doug lea - scalable io in java

PPT: Scalable IO in Java

Author: Doug Lea


  • Scalable network services
  • Event-driven processing
  • Reactor pattern
    1. Basic version
    2. Multithreaded versions
    3. Other variants
  • Walkthrough of java.nio nonblocking IO APIs

Network Services

  • Web services, Distributed Objects, etc
  • Most have same basic structure:
    1. Read request
    2. Decode request
    3. Process service
    4. Encode reply
    5. Send reply
  • But differ in nature and cost of each step
    1. XML parsing, File transfer, Web page
    2. generation, computational services, ...

Classic Service Designs

Each handler may be started in its own thread

Classic ServerSocket Loop

// Note: most exception handling elided from code examplesclass Server implements Runnable {    public void run() {        try {            ServerSocket ss = new ServerSocket(PORT);            while (!Thread.interrupted())                new Thread(new Handler(ss.accept())).start();                // 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) { /* ... */ }    }}

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

  • Divide-and-conquer is usually the best approach for achieving any scalability goal
  • 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

  • Usually more efficient than alternatives
    • Fewer resources
  • Don't usually need a thread per client
    • Less overhead
  • Less context switching, often less locking
    • But dispatching can be slower
      • Must manually bind actions to events
  • Usually harder to program
    • Must break up into simple non-blocking actions
      • Similar to GUI event-driven actions
      • Cannot eliminate all blocking: GC, page faults, etc
    • Must keep track of logical state of service

Background: Events in AWT

Event-driven IO uses similar ideas but in different designs

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
  • See Schmidt et al, Pattern-Oriented Software Architecture, Volume 2 (POSA2)
    • Also Richard Stevens's networking books, Matt Welsh's SEDA framework, etc

Basic Reactor Design

Single threaded version

java.nio Support


  • Connections to files, sockets etc that support
  • non-blocking reads


  • Array-like objects that can be directly read or written by Channels


  • Tell which of a set of Channels have IO events


Maintain IO event status and bindings

Reactor 1: Setup

class Reactor implements Runnable {    final Selector selector;    final ServerSocketChannel serverSocket;    Reactor(int port) throws IOException {        selector =;        serverSocket =;        serverSocket.socket().bind(                new InetSocketAddress(port));        serverSocket.configureBlocking(false);        SelectionKey sk =                serverSocket.register(selector, SelectionKey.OP_ACCEPT);        sk.attach(new Acceptor());    }    /**     * Alternatively, use explicit SPI provider:     * SelectorProvider p = SelectorProvider.provider();     * selector = p.openSelector();     * serverSocket = p.openServerSocketChannel();     */

Reactor 2: Dispatch Loop

// class Reactor continuedpublic void run() { // normally in a new Thread    try {        while (!Thread.interrupted()) {  ;            Set selected = selector.selectedKeys();            Iterator it = selected.iterator();            while (it.hasNext())                dispatch((SelectionKey) (;            selected.clear();        }    } catch (IOException ex) { /* ... */ }}void dispatch(SelectionKey k) {    Runnable r = (Runnable) (k.attachment());    if (r != null);}

Reactor 3: Acceptor

    // class Reactor continued    class Acceptor implements Runnable { // inner        public void run() {            try {                SocketChannel c = serverSocket.accept();                if (c != null)                    new Handler(selector, c);            } catch (IOException ex) { /* ... */ }        }    }}

Reactor4: Handler setup

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);        sk.interestOps(SelectionKey.OP_READ);        sel.wakeup();    }    boolean inputIsComplete() { /* ... */ }    boolean outputIsComplete() { /* ... */ }    void process() { /* ... */ }

Reactor 5: Request handling

    // class Handler continued    public void run() {        try {            if            (state == READING) read();            else if (state == SENDING) send();        } catch (IOException ex) { /* ... */ }    }    void read() throws IOException {;        if (inputIsComplete()) {            process();            state = SENDING;            // Normally also do first write now            sk.interestOps(SelectionKey.OP_WRITE);        }    }    void send() throws IOException {        socket.write(output);        if (outputIsComplete()) sk.cancel();    }}

Per-State Handlers

  • A simple use of GoF State-Object pattern
    • Rebind appropriate handler as attachment
class Handler { // ...    public void run() { // initial state is reader;        if (inputIsComplete()) {            process();            sk.attach(new Sender());            sk.interest(SelectionKey.OP_WRITE);            sk.selector().wakeup();        }    }    class Sender implements Runnable {        public void run() { // ...            socket.write(output);            if (outputIsComplete()) sk.cancel();        }    }}

Multithreaded Designs

  • Strategically add threads for scalability
    • Mainly applicable to multiprocessors
  • Worker Threads
    • Reactors should quickly trigger handlers
      • Handler processing slows down Reactor
    • Offload non-IO processing to other threads
  • Multiple Reactor Threads
    • Reactor threads can saturate doing IO
    • Distribute load to other reactors
      • Load-balance to match CPU and IO rates

Worker Threads

  • Offload non-IO processing to speed up Reactor thread
    • Similar to POSA2 Proactor designs
  • Simpler than reworking compute-bound processing into event-driven form
    • Should still be pure nonblocking computation
      • Enough processing to outweigh overhead
  • But harder to overlap processing with IO
    • Best when can first read all input into a buffer
  • Use thread pool so can tune and control
    • Normally need many fewer threads than clientsWorker Thread Pools

Handler with Thread Pool

class Handler implements Runnable {    // uses util.concurrent thread pool    static PooledExecutor pool = new PooledExecutor(...);    static final int PROCESSING = 3;    // ...    synchronized void read() { // ...;        if (inputIsComplete()) {            state = PROCESSING;            pool.execute(new Processer());        }    }    synchronized void processAndHandOff() {        process();        state = SENDING; // or rebind attachment        sk.interest(SelectionKey.OP_WRITE);    }    class Processer implements Runnable {        public void run() {            processAndHandOff();        }    }}

Coordinating Tasks


  • Each task enables, triggers, or calls next one
  • Usually fastest but can be brittle

Callbacks to per-handler dispatcher

  • Sets state, attachment, etc
  • A variant of GoF Mediator pattern


  • For example, passing buffers across stages


  • When each task produces a result
  • Coordination layered on top of join or wait/notify

Using PooledExecutor

  • A tunable worker thread pool
  • Main method execute(Runnable r)
  • Controls for:
    • The kind of task queue (any Channel)
    • Maximum number of threads
    • Minimum number of threads
    • "Warm" versus on-demand threads
    • Keep-alive interval until idle threads die
      • to be later replaced by new ones if necessary
    • Saturation policy
      • block, drop, producer-runs, etc

Multiple Reactor Threads

  • Using Reactor Pools
    • Use to match CPU and IO rates
    • Static or dynamic construction
      • Each with own Selector, Thread, dispatch loop
    • Main acceptor distributes to other reactors
Selector[] selectors; // also create threadsint next = 0;class Acceptor { // ...    public synchronized void run() { ...        Socket connection = serverSocket.accept();        if (connection != null)            new Handler(selectors[next], connection);        if (++next == selectors.length) next = 0;    }}

Using Multiple Reactors

Using other java.nio features

  • Multiple Selectors per Reactor
    • To bind different handlers to different IO events
    • May need careful synchronization to coordinate
  • File transfer
    • Automated file-to-net or net-to-file copying
  • Memory-mapped files
    • Access files via buffers
  • Direct buffers
    • Can sometimes achieve zero-copy transfer
    • But have setup and finalization overhead
    • Best for applications with long-lived connections

Connection-Based Extensions

  • Instead of a single service request,
    • Client connects
    • Client sends a series of messages/requests
    • Client disconnects
  • Examples
    • Databases and Transaction monitors
    • Multi-participant games, chat, etc
  • Can extend basic network service patterns
    • Handle many relatively long-lived clients
    • Track client and session state (including drops)
    • Distribute services across multiple hosts

API Walkthrough

  1. Buffer
  2. ByteBuffer (CharBuffer, LongBuffer, etc not shown.)
  3. Channel
  4. SelectableChannel
  5. SocketChannel
  6. ServerSocketChannel
  7. FileChannel
  8. Selector
  9. SelectionKey


abstract class Buffer {        int capacity();        int position();     Buffer position(int newPosition);        int limit();     Buffer limit(int newLimit);     Buffer mark();     Buffer reset();     Buffer clear();     Buffer flip();     Buffer rewind();        int remaining();    boolean hasRemaining();    boolean isReadOnly();}


abstract class ByteBuffer extends Buffer {          static ByteBuffer allocateDirect(int capacity);          static ByteBuffer allocate(int capacity);          static ByteBuffer wrap(byte[] src, int offset, int len);          static ByteBuffer wrap(byte[] src);         boolean isDirect();       ByteOrder order();      ByteBuffer order(ByteOrder bo);      ByteBuffer slice();      ByteBuffer duplicate();      ByteBuffer compact();      ByteBuffer asReadOnlyBuffer();            byte get();            byte get(int index);      ByteBuffer get(byte[] dst, int offset, int length);      ByteBuffer get(byte[] dst);      ByteBuffer put(byte b);      ByteBuffer put(int index, byte b);      ByteBuffer put(byte[] src, int offset, int length);      ByteBuffer put(ByteBuffer src);      ByteBuffer put(byte[] src);            char getChar();            char getChar(int index);      ByteBuffer putChar(char value);      ByteBuffer putChar(int index, char value);      CharBuffer asCharBuffer();           short getShort();           short getShort(int index);      ByteBuffer putShort(short value);      ByteBuffer putShort(int index, short value);     ShortBuffer asShortBuffer();             int getInt();             int getInt(int index);      ByteBuffer putInt(int value);      ByteBuffer putInt(int index, int value);       IntBuffer asIntBuffer();            long getLong();            long getLong(int index);      ByteBuffer putLong(long value);      ByteBuffer putLong(int index, long value);      LongBuffer asLongBuffer();           float getFloat();           float getFloat(int index);      ByteBuffer putFloat(float value);      ByteBuffer putFloat(int index, float value);     FloatBuffer asFloatBuffer();          double getDouble();          double getDouble(int index);      ByteBuffer putDouble(double value);      ByteBuffer putDouble(int index, double value);    DoubleBuffer asDoubleBuffer();}


interface Channel {    boolean isOpen();       void close() throws IOException;}interface ReadableByteChannel extends Channel {    int read(ByteBuffer dst) throws IOException;}interface WritableByteChannel  extends Channel {    int write(ByteBuffer src) throws IOException;}interface ScatteringByteChannel extends ReadableByteChannel {    int read(ByteBuffer[] dsts, int offset, int length) throws IOException;    int read(ByteBuffer[] dsts) throws IOException;}interface GatheringByteChannel extends WritableByteChannel {    int write(ByteBuffer[] srcs, int offset, int length) throws IOException;    int write(ByteBuffer[] srcs) throws IOException;}


abstract class SelectableChannel implements Channel {             int validOps();         boolean isRegistered();    SelectionKey keyFor(Selector sel);    SelectionKey register(Selector sel, int ops) throws ClosedChannelException;    SelectionKey register(Selector sel, int ops, Object att) throws ClosedChannelException;            void configureBlocking(boolean block) throws IOException;         boolean isBlocking();          Object blockingLock();}


abstract class SocketChannel implements ByteChannel ... {     static SocketChannel open() throws IOException;     Socket socket();        int validOps();    boolean isConnected();    boolean isConnectionPending();    boolean isInputOpen();    boolean isOutputOpen();    boolean connect(SocketAddress remote) throws IOException;    boolean finishConnect() throws IOException;       void shutdownInput() throws IOException;       void shutdownOutput() throws IOException;        int read(ByteBuffer dst) throws IOException;        int read(ByteBuffer[] dsts, int offset, int length) throws IOException;        int read(ByteBuffer[] dsts) throws IOException;        int write(ByteBuffer src) throws IOException;        int write(ByteBuffer[] srcs, int offset, int length) throws IOException;        int write(ByteBuffer[] srcs) throws IOException;}


abstract class ServerSocketChannel extends ... {           static ServerSocketChannel open() throws IOException;              int validOps();     ServerSocket socket();    SocketChannel accept() throws IOException;}


abstract class FileChannel implements ... {                 int read(ByteBuffer dst);                 int read(ByteBuffer dst, long position);                 int read(ByteBuffer[] dsts, int offset, int length);                 int read(ByteBuffer[] dsts);                 int write(ByteBuffer src);                 int write(ByteBuffer src, long position);                 int write(ByteBuffer[] srcs, int offset, int length);                 int write(ByteBuffer[] srcs);                long position();                void position(long newPosition);                long size();                void truncate(long size);                void force(boolean flushMetaDataToo);                 int transferTo(long position, int count, WritableByteChannel dst);                 int transferFrom(ReadableByteChannel src, long position, int count);            FileLock lock(long position, long size, boolean shared);            FileLock lock();            FileLock tryLock(long pos, long size, boolean shared);            FileLock tryLock();              static final int MAP_RO, MAP_RW,  MAP_COW;    MappedByteBuffer map(int mode, long position, int size);}

NOTE: ALL methods throw IOException


abstract class Selector {    static Selector open() throws IOException;       Set keys();       Set selectedKeys();       int selectNow() throws IOException;       int select(long timeout) throws IOException;       int select() throws IOException;      void wakeup();      void close() throws IOException;}


abstract class SelectionKey {    static final int  OP_READ,    OP_WRITE,                      OP_CONNECT, OP_ACCEPT;    SelectableChannel channel();             Selector selector();              boolean isValid();                 void cancel();                  int interestOps();                 void interestOps(int ops);                  int readyOps();              boolean isReadable();              boolean isWritable();              boolean isConnectable();              boolean isAcceptable();               Object attach(Object ob);               Object attachment();}


  1. Scalable IO in Java