모니터링: Actuator + Prometheus

implementation 'org.springframework.boot:spring-boot-starter-actuator'
implementation 'io.micrometer:micrometer-registry-prometheus'

management:
  endpoints:
    web:
      exposure:
        include: health,info,metrics,prometheus,env
      base-path: /actuator
  endpoint:
    health:
      show-details: when-authorized
      probes:
        enabled: true
  metrics:
    tags:
      application: ${spring.application.name}
      env: ${spring.profiles.active}
  prometheus:
    metrics:
      export:
        enabled: true

GET /actuator/health
{"status": "UP"}

GET /actuator/health/liveness
GET /actuator/health/readiness

GET /actuator/metrics
{"names": ["jvm.memory.used", "http.server.requests", ...]}

GET /actuator/metrics/jvm.memory.used
{"name": "jvm.memory.used", "measurements": [...]}

GET /actuator/prometheus
# Prometheus exposition 형식
# HELP jvm_memory_used_bytes ...
# TYPE jvm_memory_used_bytes gauge
jvm_memory_used_bytes{area="heap"} 123456789

@Component
@RequiredArgsConstructor
public class DatabaseHealthIndicator implements HealthIndicator {
    private final DataSource dataSource;

    @Override
    public Health health() {
        try (Connection conn = dataSource.getConnection()) {
            try (Statement stmt = conn.createStatement();
                 ResultSet rs = stmt.executeQuery("SELECT 1")) {
                if (rs.next()) {
                    return Health.up()
                        .withDetail("database", "PostgreSQL")
                        .build();
                }
            }
        } catch (SQLException e) {
            return Health.down(e).build();
        }
        return Health.unknown().build();
    }
}


@Component
@RequiredArgsConstructor
public class ExternalApiHealthIndicator implements HealthIndicator {
    private final RestTemplate restTemplate;

    @Override
    public Health health() {
        try {
            ResponseEntity<String> response = restTemplate.getForEntity(
                "https://api.example.com/health", String.class
            );
            if (response.getStatusCode().is2xxSuccessful()) {
                return Health.up().build();
            }
        } catch (Exception e) {
            return Health.down(e).build();
        }
        return Health.down().build();
    }
}

@Component
@RequiredArgsConstructor
public class OrderMetrics {
    private final MeterRegistry registry;

    private Counter orderCount;
    private Timer orderDuration;

    @PostConstruct
    void init() {
        orderCount = Counter.builder("orders.total")
            .description("총 주문 수")
            .tag("service", "order")
            .register(registry);

        orderDuration = Timer.builder("orders.duration")
            .description("주문 처리 시간")
            .register(registry);
    }

    public void recordOrder(String status) {
        orderCount.increment();
    }

    public <T> T timeOrderProcess(Supplier<T> action) {
        return orderDuration.record(action);
    }
}


@Service
public class OrderService {

    @Timed(value = "orders.processing", description = "주문 처리")
    public Order placeOrder(OrderRequest req) {
        // ... 자동으로 메트릭 기록
    }
}

# prometheus.yml
scrape_configs:
  - job_name: 'spring-boot'
    metrics_path: '/actuator/prometheus'
    scrape_interval: 15s
    static_configs:
      - targets: ['app:8080']

주요 패널:
- JVM Memory (heap/non-heap)
- HTTP requests rate by status (200/400/500)
- HTTP request duration P50/P95/P99
- DB connection pool (active/idle)
- Custom: orders/min, error rate
- Cache hit rate