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Reactor Pattern Explained

发布时间：2020-12-15 04:49:17 所属栏目：百科 来源：网络整理
导读：Handling concurrent events a Server receives is often thought of as a use-case for creating a separate thread for each IO event listener. Most programmers are tempted to use the famous socket loop for creating Sockets for every incoming co
Handling concurrent events a Server receives is often thought of as a use-case for creating a separate thread for each IO event listener. Most programmers are tempted to use the famous socket loop for creating Sockets for every incoming connection.
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class Server implements Runnable {
public void run()
{
try {
ServerSocket ss = new ServerSocket(PORT);
while (!Thread.interrupted())
Thread( Handler(ss.accept())).start();
// or,single-threaded,or a thread pool
} catch (IOException ex) { }
}
}
Handler Runnable {
final Socket socket;
Handler(Socket s) { socket = s; }
run() {
{
byte[] input = new byte[MAX_INPUT];
socket.getInputStream().read(input);
[] output = process(input);
socket.getOutputStream().write(output);
(IOException ex) { }
}
private [] process([] cmd) { }
}
The disadvantage of using a separate thread for each event listener isthe overhead of context switching. In the worst case,some threads handling event listeners which do not read or write data frequently,will be context switched periodically without doing useful work. Every time such a Thread is dispatched to the CPU by the scheduler,it will be blocked until an IO event occurs,in which case all the time spent waiting for an IO event will be wasted. Note thatss.accept()is a blocking call which blocks the server thread till a client connects. The server thread will not be able to callstart()method of the new Handler thread until it is returned from. To reduce the wastage of CPU time by unnecessary context switches,the concept of non blocking IO was invented.
Reactor Pattern is an event handling design pattern used to address this issue. Here,one Reactor will keep looking for events and will inform the corresponding event handler to handle it once the event gets triggered. To explain this I am using some Java code borrowed from some lecture slides byProfessor Doug Lea. To see his explanation please go throughthisset of slides.
Java provides a standard API (java.nio) which could be used to design non-blocking IO systems. I will explain the Reactor pattern with a simple client server model where the clients will shout out their names to the server while the server will respond to the corresponding client with a Hello message.
There are two important participants in the architecture of Reactor Pattern.

1. Reactor


AReactorruns in a separate thread and its job is to react to IO events by dispatching the work to the appropriate handler. Its like a telephone operator in a company who answers the calls from clients and transfers the communication line to the appropriate receiver. Don't go too far with the analogy though :).
2. Handlers Handlerperforms the actual work to be done with an IO event similar to the actual officer in the company the client who called wants to speak to.
Since we are using java.nio package,its important to understand some of the classes used to implement the system. I will simply repeat some of the explanations by Doug Lea in his lecture sides to make the readers lives easy :).
Channels These are connections to files,sockets etc. that support non blocking reads. Just like many TV channels can be watched from one physical connection to the antena,manyjava.nio.channels.SocketChannels corresponding to each client can be made from a singlejava.nio.channels.ServerSocketChannelwhich is bound to a single port.
Buffers Array-like objects that can be directly read or written to by Channels.
Selectors Selectors tell which of a set of Channels has IO events.
Selection Keys Selection Keys maintain IO event status and bindings. Its a representation of the relationship between a Selector and a Channel. By looking at the Selection Key given by the Selector,the Reactor can decide what to do with the IO event which occurs on the Channel.
Now lets try to understand what Reactor Pattern is. Take a look at this diagram.

Here,there is a singleServerSocketChannelwhich is registered with aSelector. TheSelectionKey 0for this registration has information on what to do with theif it gets an event. Obviously the ServerSocketChannel should receive events from incoming connection requests from clients. When a client requests for a connection and wants to have a dedicated SocketChannel,the ServerSocketChannel should get triggered with an IO event. What does thehave to do with this event? It simply has to Accept it to make a SocketChannel. Thereforewill be bound to anAcceptorwhich is a special handler made to accept connections so that the Reactor can figure out that the event should be dispatched to the Acceptor by looking at SelectionKey 0. Notice that,andare all in same colour ( Gray I suppose :) )
Theis made to keep looking for IO events. When the Reactor callsSelector.select()method,the Selector will provide a set ofSelectionKeysfor the channels which have pending events. Whenis selected,it means that an event has occurred on ServerSocketChannel. So the Reactor will dispatch the event to the.
When the Acceptor accepts the connection fromClient 1SocketChannel1for the client. This SocketChannel will be registered with the same Selector withSelectionKey 1. What would the client do with this SocketChannel? It will simply read from and write to the server. The server does not need to accept connections from client 1 any more since it already accepted the connection. Now what the server needs is to Read and Write data to the channel. So SelectionKey 1 will be bound toHandler 1object which handles reading and writing. Notice thatSocketChannel 1are all in Green.
The next time the Reactor callesSelectionKey Sethasin it,it means thatis triggered with an event. Now by looking at SelectionKey 1,the Reactor knows that it has to dispatch the event tosince Hander 1 is bound to SelectionKey 1. If the returned SelectionKey Set hashas received an event from another client and by looking at the SelectionKey 0 the Reactor knows that it has to dispatch the event to theagain. When the event is dispatched to the Acceptor it will makeSocketChannel 2forclient 2and register the socket channel with the Selector withSelectionKey 2So in this scenario we are interested in 3 types of events.
  1. Connection request eventswhich get triggered on theServerSocketChannelwhich we need toAccept.
  2. Read eventswhich get triggerd onSocketChannels when they have data to be read,from which we need toRead.
  3. Write eventswhich get triggered onSocketChannels when they are ready to be written with data,to which we need toWrite.
A SelectionKey will have all the information about the relationship with its corresponding Channel and the Selector. It will have information about the corresponding Handler too. Selector will just select the SelectionKeys which have pending IO events. This way the Reactor can decide how to deal with the IO events accordingly. The relationships among the Channels,Selection Keys and Handlers can be put in a table as follows.
Selection Key Channel Handler Interested Operation
SelectionKey 0 ServerSocketChannel Acceptor Accept
SelectionKey 1 SocketChannel 1 Handler 1 Read and Write
SelectionKey 2 SocketChannel 2 Handler 2 Read and Write
SelectionKey 3 SocketChannel 3 Handler 3 Read and Write
Now what does a Thread pool has to do with this? Let me explain. The beauty of non blocking architecture is that we can write the server to run in a single Thread while catering all the requests from clients. Just forget about the Thread pool for a while. Naturally when concurrency is not used to design a server it should obviously be less responsive to events. In this scenario when the system runs in a single Thread the Reactor will not respond to other events until the Handler to which the event is dispatched is done with the event. Why? Because we are using one Thread to handle all the events. We naturally have to go one by one.
We can add concurrency to our design to make the system more responsive and faster. When the Reactor dispatches the event to a Handler,it can start the Handler in a new Thread so that the Reactor can happily continue to deal with other events. This will always be a better design when performance is concerned. To limit the number of Threads in the system and to make things more organized,a Thread pool can be used.

I believe this explanation is adequate for us to get our hands dirty with some coding.


In this blog post I will explain the implementation of Reactor Pattern with a simple Client - Server system where the server will send Hello messages to each client when their names are told to the server. The server will listen to port9900and multiple clients will connect to the server to shout out their names. A thread pool will not be used here. First lets run the server in a single thread. Part 3 of this series will explain how a Thread pool is used.
First lets make the Client to connect to port

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public class Client {
String hostIp;
int hostPort;
public Client(String hostIp, hostPort) {
this .hostIp = hostIp;
.hostPort = hostPort;
}
void runClient() throws IOException {
Socket clientSocket = null ;
PrintWriter out = ;
BufferedReader in = ;
try {
clientSocket = new Socket(hostIp,hostPort);
out = PrintWriter(clientSocket.getOutputStream(),monospace!important; font-size:1em!important; min-height:auto!important">true );
in = BufferedReader( InputStreamReader(clientSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println( "Unknown host: " + hostIp);
System.exit( 1 );
(IOException e) {
"Couldn't connect to: " + hostIp);
);
}
BufferedReader stdIn = InputStreamReader(System.in));
String userInput;
System.out.println( "Client connected to host : " + hostIp + " port: " + hostPort);
"Type ("Bye" to quit)" );
"Tell what your name is to the Server....." );
while ((userInput = stdIn.readLine()) != ) {
out.println(userInput);
// Break when client says Bye.
if (userInput.equalsIgnoreCase( "Bye" ))
break ;
"Server says: " + in.readLine());
}
out.close();
in.close();
stdIn.close();
clientSocket.close();
}
static main(String[] args) IOException {
Client client = Client( "127.0.0.1" , 9900 );
client.runClient();
}
}

Notice that the client doesn't use java.nio to create the Socket. It simply uses a java.net.Socket everybody knows about.

Now lets make the Reactor in the Server.
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Reactor implements Runnable {
final Selector selector;
ServerSocketChannel serverSocketChannel;
final boolean isWithThreadPool;
Reactor( port,monospace!important; font-size:1em!important; min-height:auto!important">boolean isWithThreadPool) IOException {
.isWithThreadPool = isWithThreadPool;
selector = Selector.open();
serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.socket().bind( InetSocketAddress(port));
serverSocketChannel.configureBlocking( false );
SelectionKey selectionKey0 = serverSocketChannel.register(selector,SelectionKey.OP_ACCEPT);
selectionKey0.attach( Acceptor());
}
run() {
"Server listening to port: " + serverSocketChannel.socket().getLocalPort());
{
(!Thread.interrupted()) {
selector.select();
Set selected = selector.selectedKeys();
Iterator it = selected.iterator();
(it.hasNext()) {
dispatch((SelectionKey) (it.next()));
}
selected.clear();
}
(IOException ex) {
ex.printStackTrace();
}
}
void dispatch(SelectionKey k) {
Runnable r = (Runnable) (k.attachment());
(r != ) {
r.run();
}
}
class Acceptor Runnable {
run() {
{
SocketChannel socketChannel = serverSocketChannel.accept();
(socketChannel != ) {
(isWithThreadPool)
HandlerWithThreadPool(selector,socketChannel);
else
Handler(selector,socketChannel);
}
"Connection Accepted by Reactor" (IOException ex) {
ex.printStackTrace();
}
}
}
The Reactor is a Runnable . See the while loop in the run() method. It will call selector.select() to get the SelectionKeys which have pending IO events. When the SelectionKeys are selected,they will be dispatched one by one. See the dispatch() method. The SelectionKey will have an attatchment which is also a Runnable. This attatchement will either be an Acceptor or a Handler .
Notice how the Acceptor inner class in the Reactor accepts connections to make SocketChannels . When a SocketChannel is created a new Handler will be created as well. (HandlerWithThreadPool will be discussed in the next section)
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Handler SocketChannel socketChannel;
SelectionKey selectionKey;
ByteBuffer input = ByteBuffer.allocate( 1024 );
static final int READING = 0 ;
state = READING;
String clientName = "" ;
Handler(Selector selector,SocketChannel c) socketChannel = c;
c.configureBlocking( );
selectionKey = socketChannel.register(selector,monospace!important; font-size:1em!important; min-height:auto!important">selectionKey.attach( );
selectionKey.interestOps(SelectionKey.OP_READ);
selector.wakeup();
}
run() {
(state == READING) {
read();
else if (state == SENDING) {
send();
}
(IOException ex) {
ex.printStackTrace();
}
}
read() IOException {
readCount = socketChannel.read(input);
(readCount > ) {
readProcess(readCount);
}
state = SENDING;
// Interested in writing
selectionKey.interestOps(SelectionKey.OP_WRITE);
}
/**
* Processing of the read message. This only prints the message to stdOut.
*
* @param readCount
*/
synchronized readProcess( readCount) {
StringBuilder sb = StringBuilder();
input.flip();
byte [] subStringBytes = new byte [readCount];
[] array = input.array();
System.arraycopy(array,subStringBytes,readCount);
// Assuming ASCII (bad assumption but simplifies the example)
sb.append( String(subStringBytes));
input.clear();
clientName = sb.toString().trim();
}
send() IOException {
"Saying hello to " + clientName);
ByteBuffer output = ByteBuffer.wrap(( "Hello " + clientName + "n" ).getBytes());
socketChannel.write(output);
selectionKey.interestOps(SelectionKey.OP_READ);
state = READING;
}
A Handler has 2 states, READING and SENDING . Both cant be handled at the same time because a Channel supports only one operation at one time. Since its the client who speaks first,a server Handler will start with the READING state. Notice how this Handler is attatched to the SelectionKey and how the Interested Operation is set to OP_READ . This means that the Selector should only select this SelectionKey when a Read Event occurs. Once the read process is done,the Handler will change its state to and will change the to OP_WRITE . Now the Selector will select this SelectionKey only when it gets a Write Event from the Channel when its ready to be written with data. When a is dispatched to this Handler,it will write the Hello message to the output buffer since now the state is . Once sending is done,it will change back to state with changed to again. It should be obvious that since both are Runnables method of the Reactor can execute the method of any attatchment it gets from a selected SelectionKey.
Here is the main method. We will run it without a Thread pool for the moment.
5
IOException{
Reactor reactor = Reactor( );
Thread(reactor).start();
To see how this works first run the server. Then run several clients and see how they get connected to the server. When each client writes a name to standard in of the client,the sever will respond to the client with a Hello message. Notice that the server runs in a single Thread but responds to any number of clients which connect to the server.

In this post the usage of Thread pools in Handlers is explained. We will create an extended version of Handler class named HandlerWithThreadPool. Check this out.

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class HandlerWithThreadPool extends Handler {
static ExecutorService pool = Executors.newFixedThreadPool(2);
static final int PROCESSING = ;
public HandlerWithThreadPool(Selector sel,SocketChannel c) throws IOException {
super(sel,c);
}
void read() IOException {
int readCount = socketChannel.read(input);
if (readCount > 0) {
state = PROCESSING;
pool.execute( Processer(readCount));
}
//We are interested in writing back to the client soon after read processing is done.
selectionKey.interestOps(SelectionKey.OP_WRITE);
}
//Start processing in a new Processer Thread and Hand off to the reactor thread.
synchronized processAndHandOff( readCount) {
readProcess(readCount);
//Read processing done. Now the server is ready to send a message to the client.
state = SENDING;
}
Processer Runnable {
readCount;
Processer( readCount) {
this.readCount = readCount;
}
run() {
processAndHandOff(readCount);
}
}
Notice that there is a new statePROCESSINGintroduced and that theread()method is over-ridden. Now when aRead EventSENDING. It will create aProcesserwhich will process the message and run it in a different Thread in the Thread pool and set theInterested OperationtoOP_WRITE. At this point even if theChannelis ready to be written to and the Hander is interested in writing,the Handler will not write since its still instate. See therun()method of the Handler,it will only write when its instate. Once the Processer is done with its read process,it will change the state to. Now the Handler can send data to the client.
Lets run the Reactor with the booleanisWithThreadPoolset to true.
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static main(String[] args) IOException{
Reactor reactor = Reactor(9900true Thread(reactor).start();
Notice that the size of the Thread pool is 2 which would limit the number of Handlers to run concurrently to 2. As we already know,when the events are handled concurrenly we can easyly improve the system performance.
Done :). Hope you find this useful. Have fun with coding.
Cheers.

（编辑：李大同）

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