Scalable IO in Java

http://gee.cs.oswego.edu/dl/cpjslides/nio.pdf
Web services, Distributed Objects, etc, Most have same basic structure:
Read request
Decode request
Process service
Encode reply
Send reply

Scalability Goals
" Graceful degradation under increasing load (more clients)
" Continuous improvement with increasing resources (CPU, memory, disk, bandwidth)
" Also meet availability and performance goals
Short latencies
Meeting peak demand
Tunable quality of service
" Divide-and-conquer is usually the best approach for achieving any scalability goal

http://www.cnblogs.com/luxiaoxun/p/4331110.html

class Server implements Runnable {
    public void run() {
        try {
            ServerSocket ss = new ServerSocket(PORT);
            while (!Thread.interrupted())
            new Thread(new Handler(ss.accept())).start(); //创建新线程来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) { /* ... */ }
    }
}

对于每一个请求都分发给一个线程，每个线程中都独自处理上面的流程。

这种模型由于IO在阻塞时会一直等待，因此在用户负载增加时，性能下降的非常快。

server导致阻塞的原因：

1、serversocket的accept方法，阻塞等待client连接，直到client连接成功。

2、线程从socket inputstream读入数据，会进入阻塞状态，直到全部数据读完。

3、线程向socket outputstream写入数据，会阻塞直到全部数据写完。

client导致阻塞的原因：

1、client建立连接时会阻塞，直到连接成功。

2、线程从socket输入流读入数据，如果没有足够数据读完会进入阻塞状态，直到有数据或者读到输入流末尾。

3、线程从socket输出流写入数据，直到输出所有数据。

4、socket.setsolinger()设置socket的延迟时间，当socket关闭时，会进入阻塞状态，直到全部数据都发送完或者超时。

改进：采用基于事件驱动的设计，当有事件触发时，才会调用处理器进行数据处理。

Basic Reactor Design

Divide and Conquer
" Divide processing into small tasks
Each task performs an action without blocking
" Execute each task when it is enabled
Here, an IO event usually serves as trigger

Basic mechanisms supported in java.nio
Non-blocking reads and writes
Dispatch tasks associated with sensed IO events
" Endless variation possible
A family of event-driven designs

Event-driven Designs

Background: Events in AWT
AWT Event Queue

Reactor Pattern
" Reactor responds to IO events by dispatching
the appropriate handler
Similar to AWT thread
" Handlers perform non-blocking actions
Similar to AWT ActionListeners
" Manage by binding handlers to events
Similar to AWT addActionListener

java.nio Support
" Channels
Connections to files, sockets etc that support
non-blocking reads
" Buffers
Array-like objects that can be directly read or
written by Channels
" Selectors
Tell which of a set of Channels have IO events
" SelectionKeys
Maintain IO event status and bindings