDB 타입 → Shell 쿼리 용이 (`ISODate()`)
+- 효율적인 인덱싱 및 범위 쿼리
+- Custom Converter 불필요 (Zero overhead)
+
+### 3. 나노초 분리 저장
+- BSON Date는 ms까지만 지원
+- `nanoAdjustment` 필드로 sub-ms 나노초 보존
+- 복원: `Instant.ofEpochMilli(ms).plusNanos(nanoAdjustment)`
+
+---
+
+## 요청 시간 캡처 아키텍처
+
+### 문제
+- Controller나 Service에서 `Instant.now()` 사용 시 실제 클라이언트 클릭 시간과 차이 발생
+- 클레임 발생 가능성
+
+### 해결: 계층별 타임스탬프 캡처
+
+```
+┌─────────────────────────────────────────────────────────────────┐
+│                        Client Click                              │
+│                             │                                    │
+│                      ~50-500ms 지연                              │
+│                             ▼                                    │
+│  ┌───────────────────────────────────────────────────────────┐  │
+│  │  Ingress/LB (Nginx, ALB)                                  │  │
+│  │  → X-Request-Start-Time: $msec 헤더 추가 ⭐ 가장 정확     │  │
+│  └───────────────────────────────────────────────────────────┘  │
+│                             │                                    │
+│                             ▼                                    │
+│  ┌───────────────────────────────────────────────────────────┐  │
+│  │  RequestTimestampFilter (Servlet Filter)                  │  │
+│  │  1. X-Request-Start-Time 헤더 파싱                        │  │
+│  │  2. 없으면 Instant.now() (차선책)                         │  │
+│  │  3. ThreadLocal + MDC 저장                                │  │
+│  └───────────────────────────────────────────────────────────┘  │
+│                             │                                    │
+│                             ▼                                    │
+│  ┌───────────────────────────────────────────────────────────┐  │
+│  │  Service                                                   │  │
+│  │  → RequestTimestampHolder.get() 으로 시간 조회             │  │
+│  └───────────────────────────────────────────────────────────┘  │
+└─────────────────────────────────────────────────────────────────┘
+```
+
+### Filter vs Interceptor
+
+| 구분 | Filter | Interceptor |
+|------|--------|-------------|
+| **실행 시점** | DispatcherServlet **이전** | DispatcherServlet **이후** |
+| **소속** | Servlet 스펙 | Spring MVC |
+| **정확도** | ⭐⭐⭐ 더 정확 | ⭐⭐ 덜 정확 |
+
+**결론: Filter가 더 빠르므로 Filter 사용 권장**
+
+### Nginx 설정
+
+```nginx
+location / {
+    proxy_set_header X-Request-Start-Time $msec;
+    proxy_pass http://backend;
+}
+```
+
+### Kubernetes Ingress
+
+```yaml
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  annotations:
+    nginx.ingress.kubernetes.io/configuration-snippet: |
+      proxy_set_header X-Request-Start-Time $msec;
+```
+
+---
+
+## API 엔드포인트
+
+| Method | Path | 설명 |
+|--------|------|------|
+| **POST** | `/api/v1/event/participation` | 이벤트 참여 등록 |
+| GET | `/api/v1/event/participation/{id}` | 참여 정보 조회 |
+| GET | `/api/v1/event/participation/event/{eventId}` | 이벤트별 참여 목록 |
+| GET | `/api/v1/event/participation/user/{userId}` | 사용자별 참여 목록 |
+| GET | `/api/v1/event/participation/check` | 참여 여부 확인 |
+| GET | `/api/v1/event/participation/count/{eventId}` | 참여자 수 |
+
+### POST Request 예시
+
+```bash
+curl -X POST http://localhost:8080/api/v1/event/participation \
+  -H "Content-Type: application/json" \
+  -H "X-Timezone: Asia/Seoul" \
+  -d '{
+    "eventId": "event-001",
+    "userId": "user-001",
+    "metadata": {
+      "source": "mobile"
+    }
+  }'
+```
+
+### Response 예시
+
+```json
+{
+  "id": "678f...",
+  "eventId": "event-001",
+  "userId": "user-001",
+  "participatedAt": "2024-01-21T15:30:45.123456789+09:00[Asia/Seoul]",
+  "participatedAtEpochMilli": 1705819845123,
+  "nanoAdjustment": 456789,
+  "metadata": { "source": "mobile" }
+}
+```
+
+---
+
+## MongoDB Shell 쿼리 예시
+
+```javascript
+// 이벤트별 참여자 조회
+db.event_participations.find({ eventId: "event-123" })
+
+// 사용자별 참여 이벤트 조회
+db.event_participations.find({ userId: "user-456" })
+
+// 특정 기간 참여자 조회
+db.event_participations.find({
+    participatedAt: {
+        $gte: ISODate("2024-01-21T00:00:00Z"),
+        $lt: ISODate("2024-01-22T00:00:00Z")
+    }
+})
+
+// 참여 시간순 정렬
+db.event_participations.find({ eventId: "event-123" }).sort({ participatedAt: 1 })
+
+// 이벤트별 참여자 수
+db.event_participations.countDocuments({ eventId: "event-123" })
+
+// 전체 나노초 계산 (JavaScript)
+db.event_participations.find().forEach(function(doc) {
+    var msNanos = (doc.participatedAt.getTime() % 1000) * 1000000;
+    var fullNanos = msNanos + doc.nanoAdjustment;
+    print(doc.userId + ": " + fullNanos + " ns");
+});
+```
+
+---
+
+## 파일 구조
+
+```
+src/main/kotlin/com/myoauniverse/lab/mongodb/
+├── MongodbNanosecApplication.kt
+├── config/
+│   └── MongoConfig.kt                    # MongoDB 설정 + Custom Converters
+├── domain/
+│   └── EventParticipation.kt             # 도메인 엔티티
+├── repository/
+│   └── EventParticipationRepository.kt   # Repository
+├── service/
+│   └── EventParticipationService.kt      # 비즈니스 로직
+├── filter/
+│   └── RequestTimestampFilter.kt         # 요청 시간 캡처 Filter
+└── controller/
+    ├── EventParticipationController.kt   # REST Controller
+    └── dto/
+        └── EventParticipationDto.kt      # Request/Response DTO
+```
+
+---
+
+## 테스트 실행
+
+```bash
+# MongoDB 실행 필요 (localhost:27017)
+./gradlew test --tests "NanosecondPrecisionTest"
+```
+
+---
+
+## 성능 고려사항
+
+| 항목 | 값 |
+|------|-----|
+| 필드 수 | 2개 (participatedAt + nanoAdjustment) |
+| 메모리 오버헤드 | ~12 bytes (Date 8 + Int 4) |
+| Custom Converter | 불필요 (native BSON types) |
+| 대상 TPS | 10,000 ~ 50,000 |
diff --git a/gradle/wrapper/gradle-wrapper.jar b/gradle/wrapper/gradle-wrapper.jar
new file mode 100644
index 0000000..f8e1ee3
Binary files /dev/null and b/gradle/wrapper/gradle-wrapper.jar differ
diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
new file mode 100644
index 0000000..23449a2
--- /dev/null
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -0,0 +1,7 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.2.1-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists
diff --git a/gradlew b/gradlew
new file mode 100755
index 0000000..adff685
--- /dev/null
+++ b/gradlew
@@ -0,0 +1,248 @@
+#!/bin/sh
+
+#
+# Copyright © 2015 the original authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+##############################################################################
+#
+#   Gradle start up script for POSIX generated by Gradle.
+#
+#   Important for running:
+#
+#   (1) You need a POSIX-compliant shell to run this script. If your /bin/sh is
+#       noncompliant, but you have some other compliant shell such as ksh or
+#       bash, then to run this script, type that shell name before the whole
+#       command line, like:
+#
+#           ksh Gradle
+#
+#       Busybox and similar reduced shells will NOT work, because this script
+#       requires all of these POSIX shell features:
+#         * functions;
+#         * expansions «$var», «${var}», «${var:-default}», «${var+SET}»,
+#           «${var#prefix}», «${var%suffix}», and «$( cmd )»;
+#         * compound commands having a testable exit status, especially «case»;
+#         * various built-in commands including «command», «set», and «ulimit».
+#
+#   Important for patching:
+#
+#   (2) This script targets any POSIX shell, so it avoids extensions provided
+#       by Bash, Ksh, etc; in particular arrays are avoided.
+#
+#       The "traditional" practice of packing multiple parameters into a
+#       space-separated string is a well documented source of bugs and security
+#       problems, so this is (mostly) avoided, by progressively accumulating
+#       options in "$@", and eventually passing that to Java.
+#
+#       Where the inherited environment variables (DEFAULT_JVM_OPTS, JAVA_OPTS,
+#       and GRADLE_OPTS) rely on word-splitting, this is performed explicitly;
+#       see the in-line comments for details.
+#
+#       There are tweaks for specific operating systems such as AIX, CygWin,
+#       Darwin, MinGW, and NonStop.
+#
+#   (3) This script is generated from the Groovy template
+#       https://github.com/gradle/gradle/blob/HEAD/platforms/jvm/plugins-application/src/main/resources/org/gradle/api/internal/plugins/unixStartScript.txt
+#       within the Gradle project.
+#
+#       You can find Gradle at https://github.com/gradle/gradle/.
+#
+##############################################################################
+
+# Attempt to set APP_HOME
+
+# Resolve links: $0 may be a link
+app_path=$0
+
+# Need this for daisy-chained symlinks.
+while
+    APP_HOME=${app_path%"${app_path##*/}"}  # leaves a trailing /; empty if no leading path
+    [ -h "$app_path" ]
+do
+    ls=$( ls -ld "$app_path" )
+    link=${ls#*' -> '}
+    case $link in             #(
+      /*)   app_path=$link ;; #(
+      *)    app_path=$APP_HOME$link ;;
+    esac
+done
+
+# This is normally unused
+# shellcheck disable=SC2034
+APP_BASE_NAME=${0##*/}
+# Discard cd standard output in case $CDPATH is set (https://github.com/gradle/gradle/issues/25036)
+APP_HOME=$( cd -P "${APP_HOME:-./}" > /dev/null && printf '%s\n' "$PWD" ) || exit
+
+# Use the maximum available, or set MAX_FD != -1 to use that value.
+MAX_FD=maximum
+
+warn () {
+    echo "$*"
+} >&2
+
+die () {
+    echo
+    echo "$*"
+    echo
+    exit 1
+} >&2
+
+# OS specific support (must be 'true' or 'false').
+cygwin=false
+msys=false
+darwin=false
+nonstop=false
+case "$( uname )" in                #(
+  CYGWIN* )         cygwin=true  ;; #(
+  Darwin* )         darwin=true  ;; #(
+  MSYS* | MINGW* )  msys=true    ;; #(
+  NONSTOP* )        nonstop=true ;;
+esac
+
+
+
+# Determine the Java command to use to start the JVM.
+if [ -n "$JAVA_HOME" ] ; then
+    if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
+        # IBM's JDK on AIX uses strange locations for the executables
+        JAVACMD=$JAVA_HOME/jre/sh/java
+    else
+        JAVACMD=$JAVA_HOME/bin/java
+    fi
+    if [ ! -x "$JAVACMD" ] ; then
+        die "ERROR: JAVA_HOME is set to an invalid directory: $JAVA_HOME
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+else
+    JAVACMD=java
+    if ! command -v java >/dev/null 2>&1
+    then
+        die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+fi
+
+# Increase the maximum file descriptors if we can.
+if ! "$cygwin" && ! "$darwin" && ! "$nonstop" ; then
+    case $MAX_FD in #(
+      max*)
+        # In POSIX sh, ulimit -H is undefined. That's why the result is checked to see if it worked.
+        # shellcheck disable=SC2039,SC3045
+        MAX_FD=$( ulimit -H -n ) ||
+            warn "Could not query maximum file descriptor limit"
+    esac
+    case $MAX_FD in  #(
+      '' | soft) :;; #(
+      *)
+        # In POSIX sh, ulimit -n is undefined. That's why the result is checked to see if it worked.
+        # shellcheck disable=SC2039,SC3045
+        ulimit -n "$MAX_FD" ||
+            warn "Could not set maximum file descriptor limit to $MAX_FD"
+    esac
+fi
+
+# Collect all arguments for the java command, stacking in reverse order:
+#   * args from the command line
+#   * the main class name
+#   * -classpath
+#   * -D...appname settings
+#   * --module-path (only if needed)
+#   * DEFAULT_JVM_OPTS, JAVA_OPTS, and GRADLE_OPTS environment variables.
+
+# For Cygwin or MSYS, switch paths to Windows format before running java
+if "$cygwin" || "$msys" ; then
+    APP_HOME=$( cygpath --path --mixed "$APP_HOME" )
+
+    JAVACMD=$( cygpath --unix "$JAVACMD" )
+
+    # Now convert the arguments - kludge to limit ourselves to /bin/sh
+    for arg do
+        if
+            case $arg in                                #(
+              -*)   false ;;                            # don't mess with options #(
+              /?*)  t=${arg#/} t=/${t%%/*}              # looks like a POSIX filepath
+                    [ -e "$t" ] ;;                      #(
+              *)    false ;;
+            esac
+        then
+            arg=$( cygpath --path --ignore --mixed "$arg" )
+        fi
+        # Roll the args list around exactly as many times as the number of
+        # args, so each arg winds up back in the position where it started, but
+        # possibly modified.
+        #
+        # NB: a `for` loop captures its iteration list before it begins, so
+        # changing the positional parameters here affects neither the number of
+        # iterations, nor the values presented in `arg`.
+        shift                   # remove old arg
+        set -- "$@" "$arg"      # push replacement arg
+    done
+fi
+
+
+# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+DEFAULT_JVM_OPTS='"-Xmx64m" "-Xms64m"'
+
+# Collect all arguments for the java command:
+#   * DEFAULT_JVM_OPTS, JAVA_OPTS, and optsEnvironmentVar are not allowed to contain shell fragments,
+#     and any embedded shellness will be escaped.
+#   * For example: A user cannot expect ${Hostname} to be expanded, as it is an environment variable and will be
+#     treated as '${Hostname}' itself on the command line.
+
+set -- \
+        "-Dorg.gradle.appname=$APP_BASE_NAME" \
+        -jar "$APP_HOME/gradle/wrapper/gradle-wrapper.jar" \
+        "$@"
+
+# Stop when "xargs" is not available.
+if ! command -v xargs >/dev/null 2>&1
+then
+    die "xargs is not available"
+fi
+
+# Use "xargs" to parse quoted args.
+#
+# With -n1 it outputs one arg per line, with the quotes and backslashes removed.
+#
+# In Bash we could simply go:
+#
+#   readarray ARGS < <( xargs -n1 <<<"$var" ) &&
+#   set -- "${ARGS[@]}" "$@"
+#
+# but POSIX shell has neither arrays nor command substitution, so instead we
+# post-process each arg (as a line of input to sed) to backslash-escape any
+# character that might be a shell metacharacter, then use eval to reverse
+# that process (while maintaining the separation between arguments), and wrap
+# the whole thing up as a single "set" statement.
+#
+# This will of course break if any of these variables contains a newline or
+# an unmatched quote.
+#
+
+eval "set -- $(
+        printf '%s\n' "$DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS" |
+        xargs -n1 |
+        sed ' s~[^-[:alnum:]+,./:=@_]~\\&~g; ' |
+        tr '\n' ' '
+    )" '"$@"'
+
+exec "$JAVACMD" "$@"
diff --git a/gradlew.bat b/gradlew.bat
new file mode 100644
index 0000000..c4bdd3a
--- /dev/null
+++ b/gradlew.bat
@@ -0,0 +1,93 @@
+@rem
+@rem Copyright 2015 the original author or authors.
+@rem
+@rem Licensed under the Apache License, Version 2.0 (the "License");
+@rem you may not use this file except in compliance with the License.
+@rem You may obtain a copy of the License at
+@rem
+@rem      https://www.apache.org/licenses/LICENSE-2.0
+@rem
+@rem Unless required by applicable law or agreed to in writing, software
+@rem distributed under the License is distributed on an "AS IS" BASIS,
+@rem WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+@rem See the License for the specific language governing permissions and
+@rem limitations under the License.
+@rem
+@rem SPDX-License-Identifier: Apache-2.0
+@rem
+
+@if "%DEBUG%"=="" @echo off
+@rem ##########################################################################
+@rem
+@rem  Gradle startup script for Windows
+@rem
+@rem ##########################################################################
+
+@rem Set local scope for the variables with windows NT shell
+if "%OS%"=="Windows_NT" setlocal
+
+set DIRNAME=%~dp0
+if "%DIRNAME%"=="" set DIRNAME=.
+@rem This is normally unused
+set APP_BASE_NAME=%~n0
+set APP_HOME=%DIRNAME%
+
+@rem Resolve any "." and ".." in APP_HOME to make it shorter.
+for %%i in ("%APP_HOME%") do set APP_HOME=%%~fi
+
+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+set DEFAULT_JVM_OPTS="-Xmx64m" "-Xms64m"
+
+@rem Find java.exe
+if defined JAVA_HOME goto findJavaFromJavaHome
+
+set JAVA_EXE=java.exe
+%JAVA_EXE% -version >NUL 2>&1
+if %ERRORLEVEL% equ 0 goto execute
+
+echo. 1>&2
+echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH. 1>&2
+echo. 1>&2
+echo Please set the JAVA_HOME variable in your environment to match the 1>&2
+echo location of your Java installation. 1>&2
+
+goto fail
+
+:findJavaFromJavaHome
+set JAVA_HOME=%JAVA_HOME:"=%
+set JAVA_EXE=%JAVA_HOME%/bin/java.exe
+
+if exist "%JAVA_EXE%" goto execute
+
+echo. 1>&2
+echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME% 1>&2
+echo. 1>&2
+echo Please set the JAVA_HOME variable in your environment to match the 1>&2
+echo location of your Java installation. 1>&2
+
+goto fail
+
+:execute
+@rem Setup the command line
+
+
+
+@rem Execute Gradle
+"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -jar "%APP_HOME%\gradle\wrapper\gradle-wrapper.jar" %*
+
+:end
+@rem End local scope for the variables with windows NT shell
+if %ERRORLEVEL% equ 0 goto mainEnd
+
+:fail
+rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
+rem the _cmd.exe /c_ return code!
+set EXIT_CODE=%ERRORLEVEL%
+if %EXIT_CODE% equ 0 set EXIT_CODE=1
+if not ""=="%GRADLE_EXIT_CONSOLE%" exit %EXIT_CODE%
+exit /b %EXIT_CODE%
+
+:mainEnd
+if "%OS%"=="Windows_NT" endlocal
+
+:omega
diff --git a/settings.gradle.kts b/settings.gradle.kts
new file mode 100644
index 0000000..4cde7fd
--- /dev/null
+++ b/settings.gradle.kts
@@ -0,0 +1 @@
+rootProject.name = "mongodb-nanosec"
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/MongodbNanosecApplication.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/MongodbNanosecApplication.kt
new file mode 100644
index 0000000..93e4c5e
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/MongodbNanosecApplication.kt
@@ -0,0 +1,13 @@
+package com.myoauniverse.lab.mongodb
+
+import org.springframework.boot.autoconfigure.SpringBootApplication
+import org.springframework.boot.context.properties.EnableConfigurationProperties
+import org.springframework.boot.runApplication
+
+@SpringBootApplication
+@EnableConfigurationProperties
+class MongodbNanosecApplication
+
+fun main(args: Array<String>) {
+    runApplication<MongodbNanosecApplication>(*args)
+}
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/config/MongoConfig.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/config/MongoConfig.kt
new file mode 100644
index 0000000..3f3ff81
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/config/MongoConfig.kt
@@ -0,0 +1,189 @@
+package com.myoauniverse.lab.mongodb.config
+
+import com.mongodb.ConnectionString
+import com.mongodb.MongoClientSettings
+import com.mongodb.client.MongoClient
+import com.mongodb.client.MongoClients
+import com.mongodb.connection.ConnectionPoolSettings
+import com.mongodb.connection.SocketSettings
+import com.mongodb.management.JMXConnectionPoolListener
+import org.bson.types.Decimal128
+import org.springframework.boot.context.properties.EnableConfigurationProperties
+import org.springframework.context.annotation.Bean
+import org.springframework.context.annotation.Configuration
+import org.springframework.core.convert.converter.Converter
+import org.springframework.data.convert.ReadingConverter
+import org.springframework.data.convert.WritingConverter
+import org.springframework.data.mongodb.MongoDatabaseFactory
+import org.springframework.data.mongodb.MongoTransactionManager
+import org.springframework.data.mongodb.core.MongoTemplate
+import org.springframework.data.mongodb.core.SimpleMongoClientDatabaseFactory
+import org.springframework.data.mongodb.core.convert.*
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext
+import java.math.BigDecimal
+import java.time.Instant
+import java.time.ZoneId
+import java.time.ZonedDateTime
+import java.util.*
+import java.util.concurrent.TimeUnit
+
+/**
+ * MongoDB configuration with custom converters.
+ *
+ * Features:
+ * - Removes _class field from documents (DefaultMongoTypeMapper(null))
+ * - BigDecimal ↔ Decimal128 conversion
+ * - ZonedDateTime ↔ Date conversion (for compatibility)
+ *
+ * Note: For nanosecond precision, use Event entity with nanoAdjustment field
+ * instead of relying on ZonedDateTime converter (which loses nanoseconds).
+ */
+@Configuration
+@EnableConfigurationProperties(MongoDBProperties::class)
+class MongoConfig(
+    private val mongoDBProperties: MongoDBProperties
+) {
+
+    companion object {
+        private const val CONNECTION_POOL_MAX_SIZE = 100
+        private const val CONNECTION_POOL_MIN_SIZE = 10
+        private const val CONNECTION_POOL_MAX_LIFE_TIME = 30L
+        private const val CONNECTION_POOL_TIMEOUT = 2000L
+
+        /**
+         * Default timezone for ZonedDateTime conversion.
+         * Used when reading Date from MongoDB back to ZonedDateTime.
+         */
+        val DEFAULT_ZONE_ID: ZoneId = ZoneId.of("Asia/Seoul")
+    }
+
+    @Bean("mongoTransactionManager")
+    fun mongoTransactionManager(mongoDatabaseFactory: MongoDatabaseFactory): MongoTransactionManager {
+        return MongoTransactionManager(mongoDatabaseFactory)
+    }
+
+    @Bean
+    fun mongoTemplate(
+        mongoDatabaseFactory: MongoDatabaseFactory,
+        mongoCustomConversions: MongoCustomConversions,
+        mongoMappingContext: MongoMappingContext
+    ): MongoTemplate {
+        val resolver: DbRefResolver = DefaultDbRefResolver(mongoDatabaseFactory)
+        val converter = MappingMongoConverter(resolver, mongoMappingContext)
+        converter.setTypeMapper(DefaultMongoTypeMapper(null))
+        converter.customConversions = mongoCustomConversions
+        converter.afterPropertiesSet()
+        return MongoTemplate(mongoDatabaseFactory, converter)
+    }
+
+    @Bean
+    fun mongoDatabaseFactory(
+        mongoClient: MongoClient,
+        connectionString: ConnectionString
+    ): MongoDatabaseFactory {
+        return SimpleMongoClientDatabaseFactory(mongoClient, connectionString.database!!)
+    }
+
+
+    @Bean
+    fun mongoClient(connectionString: ConnectionString): MongoClient {
+        val clientSettings = MongoClientSettings.builder()
+                .retryWrites(true)
+                .applyConnectionString(connectionString)
+                .applyToConnectionPoolSettings { builder: ConnectionPoolSettings.Builder ->
+                    builder.maxSize(CONNECTION_POOL_MAX_SIZE)
+                            .minSize(CONNECTION_POOL_MIN_SIZE)
+                            .maxConnectionLifeTime(CONNECTION_POOL_MAX_LIFE_TIME, TimeUnit.MINUTES)
+                            .addConnectionPoolListener(JMXConnectionPoolListener())
+                }
+                .applyToSocketSettings { builder: SocketSettings.Builder ->
+                    builder.connectTimeout(CONNECTION_POOL_TIMEOUT, TimeUnit.MILLISECONDS)
+                }
+                .build()
+        return MongoClients.create(clientSettings)
+    }
+
+    @Bean
+    fun connectionString(): ConnectionString {
+        return ConnectionString(mongoDBProperties.uri)
+    }
+
+    // ==================== Custom Converters ====================
+    @Bean
+    fun mongoCustomConversions(): MongoCustomConversions {
+        val converters = listOf(
+            BigDecimalToDecimal128Converter(),
+            Decimal128ToBigDecimalConverter(),
+            ZonedDateTimeToDateConverter(),
+            DateToZonedDateTimeConverter()
+        )
+        return MongoCustomConversions(converters)
+    }
+
+    @Bean
+    fun mappingMongoConverter(
+        mongoDatabaseFactory: MongoDatabaseFactory,
+        mongoCustomConversions: MongoCustomConversions
+    ): MappingMongoConverter {
+        val dbRefResolver: DbRefResolver = DefaultDbRefResolver(mongoDatabaseFactory)
+        val mappingContext = MongoMappingContext()
+        mappingContext.setSimpleTypeHolder(mongoCustomConversions.simpleTypeHolder)
+        mappingContext.afterPropertiesSet()
+
+        val converter = MappingMongoConverter(dbRefResolver, mappingContext)
+        converter.setTypeMapper(DefaultMongoTypeMapper(null))  // Remove _class field
+        converter.customConversions = mongoCustomConversions
+        converter.afterPropertiesSet()
+        return converter
+    }
+
+    /**
+     * BigDecimal → Decimal128 (Writing)
+     */
+    @WritingConverter
+    class BigDecimalToDecimal128Converter : Converter<BigDecimal, Decimal128> {
+        override fun convert(source: BigDecimal): Decimal128 {
+            return Decimal128(source)
+        }
+    }
+
+    /**
+     * Decimal128 → BigDecimal (Reading)
+     */
+    @ReadingConverter
+    class Decimal128ToBigDecimalConverter : Converter<Decimal128, BigDecimal> {
+        override fun convert(source: Decimal128): BigDecimal {
+            return source.bigDecimalValue()
+        }
+    }
+
+    /**
+     * ZonedDateTime → Date (Writing)
+     *
+     * WARNING: This loses nanosecond precision (only milliseconds preserved).
+     * For nanosecond precision, use Event entity with nanoAdjustment field.
+     */
+    @WritingConverter
+    class ZonedDateTimeToDateConverter : Converter<ZonedDateTime, Date> {
+        override fun convert(source: ZonedDateTime): Date {
+            return Date.from(source.toInstant())
+        }
+    }
+
+    /**
+     * Date → ZonedDateTime (Reading)
+     *
+     * Uses DEFAULT_ZONE_ID since BSON Date doesn't store timezone info.
+     * WARNING: Original timezone is lost; uses configured default.
+     */
+    @ReadingConverter
+    class DateToZonedDateTimeConverter : Converter<Date, ZonedDateTime> {
+        override fun convert(source: Date): ZonedDateTime {
+            return ZonedDateTime.ofInstant(
+                Instant.ofEpochMilli(source.time),
+                DEFAULT_ZONE_ID
+            )
+        }
+    }
+
+}
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/config/MongoDBProperties.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/config/MongoDBProperties.kt
new file mode 100644
index 0000000..e065354
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/config/MongoDBProperties.kt
@@ -0,0 +1,9 @@
+package com.myoauniverse.lab.mongodb.config
+
+import org.springframework.boot.context.properties.ConfigurationProperties
+
+@ConfigurationProperties(prefix = "event.config.persistence.mongodb")
+data class MongoDBProperties(
+    var uri: String = "",
+    var autoIndexCreation: Boolean = false
+)
\ No newline at end of file
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/controller/EventParticipationController.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/controller/EventParticipationController.kt
new file mode 100644
index 0000000..169adad
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/controller/EventParticipationController.kt
@@ -0,0 +1,163 @@
+package com.myoauniverse.lab.mongodb.controller
+
+import com.myoauniverse.lab.mongodb.controller.dto.*
+import com.myoauniverse.lab.mongodb.filter.RequestTimestampHolder
+import com.myoauniverse.lab.mongodb.service.EventParticipationService
+import io.github.oshai.kotlinlogging.KotlinLogging
+import org.springframework.http.HttpStatus
+import org.springframework.http.ResponseEntity
+import org.springframework.web.bind.annotation.*
+import java.time.Instant
+import java.time.ZoneId
+import java.time.format.DateTimeFormatter
+
+private val logger = KotlinLogging.logger {}
+
+@RestController
+@RequestMapping("/api/v1/event/participation")
+class EventParticipationController(
+    private val eventParticipationService: EventParticipationService
+) {
+
+    companion object {
+        private val isoFormatter = DateTimeFormatter.ISO_INSTANT
+    }
+
+    /**
+     * POST /api/v1/event/participation
+     * 이벤트 참여 등록
+     */
+    @PostMapping
+    fun participate(
+        @RequestBody request: EventParticipationRequest,
+        @RequestHeader("X-Timezone", defaultValue = "Asia/Seoul") timezone: String
+    ): ResponseEntity<EventParticipationWithTimingResponse> {
+        // Controller 진입 시간 캡처
+        val controllerTime = Instant.now()
+        val filterTime = RequestTimestampHolder.get()
+        val clientTime = RequestTimestampHolder.getClientTime()
+
+        logger.info {
+            "Controller timing - filterTime: $filterTime, controllerTime: $controllerTime, " +
+            "diff: ${controllerTime.toEpochMilli() - filterTime.toEpochMilli()}ms"
+        }
+
+        val participation = eventParticipationService.participate(
+            eventId = request.eventId,
+            userId = request.userId,
+            metadata = request.metadata
+        )
+
+        val zoneId = ZoneId.of(timezone)
+        val participationResponse = EventParticipationResponse.from(participation, zoneId)
+
+        val timing = TimingInfo(
+            clientRequestTime = clientTime?.let { isoFormatter.format(it) },
+            clientRequestTimeEpochMilli = clientTime?.toEpochMilli(),
+            serverFilterTime = isoFormatter.format(filterTime),
+            serverFilterTimeEpochMilli = filterTime.toEpochMilli(),
+            serverControllerTime = isoFormatter.format(controllerTime),
+            serverControllerTimeEpochMilli = controllerTime.toEpochMilli(),
+            filterToControllerDiffMs = controllerTime.toEpochMilli() - filterTime.toEpochMilli(),
+            clientToFilterDiffMs = clientTime?.let { filterTime.toEpochMilli() - it.toEpochMilli() }
+        )
+
+        val response = EventParticipationWithTimingResponse(participationResponse, timing)
+
+        return ResponseEntity.status(HttpStatus.CREATED).body(response)
+    }
+
+    /**
+     * GET /api/v1/event/participation/{id}
+     * 참여 정보 조회
+     */
+    @GetMapping("/{id}")
+    fun getById(
+        @PathVariable id: String,
+        @RequestHeader("X-Timezone", defaultValue = "Asia/Seoul") timezone: String
+    ): ResponseEntity<EventParticipationResponse> {
+        val participation = eventParticipationService.findById(id)
+            ?: return ResponseEntity.notFound().build()
+
+        val zoneId = ZoneId.of(timezone)
+        return ResponseEntity.ok(EventParticipationResponse.from(participation, zoneId))
+    }
+
+    /**
+     * GET /api/v1/event/participation/event/{eventId}
+     * 이벤트별 참여 목록 조회
+     */
+    @GetMapping("/event/{eventId}")
+    fun getByEventId(
+        @PathVariable eventId: String,
+        @RequestHeader("X-Timezone", defaultValue = "Asia/Seoul") timezone: String
+    ): ResponseEntity<List<EventParticipationResponse>> {
+        val participations = eventParticipationService.findByEventId(eventId)
+        val zoneId = ZoneId.of(timezone)
+
+        val responses = participations.map { EventParticipationResponse.from(it, zoneId) }
+        return ResponseEntity.ok(responses)
+    }
+
+    /**
+     * GET /api/v1/event/participation/user/{userId}
+     * 사용자별 참여 목록 조회
+     */
+    @GetMapping("/user/{userId}")
+    fun getByUserId(
+        @PathVariable userId: String,
+        @RequestHeader("X-Timezone", defaultValue = "Asia/Seoul") timezone: String
+    ): ResponseEntity<List<EventParticipationResponse>> {
+        val participations = eventParticipationService.findByUserId(userId)
+        val zoneId = ZoneId.of(timezone)
+
+        val responses = participations.map { EventParticipationResponse.from(it, zoneId) }
+        return ResponseEntity.ok(responses)
+    }
+
+    /**
+     * GET /api/v1/event/participation/check
+     * 참여 여부 확인
+     */
+    @GetMapping("/check")
+    fun checkParticipation(
+        @RequestParam eventId: String,
+        @RequestParam userId: String,
+        @RequestHeader("X-Timezone", defaultValue = "Asia/Seoul") timezone: String
+    ): ResponseEntity<ParticipationCheckResponse> {
+        val participation = eventParticipationService.findByEventIdAndUserId(eventId, userId)
+        val zoneId = ZoneId.of(timezone)
+
+        val response = ParticipationCheckResponse(
+            eventId = eventId,
+            userId = userId,
+            hasParticipated = participation != null,
+            participation = participation?.let { EventParticipationResponse.from(it, zoneId) }
+        )
+
+        return ResponseEntity.ok(response)
+    }
+
+    /**
+     * GET /api/v1/event/participation/count/{eventId}
+     * 이벤트 참여자 수 조회
+     */
+    @GetMapping("/count/{eventId}")
+    fun getParticipationCount(
+        @PathVariable eventId: String
+    ): ResponseEntity<ParticipationCountResponse> {
+        val count = eventParticipationService.countByEventId(eventId)
+
+        return ResponseEntity.ok(ParticipationCountResponse(eventId, count))
+    }
+
+    /**
+     * Exception handler for duplicate participation
+     */
+    @ExceptionHandler(IllegalStateException::class)
+    fun handleIllegalState(e: IllegalStateException): ResponseEntity<Map<String, String>> {
+        return ResponseEntity
+            .status(HttpStatus.CONFLICT)
+            .body(mapOf("error" to (e.message ?: "Conflict")))
+    }
+}
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/controller/dto/EventParticipationDto.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/controller/dto/EventParticipationDto.kt
new file mode 100644
index 0000000..0ed56c9
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/controller/dto/EventParticipationDto.kt
@@ -0,0 +1,90 @@
+package com.myoauniverse.lab.mongodb.controller.dto
+
+import com.myoauniverse.lab.mongodb.domain.EventParticipation
+import java.time.ZoneId
+import java.time.format.DateTimeFormatter
+
+/**
+ * Request DTO for event participation.
+ */
+data class EventParticipationRequest(
+    val eventId: String,
+    val userId: String,
+    val metadata: Map<String, Any>? = null
+)
+
+/**
+ * Response DTO for event participation.
+ * Includes formatted participatedAt with full nanosecond precision.
+ */
+data class EventParticipationResponse(
+    val id: String,
+    val eventId: String,
+    val userId: String,
+    val participatedAt: String,      // ISO8601 with nanoseconds
+    val participatedAtEpochMilli: Long,
+    val nanoAdjustment: Int,
+    val metadata: Map<String, Any>?
+) {
+    companion object {
+        private val formatter = DateTimeFormatter.ISO_ZONED_DATE_TIME
+
+        fun from(
+            participation: EventParticipation,
+            zoneId: ZoneId = ZoneId.of("Asia/Seoul")
+        ): EventParticipationResponse {
+            val zonedDateTime = participation.toZonedDateTime(zoneId)
+
+            return EventParticipationResponse(
+                id = participation.id!!,
+                eventId = participation.eventId,
+                userId = participation.userId,
+                participatedAt = zonedDateTime.format(formatter),
+                participatedAtEpochMilli = participation.participatedAt.toEpochMilli(),
+                nanoAdjustment = participation.nanoAdjustment,
+                metadata = participation.metadata
+            )
+        }
+    }
+}
+
+/**
+ * Response for participation check.
+ */
+data class ParticipationCheckResponse(
+    val eventId: String,
+    val userId: String,
+    val hasParticipated: Boolean,
+    val participation: EventParticipationResponse?
+)
+
+/**
+ * Response for participation count.
+ */
+data class ParticipationCountResponse(
+    val eventId: String,
+    val count: Long
+)
+
+/**
+ * Timing information for debugging/analysis.
+ * Shows the time difference between client, filter, and controller.
+ */
+data class TimingInfo(
+    val clientRequestTime: String?,       // Client에서 보낸 시간 (ISO8601)
+    val clientRequestTimeEpochMilli: Long?, // Client epoch millis
+    val serverFilterTime: String,          // Filter에서 캡처한 시간
+    val serverFilterTimeEpochMilli: Long,
+    val serverControllerTime: String,      // Controller에서 캡처한 시간
+    val serverControllerTimeEpochMilli: Long,
+    val filterToControllerDiffMs: Long,    // Filter → Controller 차이 (ms)
+    val clientToFilterDiffMs: Long?        // Client → Filter 차이 (ms)
+)
+
+/**
+ * Response DTO with timing information.
+ */
+data class EventParticipationWithTimingResponse(
+    val participation: EventParticipationResponse,
+    val timing: TimingInfo
+)
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/domain/EventParticipation.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/domain/EventParticipation.kt
new file mode 100644
index 0000000..c980113
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/domain/EventParticipation.kt
@@ -0,0 +1,108 @@
+package com.myoauniverse.lab.mongodb.domain
+
+import org.springframework.data.annotation.Id
+import org.springframework.data.mongodb.core.index.Indexed
+import org.springframework.data.mongodb.core.mapping.Document
+import java.time.Instant
+import java.time.ZoneId
+import java.time.ZonedDateTime
+
+/**
+ * EventParticipation entity with nanosecond precision.
+ *
+ * Storage format in MongoDB:
+ * {
+ *   "_id": "...",
+ *   "eventId": "event-123",
+ *   "userId": "user-456",
+ *   "participatedAt": ISODate("2024-01-21T06:30:45.123Z"),  // BSON Date (ms)
+ *   "nanoAdjustment": 456789,                               // 0-999999 (sub-ms nanos)
+ *   "metadata": { ... }
+ * }
+ *
+ * Note: Instant is used instead of Date for cleaner domain model.
+ * Spring Data MongoDB automatically converts Instant ↔ BSON Date.
+ * However, BSON Date only stores milliseconds, so nanoAdjustment is still needed.
+ */
+@Document(collection = "event_participations")
+data class EventParticipation(
+    @Id
+    val id: String? = null,
+    @Indexed
+    val eventId: String,
+    @Indexed
+    val userId: String,
+    @Indexed
+    val participatedAt: Instant,      // Spring Data MongoDB가 자동으로 BSON Date로 변환
+    val nanoAdjustment: Int,          // 0-999999 (나노초 중 밀리초 이하 부분 보존)
+    val metadata: Map<String, Any>? = null
+) {
+    init {
+        require(nanoAdjustment in 0..999_999) {
+            "nanoAdjustment must be between 0 and 999999, was: $nanoAdjustment"
+        }
+    }
+
+    /**
+     * Reconstructs Instant with full nanosecond precision.
+     *
+     * participatedAt에서 ms 정보 + nanoAdjustment에서 sub-ms 정보를 합침
+     */
+    fun toFullPrecisionInstant(): Instant {
+        // participatedAt은 ms 정밀도로 저장되어 있으므로, nano 부분을 다시 더해줌
+        val truncatedToMillis = Instant.ofEpochMilli(participatedAt.toEpochMilli())
+        return truncatedToMillis.plusNanos(nanoAdjustment.toLong())
+    }
+
+    /**
+     * Converts to ZonedDateTime with specified zone and full nanosecond precision.
+     */
+    fun toZonedDateTime(zoneId: ZoneId): ZonedDateTime {
+        return ZonedDateTime.ofInstant(toFullPrecisionInstant(), zoneId)
+    }
+
+    /**
+     * Returns full nanoseconds (0-999999999).
+     */
+    fun getNano(): Int {
+        val msNanos = (participatedAt.toEpochMilli() % 1000).toInt() * 1_000_000
+        return msNanos + nanoAdjustment
+    }
+
+    companion object {
+        /**
+         * Creates EventParticipation from Instant with nanosecond precision preserved.
+         */
+        fun create(
+            eventId: String,
+            userId: String,
+            participatedAt: Instant = Instant.now(),
+            metadata: Map<String, Any>? = null,
+            id: String? = null
+        ): EventParticipation {
+            // Instant을 ms로 truncate하고, sub-ms 나노초는 별도 저장
+            val truncatedToMillis = Instant.ofEpochMilli(participatedAt.toEpochMilli())
+            val nanoAdjustment = participatedAt.nano % 1_000_000
+
+            return EventParticipation(
+                id = id,
+                eventId = eventId,
+                userId = userId,
+                participatedAt = truncatedToMillis,  // ms 정밀도로 저장
+                nanoAdjustment = nanoAdjustment,     // sub-ms 나노초 보존
+                metadata = metadata
+            )
+        }
+
+        /**
+         * Creates EventParticipation from ZonedDateTime.
+         */
+        fun create(
+            eventId: String,
+            userId: String,
+            participatedAt: ZonedDateTime,
+            metadata: Map<String, Any>? = null,
+            id: String? = null
+        ): EventParticipation = create(eventId, userId, participatedAt.toInstant(), metadata, id)
+    }
+}
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/filter/RequestTimestampFilter.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/filter/RequestTimestampFilter.kt
new file mode 100644
index 0000000..c39cb2a
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/filter/RequestTimestampFilter.kt
@@ -0,0 +1,156 @@
+package com.myoauniverse.lab.mongodb.filter
+
+import io.github.oshai.kotlinlogging.KotlinLogging
+import jakarta.servlet.Filter
+import jakarta.servlet.FilterChain
+import jakarta.servlet.ServletRequest
+import jakarta.servlet.ServletResponse
+import jakarta.servlet.http.HttpServletRequest
+import org.slf4j.MDC
+import org.springframework.core.Ordered
+import org.springframework.core.annotation.Order
+import org.springframework.stereotype.Component
+import java.time.Instant
+
+private val logger = KotlinLogging.logger {}
+
+/**
+ * Servlet Filter to capture request timestamp at the earliest application level.
+ *
+ * Execution order:
+ *   Request → Filter.doFilter() → DispatcherServlet → Interceptor → Controller
+ *
+ * This filter:
+ * 1. Checks for upstream timestamp header (from Ingress/LB)
+ * 2. Falls back to capturing time at filter entry
+ * 3. Stores in both MDC and ThreadLocal for access anywhere
+ */
+@Component
+@Order(Ordered.HIGHEST_PRECEDENCE)  // 가장 먼저 실행
+class RequestTimestampFilter : Filter {
+
+    override fun doFilter(
+        request: ServletRequest,
+        response: ServletResponse,
+        chain: FilterChain
+    ) {
+        val httpRequest = request as HttpServletRequest
+
+        try {
+            // 1. Ingress/LB에서 주입한 헤더 확인 (가장 정확)
+            val upstreamTimestamp = httpRequest.getHeader(HEADER_REQUEST_START_TIME)
+                ?: httpRequest.getHeader(HEADER_X_REQUEST_TIMESTAMP)
+
+            // Client에서 보낸 시간 (JavaScript Date.now())
+            val clientTimestamp = httpRequest.getHeader(HEADER_CLIENT_TIMESTAMP)
+            val clientInstant = clientTimestamp?.let { parseClientTimestamp(it) }
+
+            val requestInstant = if (upstreamTimestamp != null) {
+                parseUpstreamTimestamp(upstreamTimestamp)
+            } else {
+                // 2. Filter 진입 시점 캡처 (차선책)
+                Instant.now()
+            }
+
+            logger.info { "Filter captured: filterTime=$requestInstant, clientTime=$clientInstant" }
+
+            // 3. ThreadLocal과 MDC에 저장
+            RequestTimestampHolder.set(requestInstant)
+            clientInstant?.let { RequestTimestampHolder.setClientTime(it) }
+            MDC.put(MDC_KEY_REQUEST_TIMESTAMP, requestInstant.toString())
+            MDC.put(MDC_KEY_REQUEST_EPOCH_MILLI, requestInstant.toEpochMilli().toString())
+            MDC.put(MDC_KEY_REQUEST_NANO, requestInstant.nano.toString())
+
+            // Request attribute에도 저장 (JSP 등에서 접근용)
+            request.setAttribute(ATTR_REQUEST_TIMESTAMP, requestInstant)
+
+            chain.doFilter(request, response)
+
+        } finally {
+            // 4. 정리
+            RequestTimestampHolder.clear()
+            MDC.remove(MDC_KEY_REQUEST_TIMESTAMP)
+            MDC.remove(MDC_KEY_REQUEST_EPOCH_MILLI)
+            MDC.remove(MDC_KEY_REQUEST_NANO)
+        }
+    }
+
+    private fun parseUpstreamTimestamp(value: String): Instant {
+        return try {
+            // Nginx $msec 형식: "1705819845.123" (초.밀리초)
+            if (value.contains(".")) {
+                val parts = value.split(".")
+                val seconds = parts[0].toLong()
+                val millis = parts.getOrElse(1) { "0" }.take(3).padEnd(3, '0').toInt()
+                Instant.ofEpochSecond(seconds, millis * 1_000_000L)
+            } else {
+                // epoch millis 형식: "1705819845123"
+                Instant.ofEpochMilli(value.toLong())
+            }
+        } catch (e: Exception) {
+            // 파싱 실패 시 현재 시간
+            Instant.now()
+        }
+    }
+
+    private fun parseClientTimestamp(value: String): Instant? {
+        return try {
+            // JavaScript Date.now() epoch millis 형식
+            Instant.ofEpochMilli(value.toLong())
+        } catch (e: Exception) {
+            logger.warn { "Failed to parse client timestamp: $value" }
+            null
+        }
+    }
+
+    companion object {
+        // Upstream headers (Ingress/LB에서 설정)
+        const val HEADER_REQUEST_START_TIME = "X-Request-Start-Time"
+        const val HEADER_X_REQUEST_TIMESTAMP = "X-Request-Timestamp"
+
+        // Client timestamp header (JavaScript Date.now())
+        const val HEADER_CLIENT_TIMESTAMP = "X-Client-Timestamp"
+
+        // MDC keys
+        const val MDC_KEY_REQUEST_TIMESTAMP = "requestTimestamp"
+        const val MDC_KEY_REQUEST_EPOCH_MILLI = "requestEpochMilli"
+        const val MDC_KEY_REQUEST_NANO = "requestNano"
+
+        // Request attribute
+        const val ATTR_REQUEST_TIMESTAMP = "requestTimestamp"
+    }
+}
+
+/**
+ * ThreadLocal holder for request timestamp.
+ * Use this to access the timestamp from anywhere in the request context.
+ */
+object RequestTimestampHolder {
+    private val filterTimeHolder = ThreadLocal<Instant>()
+    private val clientTimeHolder = ThreadLocal<Instant>()
+
+    fun set(instant: Instant) {
+        filterTimeHolder.set(instant)
+    }
+
+    fun setClientTime(instant: Instant) {
+        clientTimeHolder.set(instant)
+    }
+
+    fun get(): Instant {
+        return filterTimeHolder.get() ?: Instant.now()
+    }
+
+    fun getClientTime(): Instant? {
+        return clientTimeHolder.get()
+    }
+
+    fun getOrNull(): Instant? {
+        return filterTimeHolder.get()
+    }
+
+    fun clear() {
+        filterTimeHolder.remove()
+        clientTimeHolder.remove()
+    }
+}
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/repository/EventParticipationRepository.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/repository/EventParticipationRepository.kt
new file mode 100644
index 0000000..bca9a65
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/repository/EventParticipationRepository.kt
@@ -0,0 +1,34 @@
+package com.myoauniverse.lab.mongodb.repository
+
+import com.myoauniverse.lab.mongodb.domain.EventParticipation
+import org.springframework.data.mongodb.repository.MongoRepository
+import java.time.Instant
+
+/**
+ * Repository for EventParticipation entity.
+ *
+ * MongoDB Shell query examples:
+ *   db.event_participations.find({ eventId: "event-123" })
+ *   db.event_participations.find({ userId: "user-456" })
+ *   db.event_participations.find({ participatedAt: { $gte: ISODate("2024-01-21") } })
+ */
+interface EventParticipationRepository : MongoRepository<EventParticipation, String> {
+
+    fun findByEventId(eventId: String): List<EventParticipation>
+
+    fun findByUserId(userId: String): List<EventParticipation>
+
+    fun findByEventIdAndUserId(eventId: String, userId: String): EventParticipation?
+
+    fun findByParticipatedAtBetween(start: Instant, end: Instant): List<EventParticipation>
+
+    fun findByEventIdAndParticipatedAtBetween(
+        eventId: String,
+        start: Instant,
+        end: Instant
+    ): List<EventParticipation>
+
+    fun countByEventId(eventId: String): Long
+
+    fun existsByEventIdAndUserId(eventId: String, userId: String): Boolean
+}
diff --git a/src/main/kotlin/com/myoauniverse/lab/mongodb/service/EventParticipationService.kt b/src/main/kotlin/com/myoauniverse/lab/mongodb/service/EventParticipationService.kt
new file mode 100644
index 0000000..0388ff1
--- /dev/null
+++ b/src/main/kotlin/com/myoauniverse/lab/mongodb/service/EventParticipationService.kt
@@ -0,0 +1,67 @@
+package com.myoauniverse.lab.mongodb.service
+
+import com.myoauniverse.lab.mongodb.domain.EventParticipation
+import com.myoauniverse.lab.mongodb.filter.RequestTimestampHolder
+import com.myoauniverse.lab.mongodb.repository.EventParticipationRepository
+import org.springframework.stereotype.Service
+
+@Service
+class EventParticipationService(
+    private val eventParticipationRepository: EventParticipationRepository
+) {
+
+    /**
+     * 이벤트 참여 등록
+     *
+     * 참여 시간은 RequestTimestampHolder에서 가져옴:
+     * 1. Ingress/LB에서 주입한 시간 (가장 정확)
+     * 2. Filter 진입 시점 (차선책)
+     */
+    fun participate(
+        eventId: String,
+        userId: String,
+        metadata: Map<String, Any>? = null
+    ): EventParticipation {
+        // 중복 참여 체크
+        if (eventParticipationRepository.existsByEventIdAndUserId(eventId, userId)) {
+            throw IllegalStateException("User $userId already participated in event $eventId")
+        }
+
+        // RequestTimestampHolder에서 요청 시간 가져오기
+        // (Filter에서 Ingress 헤더 또는 Filter 진입 시점으로 설정됨)
+        val participatedAt = RequestTimestampHolder.get()
+
+        val participation = EventParticipation.create(
+            eventId = eventId,
+            userId = userId,
+            participatedAt = participatedAt,
+            metadata = metadata
+        )
+
+        return eventParticipationRepository.save(participation)
+    }
+
+    fun findById(id: String): EventParticipation? {
+        return eventParticipationRepository.findById(id).orElse(null)
+    }
+
+    fun findByEventId(eventId: String): List<EventParticipation> {
+        return eventParticipationRepository.findByEventId(eventId)
+    }
+
+    fun findByUserId(userId: String): List<EventParticipation> {
+        return eventParticipationRepository.findByUserId(userId)
+    }
+
+    fun findByEventIdAndUserId(eventId: String, userId: String): EventParticipation? {
+        return eventParticipationRepository.findByEventIdAndUserId(eventId, userId)
+    }
+
+    fun countByEventId(eventId: String): Long {
+        return eventParticipationRepository.countByEventId(eventId)
+    }
+
+    fun hasParticipated(eventId: String, userId: String): Boolean {
+        return eventParticipationRepository.existsByEventIdAndUserId(eventId, userId)
+    }
+}
diff --git a/src/main/resources/application.yaml b/src/main/resources/application.yaml
new file mode 100644
index 0000000..5eab177
--- /dev/null
+++ b/src/main/resources/application.yaml
@@ -0,0 +1,10 @@
+spring:
+  application:
+    name: mongodb-nanosec
+
+event:
+  config:
+    persistence:
+      mongodb:
+        uri: mongodb://fancommerce:fancommerce@localhost:27017/fan_event?authSource=admin
+        auto-index-creation: true
diff --git a/src/main/resources/static/index.html b/src/main/resources/static/index.html
new file mode 100644
index 0000000..2fc6fad
--- /dev/null
+++ b/src/main/resources/static/index.html
@@ -0,0 +1,326 @@
+<!DOCTYPE html>
+<html lang="ko">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Event Participation - Timing Test</title>
+    <style>
+        * {
+            box-sizing: border-box;
+            margin: 0;
+            padding: 0;
+        }
+        body {
+            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+            background: #f5f5f5;
+            padding: 20px;
+            line-height: 1.6;
+        }
+        .container {
+            max-width: 800px;
+            margin: 0 auto;
+        }
+        h1 {
+            color: #333;
+            margin-bottom: 20px;
+        }
+        .card {
+            background: white;
+            border-radius: 8px;
+            padding: 20px;
+            margin-bottom: 20px;
+            box-shadow: 0 2px 4px rgba(0,0,0,0.1);
+        }
+        .card h2 {
+            color: #555;
+            font-size: 1.1rem;
+            margin-bottom: 15px;
+            border-bottom: 1px solid #eee;
+            padding-bottom: 10px;
+        }
+        .form-group {
+            margin-bottom: 15px;
+        }
+        label {
+            display: block;
+            margin-bottom: 5px;
+            font-weight: 500;
+            color: #555;
+        }
+        input[type="text"] {
+            width: 100%;
+            padding: 10px;
+            border: 1px solid #ddd;
+            border-radius: 4px;
+            font-size: 14px;
+        }
+        input[type="text"]:focus {
+            outline: none;
+            border-color: #007bff;
+        }
+        button {
+            background: #007bff;
+            color: white;
+            border: none;
+            padding: 12px 24px;
+            border-radius: 4px;
+            font-size: 14px;
+            cursor: pointer;
+            transition: background 0.2s;
+        }
+        button:hover {
+            background: #0056b3;
+        }
+        button:disabled {
+            background: #ccc;
+            cursor: not-allowed;
+        }
+        .timing-grid {
+            display: grid;
+            grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
+            gap: 15px;
+        }
+        .timing-item {
+            background: #f8f9fa;
+            padding: 15px;
+            border-radius: 4px;
+        }
+        .timing-item .label {
+            font-size: 12px;
+            color: #666;
+            margin-bottom: 5px;
+        }
+        .timing-item .value {
+            font-family: 'Courier New', monospace;
+            font-size: 13px;
+            color: #333;
+            word-break: break-all;
+        }
+        .timing-item .epoch {
+            font-size: 11px;
+            color: #888;
+            margin-top: 3px;
+        }
+        .diff-section {
+            margin-top: 15px;
+            padding: 15px;
+            background: #e8f4fd;
+            border-radius: 4px;
+        }
+        .diff-item {
+            display: flex;
+            justify-content: space-between;
+            margin-bottom: 8px;
+        }
+        .diff-item:last-child {
+            margin-bottom: 0;
+        }
+        .diff-label {
+            color: #555;
+        }
+        .diff-value {
+            font-weight: 600;
+            color: #007bff;
+        }
+        .response-section {
+            margin-top: 15px;
+        }
+        .response-json {
+            background: #1e1e1e;
+            color: #d4d4d4;
+            padding: 15px;
+            border-radius: 4px;
+            font-family: 'Courier New', monospace;
+            font-size: 12px;
+            overflow-x: auto;
+            white-space: pre-wrap;
+            word-break: break-all;
+        }
+        .error {
+            color: #dc3545;
+            background: #f8d7da;
+            padding: 15px;
+            border-radius: 4px;
+            margin-top: 10px;
+        }
+        .success {
+            color: #155724;
+            background: #d4edda;
+            padding: 10px 15px;
+            border-radius: 4px;
+            margin-bottom: 15px;
+        }
+        #result {
+            display: none;
+        }
+        #result.show {
+            display: block;
+        }
+    </style>
+</head>
+<body>
+    <div class="container">
+        <h1>Event Participation - Timing Test</h1>
+
+        <div class="card">
+            <h2>Participation Form</h2>
+            <form id="participationForm">
+                <div class="form-group">
+                    <label for="eventId">Event ID</label>
+                    <input type="text" id="eventId" name="eventId" value="event-001" required>
+                </div>
+                <div class="form-group">
+                    <label for="userId">User ID</label>
+                    <input type="text" id="userId" name="userId" value="user-001" required>
+                </div>
+                <button type="submit" id="submitBtn">Submit Participation</button>
+            </form>
+        </div>
+
+        <div id="result" class="card">
+            <h2>Result</h2>
+            <div id="successMsg" class="success"></div>
+
+            <h3 style="margin-bottom: 10px; color: #555;">Timing Information</h3>
+            <div class="timing-grid">
+                <div class="timing-item">
+                    <div class="label">Client Request Time</div>
+                    <div class="value" id="clientTime">-</div>
+                    <div class="epoch" id="clientTimeEpoch">-</div>
+                </div>
+                <div class="timing-item">
+                    <div class="label">Server Filter Time</div>
+                    <div class="value" id="filterTime">-</div>
+                    <div class="epoch" id="filterTimeEpoch">-</div>
+                </div>
+                <div class="timing-item">
+                    <div class="label">Server Controller Time</div>
+                    <div class="value" id="controllerTime">-</div>
+                    <div class="epoch" id="controllerTimeEpoch">-</div>
+                </div>
+            </div>
+
+            <div class="diff-section">
+                <div class="diff-item">
+                    <span class="diff-label">Client → Filter</span>
+                    <span class="diff-value" id="clientToFilterDiff">-</span>
+                </div>
+                <div class="diff-item">
+                    <span class="diff-label">Filter → Controller</span>
+                    <span class="diff-value" id="filterToControllerDiff">-</span>
+                </div>
+            </div>
+
+            <div class="response-section">
+                <h3 style="margin-bottom: 10px; color: #555;">Full Response</h3>
+                <pre class="response-json" id="responseJson"></pre>
+            </div>
+        </div>
+
+        <div id="errorSection" class="card" style="display: none;">
+            <h2>Error</h2>
+            <div id="errorMsg" class="error"></div>
+        </div>
+    </div>
+
+    <script>
+        const form = document.getElementById('participationForm');
+        const submitBtn = document.getElementById('submitBtn');
+        const resultSection = document.getElementById('result');
+        const errorSection = document.getElementById('errorSection');
+
+        form.addEventListener('submit', async (e) => {
+            e.preventDefault();
+
+            // Reset UI
+            resultSection.classList.remove('show');
+            errorSection.style.display = 'none';
+            submitBtn.disabled = true;
+            submitBtn.textContent = 'Submitting...';
+
+            const eventId = document.getElementById('eventId').value;
+            const userId = document.getElementById('userId').value;
+
+            // Capture client time right before the request
+            const clientTimestamp = Date.now();
+
+            try {
+                const response = await fetch('/api/v1/event/participation', {
+                    method: 'POST',
+                    headers: {
+                        'Content-Type': 'application/json',
+                        'X-Timezone': 'Asia/Seoul',
+                        'X-Client-Timestamp': clientTimestamp.toString()
+                    },
+                    body: JSON.stringify({
+                        eventId: eventId,
+                        userId: userId
+                    })
+                });
+
+                const data = await response.json();
+
+                if (!response.ok) {
+                    throw new Error(data.error || 'Request failed');
+                }
+
+                // Display success
+                document.getElementById('successMsg').textContent =
+                    `Participation registered! ID: ${data.participation.id}`;
+
+                // Display timing info
+                const timing = data.timing;
+
+                // Client time
+                if (timing.clientRequestTime) {
+                    document.getElementById('clientTime').textContent = formatTime(timing.clientRequestTime);
+                    document.getElementById('clientTimeEpoch').textContent = `Epoch: ${timing.clientRequestTimeEpochMilli}`;
+                } else {
+                    document.getElementById('clientTime').textContent = 'Not captured';
+                    document.getElementById('clientTimeEpoch').textContent = '-';
+                }
+
+                // Filter time
+                document.getElementById('filterTime').textContent = formatTime(timing.serverFilterTime);
+                document.getElementById('filterTimeEpoch').textContent = `Epoch: ${timing.serverFilterTimeEpochMilli}`;
+
+                // Controller time
+                document.getElementById('controllerTime').textContent = formatTime(timing.serverControllerTime);
+                document.getElementById('controllerTimeEpoch').textContent = `Epoch: ${timing.serverControllerTimeEpochMilli}`;
+
+                // Differences
+                document.getElementById('clientToFilterDiff').textContent =
+                    timing.clientToFilterDiffMs !== null ? `${timing.clientToFilterDiffMs} ms` : 'N/A';
+                document.getElementById('filterToControllerDiff').textContent =
+                    `${timing.filterToControllerDiffMs} ms`;
+
+                // Full response JSON
+                document.getElementById('responseJson').textContent = JSON.stringify(data, null, 2);
+
+                resultSection.classList.add('show');
+
+                // Auto-increment userId for next test
+                const currentUserId = document.getElementById('userId').value;
+                const match = currentUserId.match(/^(.+-)(\d+)$/);
+                if (match) {
+                    document.getElementById('userId').value = match[1] + (parseInt(match[2]) + 1);
+                }
+
+            } catch (error) {
+                document.getElementById('errorMsg').textContent = error.message;
+                errorSection.style.display = 'block';
+            } finally {
+                submitBtn.disabled = false;
+                submitBtn.textContent = 'Submit Participation';
+            }
+        });
+
+        function formatTime(isoString) {
+            if (!isoString) return '-';
+            // Convert ISO string to readable format
+            const date = new Date(isoString);
+            return date.toISOString().replace('T', ' ').replace('Z', ' UTC');
+        }
+    </script>
+</body>
+</html>
diff --git a/src/test/kotlin/com/myoauniverse/lab/mongodb/MongodbNanosecApplicationTests.kt b/src/test/kotlin/com/myoauniverse/lab/mongodb/MongodbNanosecApplicationTests.kt
new file mode 100644
index 0000000..321e1f4
--- /dev/null
+++ b/src/test/kotlin/com/myoauniverse/lab/mongodb/MongodbNanosecApplicationTests.kt
@@ -0,0 +1,13 @@
+package com.myoauniverse.lab.mongodb
+
+import org.junit.jupiter.api.Test
+import org.springframework.boot.test.context.SpringBootTest
+
+@SpringBootTest
+class MongodbNanosecApplicationTests {
+
+	@Test
+	fun contextLoads() {
+	}
+
+}
diff --git a/src/test/kotlin/com/myoauniverse/lab/mongodb/NanosecondPrecisionTest.kt b/src/test/kotlin/com/myoauniverse/lab/mongodb/NanosecondPrecisionTest.kt
new file mode 100644
index 0000000..259dd01
--- /dev/null
+++ b/src/test/kotlin/com/myoauniverse/lab/mongodb/NanosecondPrecisionTest.kt
@@ -0,0 +1,201 @@
+package com.myoauniverse.lab.mongodb
+
+import com.myoauniverse.lab.mongodb.domain.EventParticipation
+import com.myoauniverse.lab.mongodb.repository.EventParticipationRepository
+import org.junit.jupiter.api.*
+import org.junit.jupiter.api.Assertions.*
+import org.springframework.beans.factory.annotation.Autowired
+import org.springframework.boot.test.context.SpringBootTest
+import org.springframework.data.mongodb.core.MongoTemplate
+import java.time.Instant
+import java.time.ZoneId
+import java.util.Date
+
+/**
+ * MongoDB nanosecond precision storage test for EventParticipation.
+ */
+@SpringBootTest
+@TestInstance(TestInstance.Lifecycle.PER_CLASS)
+@TestMethodOrder(MethodOrderer.OrderAnnotation::class)
+class NanosecondPrecisionTest {
+
+    @Autowired
+    lateinit var mongoTemplate: MongoTemplate
+
+    @Autowired
+    lateinit var eventParticipationRepository: EventParticipationRepository
+
+    private val testInstant = Instant.parse("2024-01-21T06:30:45.123456789Z")
+
+    @BeforeAll
+    fun setup() {
+        mongoTemplate.dropCollection(EventParticipation::class.java)
+    }
+
+    @Test
+    @Order(1)
+    @DisplayName("나노초 정밀도 보존 테스트")
+    fun `nanosecond precision is preserved`() {
+        // Given
+        val participation = EventParticipation.create(
+            eventId = "event-001",
+            userId = "user-001",
+            participatedAt = testInstant
+        )
+
+        // When
+        val saved = eventParticipationRepository.save(participation)
+        val retrieved = eventParticipationRepository.findById(saved.id!!).orElseThrow()
+        val reconstructedInstant = retrieved.toFullPrecisionInstant()
+
+        // Then
+        println("=== 나노초 정밀도 테스트 ===")
+        println("원본 Instant: $testInstant")
+        println("원본 나노초: ${testInstant.nano}")
+        println("저장된 필드:")
+        println("  - participatedAt (Instant→BSON Date): ${retrieved.participatedAt}")
+        println("  - nanoAdjustment: ${retrieved.nanoAdjustment}")
+        println("복원된 Instant: $reconstructedInstant")
+        println("복원된 나노초: ${reconstructedInstant.nano}")
+
+        assertEquals(testInstant.nano, reconstructedInstant.nano)
+        assertEquals(testInstant, reconstructedInstant)
+    }
+
+    @Test
+    @Order(2)
+    @DisplayName("다양한 나노초 값 테스트")
+    fun `various nanosecond values are preserved`() {
+        val testCases = listOf(0, 1, 999_999, 1_000_000, 123_456_789, 999_999_999)
+
+        println("\n=== 다양한 나노초 값 테스트 ===")
+
+        for (nanos in testCases) {
+            val instant = Instant.parse("2024-01-21T12:00:00Z").plusNanos(nanos.toLong())
+            val participation = EventParticipation.create(
+                eventId = "event-nano-$nanos",
+                userId = "user-nano",
+                participatedAt = instant
+            )
+            val saved = eventParticipationRepository.save(participation)
+            val retrieved = eventParticipationRepository.findById(saved.id!!).orElseThrow()
+
+            println("입력: $nanos ns → 복원: ${retrieved.toFullPrecisionInstant().nano} ns")
+            assertEquals(nanos, retrieved.toFullPrecisionInstant().nano)
+        }
+
+        println("✅ 모든 나노초 값이 정확히 보존됨")
+    }
+
+    @Test
+    @Order(10)
+    @DisplayName("클라이언트 타임존 변환 테스트")
+    fun `client timezone conversion works correctly`() {
+        // Given
+        val participation = EventParticipation.create(
+            eventId = "event-tz",
+            userId = "user-tz",
+            participatedAt = testInstant
+        )
+        val saved = eventParticipationRepository.save(participation)
+        val retrieved = eventParticipationRepository.findById(saved.id!!).orElseThrow()
+
+        // When
+        val koreaTime = retrieved.toZonedDateTime(ZoneId.of("Asia/Seoul"))
+        val utcTime = retrieved.toZonedDateTime(ZoneId.of("UTC"))
+
+        // Then
+        println("\n=== 클라이언트 타임존 변환 테스트 ===")
+        println("저장된 시간 (UTC): $testInstant")
+        println("Korea: $koreaTime (hour: ${koreaTime.hour})")
+        println("UTC: $utcTime (hour: ${utcTime.hour})")
+
+        assertEquals(testInstant, koreaTime.toInstant())
+        assertEquals(testInstant, utcTime.toInstant())
+        assertEquals(15, koreaTime.hour)  // +09:00
+        assertEquals(6, utcTime.hour)
+
+        println("✅ 타임존 변환 정상 동작")
+    }
+
+    @Test
+    @Order(20)
+    @DisplayName("이벤트/사용자 조회 테스트")
+    fun `find by eventId and userId works`() {
+        // Given
+        mongoTemplate.dropCollection(EventParticipation::class.java)
+
+        eventParticipationRepository.saveAll(listOf(
+            EventParticipation.create("event-A", "user-1", Instant.now()),
+            EventParticipation.create("event-A", "user-2", Instant.now()),
+            EventParticipation.create("event-B", "user-1", Instant.now())
+        ))
+
+        // When & Then
+        assertEquals(2, eventParticipationRepository.findByEventId("event-A").size)
+        assertEquals(2, eventParticipationRepository.findByUserId("user-1").size)
+        assertNotNull(eventParticipationRepository.findByEventIdAndUserId("event-A", "user-1"), "Should find participation")
+        assertNull(eventParticipationRepository.findByEventIdAndUserId("event-A", "user-3"), "Should not find participation")
+
+        println("\n=== 이벤트/사용자 조회 테스트 ===")
+        println("event-A 참여자: 2명")
+        println("user-1 참여 이벤트: 2개")
+        println("✅ 조회 정상 동작")
+    }
+
+    @Test
+    @Order(30)
+    @DisplayName("MongoDB 저장 데이터 형식 확인")
+    fun `verify MongoDB data format`() {
+        // Given
+        mongoTemplate.dropCollection(EventParticipation::class.java)
+        val participation = EventParticipation.create(
+            eventId = "event-format",
+            userId = "user-format",
+            participatedAt = testInstant,
+            metadata = mapOf("source" to "test", "version" to 1)
+        )
+        val saved = eventParticipationRepository.save(participation)
+
+        // When
+        val rawDoc = mongoTemplate.findById(saved.id!!, org.bson.Document::class.java, "event_participations")
+
+        // Then
+        println("\n=== MongoDB 저장 데이터 형식 ===")
+        println("Raw Document:")
+        println(rawDoc?.toJson())
+
+        assertNotNull(rawDoc, "Document should not be null")
+        // Instant은 BSON Date로 저장됨
+        assertTrue(rawDoc!!.get("participatedAt") is Date, "participatedAt should be BSON Date")
+        assertEquals(456789, rawDoc.get("nanoAdjustment"))
+    }
+
+    @Test
+    @Order(31)
+    @DisplayName("MongoDB Shell 쿼리 예시")
+    fun `print MongoDB shell query examples`() {
+        println("\n=== MongoDB Shell 쿼리 예시 ===")
+        println("""
+            // 이벤트별 참여자 조회
+            db.event_participations.find({ eventId: "event-123" })
+
+            // 사용자별 참여 이벤트 조회
+            db.event_participations.find({ userId: "user-456" })
+
+            // 특정 기간 참여자 조회
+            db.event_participations.find({
+                participatedAt: {
+                    ${'$'}gte: ISODate("2024-01-21T00:00:00Z"),
+                    ${'$'}lt: ISODate("2024-01-22T00:00:00Z")
+                }
+            })
+
+            // 참여 시간순 정렬
+            db.event_participations.find({ eventId: "event-123" }).sort({ participatedAt: 1 })
+
+            // 이벤트별 참여자 수
+            db.event_participations.countDocuments({ eventId: "event-123" })
+        """.trimIndent())
+    }
+}