netty Architectural Overview --reference

Netty uses its own buffer API instead of NIO ByteBuffer to represent a sequence of bytes. This approach has significant advantage over using ByteBuffer. Netty‘s new buffer type, ChannelBuffer has been designed from ground up to address the problems of ByteBuffer and to meet the daily needs of network application developers. To list a few cool features:

For more information, please refer to the org.jboss.netty.buffer package description.

Traditional I/O APIs in Java provided different types and methods for different transport types. For example,java.net.Socket and java.net.DatagramSocket do not have any common super type and therefore they have very different ways to perform socket I/O.

This mismatch makes porting a network application from one transport to the other tedious and difficult. The lack of portability between transports becomes a problem when you need to support more transports not rewriting the network layer of the application. Logically, many protocols can run on more than one transport such as TCP/IP, UDP/IP, SCTP, and serial port communication.

To make the matter worse, Java New I/O (NIO) API introduced the incompatibility with the old blocking I/O (OIO) API, and so will NIO.2 (AIO). Because all these APIs are different from each other in design and performance characteristics, you are often forced to determine which API your application will depend on before you even begin the implementation phase.

For instance, you might want to start with OIO because the number of clients you are going to serve will be very small and writing a socket server using OIO is much easier than using NIO. However, you are going to be in trouble when your business grows up exponentially and your server starts to serve tens of thousand clients simultaneously. You could start with NIO, but it might take much longer time to implement due to the complexity of the NIO Selector API, hindering rapid development.

Netty has a universal asynchronous I/O interface called Channel, which abstracts away all operations required to point-to-point communication. That is, once you wrote your application on one Netty transport, your application can run on other Netty transports. Netty provides a number of essential transports via one universal API:

Switching from one transport from the other usually takes just a couple lines of changes such as choosing a different ChannelFactory implementation.

Also, you are even able to take advantage of a new transport which is not written yet, serial port communication transport for instance, again by replacing just a couple lines of constructor calls. Moreover, you can write your own transport by extending the core API because it is highly extensible.

Well-defined and extensible event model is a must for an event-driven application. Netty does have a well-defined event model focused on I/O. It also allows you to implement your own event type without breaking the existing code at all because each event type is distinguished from each other by strict type hierarchy. This is another differentiator against other frameworks. Many NIO frameworks have no or very limited notion of event model; they often break the existing code when you try to add a new custom event type, or just do not allow extension.

A ChannelEvent is handled by a list of ChannelHandlers in a ChannelPipeline. The pipeline implements an advanced form of the Intercepting Filter pattern to give a user full control over how an event is handled and how the handlers in the pipeline interact with each other. For example, you can define what to do when a data is read from a socket:

public class MyReadHandler implements SimpleChannelHandler {
    public void messageReceived(ChannelHandlerContext ctx, MessageEvent evt) {
        Object message = evt.getMessage();
        // Do something with the received message.
        ...

        // And forward the event to the next handler.
        ctx.sendUpstream(evt);
    }
}

You can also define what to do when other handler requested a write operation:

public class MyWriteHandler implements SimpleChannelHandler {
    public void writeRequested(ChannelHandlerContext ctx, MessageEvent evt) {
        Object message = evt.getMessage();
        // Do something with the message to be written.
        ...

        // And forward the event to the next handler.
        ctx.sendDownstream(evt);
    }
}

For more information about the event model, please refer to the API documentation of ChannelEvent andChannelPipeline.

On top of the core components mentioned above, that already enable the implementation of all types of network applications, Netty provides a set of advanced features to accelerate the development pace even more.

In this chapter, we reviewed the overall architecture of Netty from the feature-wise standpoint. Netty has simple yet powerful architecture. It is composed of three components - buffer, channel, and event model - and all advanced features are built on top of the three core components. Once you understood how these three work together, it should not be difficult to understand more advanced features which were covered briefly in this chapter.

You might still have an unanswered question about what the overall architecture looks exactly like and how each feature work together. If so, it is a good idea to talk to us to improve this guide.