Native Transport — epoll & kqueue

// JDK NIO의 select 루프 — 매 반복마다 JNI 호출 + Set 복사
while (!stopped) {
    int ready = selector.select();          // JNI 호출 (epoll_wait 래핑)
    Set<SelectionKey> keys = selector.selectedKeys(); // Set 객체 할당
    for (SelectionKey key : keys) {
        handleEvent(key);
    }
    keys.clear();                           // GC 부담
}

┌─────────────────────────────────────────────────────┐
│                    Netty Application                 │
├──────────────┬──────────────┬───────────────────────┤
│  NIO (JDK)   │  epoll       │  kqueue               │
│  Selector    │  (Linux)     │  (macOS/BSD)           │
├──────────────┼──────────────┼───────────────────────┤
│  JNI → OS    │  직접 JNI    │  직접 JNI              │
│  (범용 래핑)  │  (최적화)    │  (최적화)              │
└──────────────┴──────────────┴───────────────────────┘

// Netty의 epoll 구현 (개념적 의사 코드)
// JNI 호출 1번으로 준비된 이벤트를 네이티브 배열에 직접 채운다
int ready = Native.epollWait(epollFd, events, timerFd, timeoutMillis);
// events는 JNI 쪽에서 직접 채운 배열 — Set 할당/복사 없음
for (int i = 0; i < ready; i++) {
    processEvent(events[i]);
}

Level-triggered (JDK NIO):
  데이터 도착 → 알림 → 일부 읽음 → 알림 → 나머지 읽음 → 알림 (데이터 없으면 중단)

Edge-triggered (Native Transport):
  데이터 도착 → 알림 → 전부 읽음 (EAGAIN까지) → 끝

// SO_REUSEPORT 활성화 — 커널이 들어오는 연결을 여러 스레드에 분산
ServerBootstrap b = new ServerBootstrap();
b.group(new EpollEventLoopGroup())
 .channel(EpollServerSocketChannel.class)
 .option(EpollChannelOption.SO_REUSEPORT, true);  // Native Transport에서만 가능

<!-- Maven — Linux epoll -->
<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-transport-native-epoll</artifactId>
    <version>${netty.version}</version>
    <classifier>linux-x86_64</classifier>  <!-- 플랫폼에 맞는 classifier -->
</dependency>

<!-- Maven — macOS kqueue -->
<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-transport-native-kqueue</artifactId>
    <version>${netty.version}</version>
    <classifier>osx-x86_64</classifier>
</dependency>

// Gradle — Linux epoll
implementation "io.netty:netty-transport-native-epoll:${nettyVersion}:linux-x86_64"

// Gradle — macOS kqueue
implementation "io.netty:netty-transport-native-kqueue:${nettyVersion}:osx-x86_64"

// 변경 전 — JDK NIO
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();

ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
 .channel(NioServerSocketChannel.class);

// 변경 후 — Linux Native Transport
EventLoopGroup bossGroup = new EpollEventLoopGroup(1);
EventLoopGroup workerGroup = new EpollEventLoopGroup();

ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
 .channel(EpollServerSocketChannel.class);

public class TransportUtil {

    // 현재 OS에 따라 적절한 EventLoopGroup을 반환
    public static EventLoopGroup createEventLoopGroup(int nThreads) {
        if (Epoll.isAvailable()) {
            return new EpollEventLoopGroup(nThreads);
        } else if (KQueue.isAvailable()) {
            return new KQueueEventLoopGroup(nThreads);
        }
        return new NioEventLoopGroup(nThreads);
    }

    // 현재 OS에 따라 적절한 ServerSocketChannel 클래스를 반환
    public static Class<? extends ServerSocketChannel> serverSocketChannelClass() {
        if (Epoll.isAvailable()) {
            return EpollServerSocketChannel.class;
        } else if (KQueue.isAvailable()) {
            return KQueueServerSocketChannel.class;
        }
        return NioServerSocketChannel.class;
    }
}

// 사용 예시 — OS와 무관하게 최적의 Transport가 선택됨
EventLoopGroup bossGroup = TransportUtil.createEventLoopGroup(1);
EventLoopGroup workerGroup = TransportUtil.createEventLoopGroup(0);

ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
 .channel(TransportUtil.serverSocketChannelClass());

epoll 방식:
  앱 → epoll_wait (시스템 콜) → "fd 준비됨" → read (시스템 콜) → 데이터

io_uring 방식:
  앱 → SQ에 read 요청 기록 (시스템 콜 없음) → 커널이 처리 → CQ에서 결과 수확 (시스템 콜 없음)

<!-- io_uring 의존성 (인큐베이터) -->
<dependency>
    <groupId>io.netty.incubator</groupId>
    <artifactId>netty-incubator-transport-native-io_uring</artifactId>
    <version>${iouring.version}</version>
    <classifier>linux-x86_64</classifier>
</dependency>

// io_uring 사용 예시
EventLoopGroup group = new IOUringEventLoopGroup();

ServerBootstrap b = new ServerBootstrap();
b.group(group)
 .channel(IOUringServerSocketChannel.class)
 .childHandler(new MyChannelInitializer());