Netty in Action - Chapter 3

Chapter 3 - Netty components and design

Channel - Sockets
EventLoop - Control flow, multithreading, concurrency
ChannelFuture - Asynchronous notification, a placeholder for the result of an operation that's to be executed in the future.

Interface EventLoop

1. An EventLoopGroup can contain multiple EventLoops
2. An EventLoop is bound to a single Thread for its lifetime
3. All I/O events processed by an EventLoop are handled on its dedicated Thread
4. A channel is registered for its lifetime with a single EventLoop
5. A single EventLoop may be assigned to one or more Channels

Interface ChannelHandler

Primary component of Netty which serves as the container for all application logic that applies to inbound and outbound data.

Interface ChannelPipeline

ChannelHandlers are installed in ChannelPipeline as follows:

1. The ChannelInitializer implementation is registered with a ServerBootstrap
2. When ChannelInitializer.initChannel() is called, it will install a custom set of ChannelHandlers in the pipeline
3. The ChannelInitializer removes itself from the ChannelPipeline

There are two ways of sending messages in Netty:

1. Write directly to the Channel, which will cause the message to start from the tail of the pipeline
2. Write to a ChannelHandlerContext, which will cause the message to start from the next handler in the pipeline

Encoder and decoder

When inbound message comes in, it will be decoded
When outbound message goes out, it will be encoded

The encoder/decoder adapter classes provided by Netty implement either ChannelInboundHandler or ChannelOutboundHandler
For each inbound message, it will call decode() and forward the decoded bytes to the next ChannelInboundHandler in the pipeline; An encoder converts the message to bytes and forwards them to the next ChannelOutboundHandler

Bootstrapping

bootstrapping a server: bind a process to a given port (ServerBootstrap)
bootstrapping a client: connect one process to another one running on a host at a specific port (Bootstrap)

A server needs two distinct set of Channels:

1. The first set will contain a single ServerChannel representing the server's own listening socket, bound to a local port.
2. The second set will contain all of the Channels that have been created to handle incoming client connections - one for each connection the server accepted.

This explains why two distinct EventLoopGroups are required

The first group will assign an EventLoop that is responsible for creating Channels for incoming connection requests. The second group will assign EventLoop to handle the Channel