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## Chapter 2. Architectural Overview

In this chapter, we will examine what core functionalities are provided in Netty and how they constitute a complete network application development stack on top of the core. Please keep this diagram in mind as you read this chapter.

## 2.1. Rich Buffer Data Structure

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:

- You can define your buffer type if necessary.

- Transparent zero copy is achieved by built-in composite buffer type.

- A dynamic buffer type is provided out-of-the-box, whose capacity is expanded on demand, just like `StringBuffer`.

- There's no need to call `flip()` anymore.

- It is often faster than `ByteBuffer`.

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

## 2.2. Universal Asynchronous I/O API

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:

- NIO-based TCP/IP transport (See `org.jboss.netty.channel.socket.nio`),

- OIO-based TCP/IP transport (See `org.jboss.netty.channel.socket.oio`),

- OIO-based UDP/IP transport, and

- Local transport (See `org.jboss.netty.channel.local`).

Switching from one transport to 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.

## 2.3. Event Model based on the Interceptor Chain Pattern

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 `ChannelHandler`s 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` and `ChannelPipeline`.

## 2.4. Advanced Components for More Rapid Development

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.

### 2.4.1. Codec framework

As demonstrated in Section 1.8, “ Speaking in POJO instead of ChannelBuffer ”, it is always a good idea to separate a protocol codec from a business logic. However, there are some complications when implementing this idea from scratch. You have to deal with the fragmentation of messages. Some protocols are multi-layered (i.e. built on top of other lower level protocol). Some are too complicated to be implemented in a single state machine.

Consequently, a good network application framework should provide an extensible, reusable, unit-testable, and multi-layered codec framework that generates maintainable user codec.

Netty provides a number of basic and advanced codecs built on top of its core to address most issues you will encounter when you write a protocol codec regardless if it is simple or not, binary or text - simply whatever.

### 2.4.2. SSL / TLS Support

Unlike old blocking I/O, it is a non-trivial task to support SSL in NIO. You can't simply wrap a stream to encrypt or decrypt data but you have to use `javax.net.ssl.SSLEngine`. `SSLEngine` is a state machine which is as complex as SSL is. You have to manage all possible states such as cipher suite and encryption key negotiation (or re-negotiation), certificate exchange and validation. Moreover, `SSLEngine` is not even completely thread-safe unlike usual expectation.

In Netty, `SslHandler` takes care of all the gory details and pitfalls of `SSLEngine`. All you need to do is to configure and insert the `SslHandler` to your `ChannelPipeline`. It also allows you to implement advanced features like StartTLS very easily.

### 2.4.3. HTTP Implementation

HTTP is definitely the most popular protocol in the Internet. There are already a number of HTTP implementations such as a Servlet container. Then why does Netty have HTTP on top of its core?

Netty's HTTP support is very different from the existing HTTP libraries. It gives you complete control over how HTTP messages are exchanged in a low level. Because it is basically the combination of HTTP codec and HTTP message classes, there is no restriction such as enforced thread model. That is, you can write your own HTTP client or server that works exactly the way you want. You have full control over thread model, connection life cycle, chunked encoding, and as much as what HTTP specification allows you to do.

Thanks to its highly customizable nature, you can write a very efficient HTTP server such as:

- Chat server that requires persistent connections and server push technology (e.g. Comet and WebSockets)

- Media streaming server that needs to keep the connection open until the whole media is streamed (e.g. 2 hours of movie)

- File server that allows the upload of large files without memory pressure (e.g. uploading 1GB per request)

- Scalable mash-up client that connects to tens of thousand 3rd party web services asynchronously

### 2.4.4. Google Protocol Buffer Integration

Google Protocol Buffers are an ideal solution for the rapid implementation of a highly efficient binary protocol that evolves over time. With `ProtobufEncoder` and `ProtobufDecoder`, you can turn the message classes generated by Google Protocol Buffers Compiler (protoc) into Netty codec. Please take a look into the 'LocalTime' example that shows how easily you can create a high-performing binary protocol client and server from the sample protocol definition.

## 2.5. Summary

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.

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