feat: 리다이렉트 캐시 및 DB 조회 지표 추가

docs/04-experiment.md

@@ -179,6 +179,21 @@ Redis 조회
 | 100 | p95 | 32.96ms | 11.93ms | 63.80% 감소 |
 | 100 | p99 | 49.88ms | 21.52ms | 56.86% 감소 |
 
+### 캐시 동작 확인
+
+100 VU 부하 테스트 후 Micrometer 지표를 확인했다.
+
+| 지표 | 결과 |
+|---|---:|
+| Cache Hit | 955,178 |
+| Cache Miss | 1 |
+| Cache Hit Ratio | 약 99.9999% |
+| DB Lookup | 1 |
+
+`setup()`에서 수행한 첫 조회는 Cache Miss로 MySQL을 조회했고,
+이후 반복 요청은 Redis Cache Hit로 처리됐다.
+
+따라서 부하 테스트 구간에서는 대부분의 요청이 MySQL 조회 없이 처리된 것을 확인했다.
 
 ## 12. 결과 분석
 

src/main/java/com/backendsystemdesignlab/urlshortener/cache/ShortUrlCache.java

@@ -1,28 +1,34 @@
 package com.backendsystemdesignlab.urlshortener.cache;
 
+import com.backendsystemdesignlab.urlshortener.metrics.RedirectMetrics;
+import lombok.RequiredArgsConstructor;
 import org.springframework.data.redis.core.StringRedisTemplate;
 import org.springframework.stereotype.Component;
 
 import java.time.Duration;
 import java.util.Optional;
 
 @Component
+@RequiredArgsConstructor
 public class ShortUrlCache {
 
     private static final String KEY_PREFIX = "short-url:";
     private static final Duration TTL = Duration.ofHours(1);
 
     private final StringRedisTemplate redisTemplate;
-
-    public ShortUrlCache(StringRedisTemplate redisTemplate) {
-        this.redisTemplate = redisTemplate;
-    }
+    private final RedirectMetrics redirectMetrics;
 
     public Optional<String> find(String shortCode) {
         String longUrl = redisTemplate.opsForValue()
                 .get(KEY_PREFIX +  shortCode);
 
-        return Optional.ofNullable(longUrl);
+        if (longUrl != null) {
+            redirectMetrics.recordCacheHit();
+            return Optional.of(longUrl);
+        }
+
+        redirectMetrics.recordCacheMiss();
+        return Optional.empty();
     }
 
     public void save(String shortCode, String longUrl) {

src/main/java/com/backendsystemdesignlab/urlshortener/metrics/RedirectMetrics.java

@@ -0,0 +1,37 @@
+package com.backendsystemdesignlab.urlshortener.metrics;
+
+import io.micrometer.core.instrument.Counter;
+import io.micrometer.core.instrument.MeterRegistry;
+import org.springframework.stereotype.Component;
+
+@Component
+public class RedirectMetrics {
+
+    private final Counter cacheHitCounter;
+    private final Counter cacheMissCounter;
+    private final Counter dbLookupCounter;
+
+    public RedirectMetrics(MeterRegistry meterRegistry) {
+        this.cacheHitCounter = Counter.builder("short_url.cache.hit")
+                .description("Redis cache hit count")
+                .register(meterRegistry);
+
+        this.cacheMissCounter = Counter.builder("short_url.cache.miss")
+                .description("Redis cache miss count")
+                .register(meterRegistry);
+
+        this.dbLookupCounter = Counter.builder("short_url.db.lookup")
+                .description("Short URL database lookup count")
+                .register(meterRegistry);
+    }
+
+    public void recordCacheHit() {
+        cacheHitCounter.increment();
+    }
+    public void recordCacheMiss() {
+        cacheMissCounter.increment();
+    }
+    public void recordDbLookup() {
+        dbLookupCounter.increment();
+    }
+}

src/main/java/com/backendsystemdesignlab/urlshortener/service/RedirectService.java

@@ -3,6 +3,7 @@
 import com.backendsystemdesignlab.urlshortener.cache.ShortUrlCache;
 import com.backendsystemdesignlab.urlshortener.encoding.Base62Encoder;
 import com.backendsystemdesignlab.urlshortener.exception.ShortUrlNotFoundException;
+import com.backendsystemdesignlab.urlshortener.metrics.RedirectMetrics;
 import com.backendsystemdesignlab.urlshortener.url.domain.ShortUrl;
 import com.backendsystemdesignlab.urlshortener.url.repository.ShortUrlRepository;
 import lombok.RequiredArgsConstructor;
@@ -17,6 +18,7 @@ public class RedirectService {
     private final ShortUrlRepository shortUrlRepository;
     private final ShortUrlCache shortUrlCache;
     private final Base62Encoder base62Encoder;
+    private final RedirectMetrics redirectMetrics;
 
     public String findLongUrl(String shortCode) {
         return shortUrlCache.find(shortCode)
@@ -26,6 +28,8 @@ public String findLongUrl(String shortCode) {
     private String findFromDatabase(String shortCode) {
         long id = base62Encoder.decode(shortCode);
 
+        redirectMetrics.recordDbLookup();
+
         ShortUrl shortUrl = shortUrlRepository.findById(id)
                 .orElseThrow(() -> new ShortUrlNotFoundException(shortCode));
 

src/test/java/com/backendsystemdesignlab/urlshortener/service/RedirectServiceTest.java

@@ -3,6 +3,7 @@
 import com.backendsystemdesignlab.urlshortener.cache.ShortUrlCache;
 import com.backendsystemdesignlab.urlshortener.encoding.Base62Encoder;
 import com.backendsystemdesignlab.urlshortener.exception.ShortUrlNotFoundException;
+import com.backendsystemdesignlab.urlshortener.metrics.RedirectMetrics;
 import com.backendsystemdesignlab.urlshortener.url.domain.ShortUrl;
 import com.backendsystemdesignlab.urlshortener.url.repository.ShortUrlRepository;
 import org.junit.jupiter.api.Test;
@@ -30,6 +31,8 @@ class RedirectServiceTest {
     private ShortUrlRepository shortUrlRepository;
     @Mock
     private Base62Encoder base62Encoder;
+    @Mock
+    private RedirectMetrics redirectMetrics;
 
     @Test
     void 캐시에_URL이_있으면_DB를_조회하지_않는다() {