로깅 전략: SLF4J + Logback

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class UserService {
    private static final Logger log = LoggerFactory.getLogger(UserService.class);

    // Lombok @Slf4j 사용 시 위 코드 자동
    public User signup(SignupDto dto) {
        log.debug("Signup attempt: {}", dto.email());

        try {
            User user = userRepo.save(new User(dto));
            log.info("User signed up: id={}, email={}", user.getId(), user.getEmail());
            return user;
        } catch (DuplicateEmailException e) {
            log.warn("Duplicate email: {}", dto.email());
            throw e;
        } catch (Exception e) {
            log.error("Signup failed for {}: {}", dto.email(), e.getMessage(), e);
            throw e;
        }
    }
}

TRACE: 매우 상세 — 개발만 (보통 비활성)
DEBUG: 진단 — 개발/스테이징
INFO:  일반 — 프로덕션 기본
WARN:  비정상이지만 복구 가능
ERROR: 에러 + 즉시 대응 필요

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

    <!-- 콘솔 -->
    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>

    <!-- 파일 — 일별 롤링 -->
    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>logs/app.log</file>
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <fileNamePattern>logs/app.%d{yyyy-MM-dd}.%i.log</fileNamePattern>
            <maxFileSize>100MB</maxFileSize>
            <maxHistory>30</maxHistory>
            <totalSizeCap>10GB</totalSizeCap>
        </rollingPolicy>
        <encoder>
            <pattern>%d{ISO8601} [%thread] %-5level %logger - %msg%n</pattern>
        </encoder>
    </appender>

    <!-- JSON 포맷 — ELK용 -->
    <appender name="JSON" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>logs/app.json</file>
        <encoder class="net.logstash.logback.encoder.LogstashEncoder">
            <customFields>{"service":"ohs-api","env":"prod"}</customFields>
        </encoder>
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <fileNamePattern>logs/app.json.%d{yyyy-MM-dd}.gz</fileNamePattern>
        </rollingPolicy>
    </appender>

    <springProfile name="dev">
        <root level="DEBUG"><appender-ref ref="CONSOLE"/></root>
    </springProfile>

    <springProfile name="prod">
        <root level="INFO">
            <appender-ref ref="FILE"/>
            <appender-ref ref="JSON"/>
        </root>
    </springProfile>

</configuration>

@Component
public class RequestLoggingFilter extends OncePerRequestFilter {

    @Override
    protected void doFilterInternal(HttpServletRequest req,
                                     HttpServletResponse res,
                                     FilterChain chain) throws ServletException, IOException {
        String requestId = req.getHeader("X-Request-Id");
        if (requestId == null) requestId = UUID.randomUUID().toString();

        MDC.put("requestId", requestId);
        MDC.put("path", req.getRequestURI());
        MDC.put("method", req.getMethod());

        try {
            chain.doFilter(req, res);
        } finally {
            MDC.clear();
        }
    }
}


// 로그 패턴에 MDC 포함
// %d{ISO8601} [%X{requestId}] %-5level %logger - %msg%n

앱 → JSON 로그 → Filebeat → Logstash → Elasticsearch → Kibana

조회 예:
- 특정 사용자의 모든 액션
- 에러 폭주 시간대 분석
- API별 P95 응답 시간
- 트래픽 급증 패턴