Netty Reator（二）Scalable IO in Java

Netty Reator（二）Scalable IO in Java

Netty 系列目錄 (http://www.javashuo.com/article/p-hskusway-em.html)

Doug Lea 大神的《Scalable IO in Java》http://gee.cs.oswego.edu/dl/cpjslides/nio.pdf：可伸縮的 IO 模型

大部分 IO 都是下面這個步驟,

• Read request
• Decode request
• Process service
• Encode reply
• Send reply

1、經典的網絡 IO 模型

傳統的 IO 模型是一個 socket 一個線程，代碼以下：

class Server implements Runnable {
    public void run() {
        try {
            ServerSocket ss = new ServerSocket(PORT);
            while (!Thread.interrupted())
            new Thread(new Handler(ss.accept())).start(); //建立新線程來handle
            // or, single-threaded, or a thread pool
        } catch (IOException ex) { /* ... */ }
    }
    
    static class Handler implements Runnable {
        final Socket socket;
        Handler(Socket s) { socket = s; }
        public void run() {
            try {
                byte[] input = new byte[MAX_INPUT];
                socket.getInputStream().read(input);
                byte[] output = process(input);
                socket.getOutputStream().write(output);
            } catch (IOException ex) { /* ... */ }
        }       
        private byte[] process(byte[] cmd) { /* ... */ }
    }
}

顯然簡單的多線程會帶來擴展性問題，當 client 數量變的不少的時候，還其餘的可用性、性能的問題。解決方法就是 Divide-and-conquer，分開後，就須要 Event-driven Designs 來串聯起來...

二。單線程( BasicReactor Design)

全部事情 read、process、write 都由單個線程完成，完成一步從新設置下一步的 event。問題固然就是，其中任何步驟阻塞其它任務就阻塞了，由於只有一個線程。

class Reactor implements Runnable { 
    final Selector selector;
    final ServerSocketChannel serverSocket;

    Reactor(int port) throws IOException { // Reactor 初始化
        selector = Selector.open();
        serverSocket = ServerSocketChannel.open();
        serverSocket.socket().bind(new InetSocketAddress(port));
        serverSocket.configureBlocking(false); // 非阻塞
        SelectionKey sk = serverSocket.register(selector, SelectionKey.OP_ACCEPT); // 分步處理,第一步,接收accept事件
        sk.attach(new Acceptor()); //attach callback object, Acceptor
    }
    
    public void run() { 
        try {
            while (!Thread.interrupted()) {
                selector.select();
                Set selected = selector.selectedKeys();
                Iterator it = selected.iterator();
                while (it.hasNext())
                    dispatch((SelectionKey)(it.next()); //Reactor負責dispatch收到的事件
                selected.clear();
            }
        } catch (IOException ex) { /* ... */ }
    }
    
    void dispatch(SelectionKey k) {
        Runnable r = (Runnable)(k.attachment()); //調用以前註冊的callback對象
        if (r != null)
            r.run();
    }
    
    class Acceptor implements Runnable { // inner
        public void run() {
            try {
                SocketChannel c = serverSocket.accept();
                if (c != null)
                new Handler(selector, c);
            }
            catch(IOException ex) { /* ... */ }
        }
    }
}

final class Handler implements Runnable {
    final SocketChannel socket;
    final SelectionKey sk;
    ByteBuffer input = ByteBuffer.allocate(MAXIN);
    ByteBuffer output = ByteBuffer.allocate(MAXOUT);
    static final int READING = 0, SENDING = 1;
    int state = READING;
    
    Handler(Selector sel, SocketChannel c) throws IOException {
        socket = c; c.configureBlocking(false);
        // Optionally try first read now
        sk = socket.register(sel, 0);
        sk.attach(this); //將Handler做爲callback對象
        sk.interestOps(SelectionKey.OP_READ); //第二步,接收Read事件
        sel.wakeup();
    }
    boolean inputIsComplete() { /* ... */ }
    boolean outputIsComplete() { /* ... */ }
    void process() { /* ... */ }
    
    public void run() {
        try {
            if (state == READING) read();
            else if (state == SENDING) send();
        } catch (IOException ex) { /* ... */ }
    }
    
    void read() throws IOException {
        socket.read(input);
        if (inputIsComplete()) {
            process();
            state = SENDING;
            // Normally also do first write now
            sk.interestOps(SelectionKey.OP_WRITE); //第三步,接收write事件
        }
    }
    void send() throws IOException {
        socket.write(output);
        if (outputIsComplete()) sk.cancel(); //write完就結束了, 關閉select key
    }
}

//上面 的實現用Handler來同時處理Read和Write事件, 因此裏面出現狀態判斷
//咱們能夠用State-Object pattern來更優雅的實現
class Handler { // ...
    public void run() { // initial state is reader
        socket.read(input);
        if (inputIsComplete()) {
            process();
            sk.attach(new Sender());  //狀態遷移, Read後變成write, 用Sender做爲新的callback對象
              sk.interest(SelectionKey.OP_WRITE);
            sk.selector().wakeup();
        }
    }
    class Sender implements Runnable {
        public void run(){ // ...
            socket.write(output);
            if (outputIsComplete()) sk.cancel();
        }
    }
}

單線程模式的侷限仍是比較明顯的。因此改進是將比較耗時的部分，從 reactor 線程中分離出去，讓 reactor 專門負責 IO，而另外建立 Thread Pool 和 queue 來緩存和處理任務。因此其實已經進化成 Proactor 模式，異步模式。

3、多線程(Worker Threads)

class Handler implements Runnable {
    // uses util.concurrent thread pool
    static PooledExecutor pool = new PooledExecutor(...);
    static final int PROCESSING = 3;
    // ...
    synchronized void read() { // ...
        socket.read(input);
        if (inputIsComplete()) {
            state = PROCESSING;
            pool.execute(new Processer()); //使用線程pool異步執行
        }
    }
    
    synchronized void processAndHandOff() {
        process();
        state = SENDING; // or rebind attachment
        sk.interest(SelectionKey.OP_WRITE); //process完,開始等待write事件
    }
    
    class Processer implements Runnable {
        public void run() { processAndHandOff(); }
    }
}

使用多個 reactor 進程，主 reactor 只負責 accept，而後將接收到的 socketchannel 交給 Thread Pool 去處理。

4、主從(Multiple Reactor Threads)

Selector[] selectors; // 一個 selector 表明一個 subReactor
int next = 0;
class Acceptor { // ...
    public synchronized void run() { ...
        Socket connection = serverSocket.accept(); // 主 selector 負責 accept
        if (connection != null)
            new Handler(selectors[next], connection); //選個 subReactor 去負責接收到的 connection
        if (++next == selectors.length) next = 0;
    }
}

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天天用心記錄一點點。內容也許不重要，但習慣很重要！