But what is event-driven (asynchronous) architecture really? To simplify, let's talk about Asynchronous I/O. Asynchronous I/O is also known as Non-blocking I/O. It can be best describe by the reactor design pattern.
Wikipedia explains, the reactor design pattern is a concurrent programming pattern for handling service requests delivered concurrently to a service handler by one or more inputs. The service handler then demultiplexes the incoming requests and dispatches them synchronously to the associated request handlers. (The Reactor pattern is closely related to the Observer/Observable pattern in this aspect: all dependents are informed when a single subject changes. The Observer pattern is associated with a single source of events, however, whereas the Reactor pattern is associated with multiple sources of events.)
Reactor design pattern is easier to understand by examples and diagrams. In this illustration, I will be using the java nio.
Until JDK 1.4, the Java platform did not support nonblocking I/O calls. With an almost one-to-one ratio of threads to clients, servers written in the Java language were susceptible to enormous thread overhead, which resulted in both performance problems and lack of scalability.
By this time, you have the basic idea of how the reactor pattern works! The key component are the Selector, Channels (and buffers) and the handler. Let's investigate them one by one.
Channels and Buffers
Channels are like streams in the original I/O package. All data that goes anywhere (or comes from anywhere) must pass through a Channel object. A Buffer is a container object. All data that is sent to a channel must first be placed in a buffer; likewise, any data that is read from a channel is read into a buffer.
A Buffer is an object, which holds some data, that is to be written to or that has just been read from. The addition of the Buffer object in NIO marks one of the most significant differences between the new library and original I/O. In stream-oriented I/O, you wrote data directly to, and read data directly from, Stream objects.
In the NIO library, all data is handled with buffers. When data is read, it is read directly into a buffer. When data is written, it is written into a buffer. Anytime you access data in NIO, you are pulling it out of the buffer.
The most commonly used kind of buffer is the ByteBuffer. A ByteBuffer allows get/set operations (that is, the getting and setting of bytes) on its underlying byte array. (There are other buffers as well. CharBuffer ShortBuffer, IntBuffer, LongBuffer, FloatBuffer, and DoubleBuffer). *NOTE: StringBuffer was added in Java 5 and it's not even part of nio package.
Basic Example on reading data from a Channel.
//getting the channel. FileInputStream fin = new FileInputStream( "readandshow.txt" ); FileChannel fc = fin.getChannel(); //creating a buffer. ByteBuffer buffer = ByteBuffer.allocate( 1024 ); fc.read( buffer );
You'll notice that we didn't need to tell the channel how much to read into the buffer. Each buffer has a sophisticated internal accounting system that keeps track of how much data has been read and how much room there is for more data.
Writing to a file.
FileOutputStream fout = new FileOutputStream( "writesomebytes.txt" );
FileChannel fc = fout.getChannel();
//create a buffer, and put some data in it.
ByteBuffer buffer = ByteBuffer.allocate( 1024 );
for (int i=0; i<100; ++i) {
buffer.put( i );
}
//The flip() method
//prepares the buffer to have the
//newly-read data written to another channel
buffer.flip();
//write data of the buffer.
fc.write( buffer );
Selector The central object in asynchronous I/O is called the Selector. A Selector is where you register your interest in various I/O events, and it is the object that tells you when those events occur. Example: Selector selector = Selector.open(); //another way of getting selector instance. Selector selector = SelectorProvider.provider().openSelector();Handler The handler are your worker threads. They are responsible for the data that you read, and also for writing your data. You can pre-define a thread pool to handle all your request. There's nothing fancy about the handler. Check the link I provided below, for detailed example. Now that you are familiar with Selectors, Channels, buffers and Selector, we need to tie them together. But first, to accept connection from a client, you need a ServerSocketChannel. ServerSocketChannel is the nio version of ServerSocket that uses channeling and buffering methodology. Example:
// Create a new non-blocking server socket channel serverChannel = ServerSocketChannel.open(); serverChannel.configureBlocking(false); // Bind the server socket to the specified address and port // NOTE: HOST_ADDRESS is a type InetAddress and PORT is an int. InetSocketAddress isa = new InetSocketAddress(HOST_ADDRESS, PORT); serverChannel.socket().bind(isa); //registering the ServerSocketChannel to the selector. SelectionKey key = ssc.register( selector, SelectionKey.OP_ACCEPT );
The first argument to register() is always the Selector. The second argument, OP_ACCEPT, here specifies that we want to listen for accept events -- that is, the events that occur when a new connection is made. This is the only kind of event that is appropriate for a ServerSocketChannel.
Note the return value of the call to register(). A SelectionKey represents this registration of this channel with this Selector. When a Selector notifies you of an incoming event, it does this by supplying the SelectionKey that corresponds to that event. The SelectionKey can also be used to de-register the channel.
I'm sure by this time, you are ready to see a working sample code to implement this whole theory. I thought of writing an example, but this site did it very well. http://rox-xmlrpc.sourceforge.net/niotut/