netty-incubator-buffer-api/README.adoc

= Netty Incubator Buffer API

This repository is incubating a new buffer API proposed for Netty 5.

== Building and Testing

Short version: just run `make`.

The project currently relies on snapshot versions of the https://github.com/openjdk/panama-foreign[Panama Foreign] fork of OpenJDK.
This allows us to test out the most recent version of the `jdk.incubator.foreign` APIs, but also make building, and local development more involved.
To simplify things, we have a Docker based build, controlled via a Makefile with the following commands:

* `image` – build the docker image.This includes building a snapshot of OpenJDK, and download all relevant Maven dependencies.
* `test` – run all tests in a docker container.This implies `image`.The container is automatically deleted afterwards.
* `dbg` – drop into a shell in the build container, without running the build itself.The debugging container is not deleted afterwards.
* `clean` – remove the leftover containers created by `dbg`, `test`, and `build`.
* `build` – build binaries and run all tests in a container, and copy the `target` directory out of the container afterwards.This is the default build target.

== Example: Echo Client and Server

Making use of this new buffer API on the client side is quite easy.
Even though Netty 5 does not have native support for these buffers, it is able to convert them to the old `ByteBuf` API as needed.
This means we are able to send incubator buffers through a Netty pipeline, and have it work as if we were sending `ByteBuf` instances.

[source,java]
----
public final class Client {
    public static void main(String[] args) throws Exception {
        EventLoopGroup group = new MultithreadEventLoopGroup(NioHandler.newFactory());
        try (BufferAllocator allocator = BufferAllocator.pooledDirect()) { // <1>
            Bootstrap b = new Bootstrap();
            b.group(group)
             .channel(NioSocketChannel.class)
             .option(ChannelOption.TCP_NODELAY, true)
             .handler(new ChannelInitializer<SocketChannel>() {
                 @Override
                 public void initChannel(SocketChannel ch) throws Exception {
                     ch.pipeline().addLast(new ChannelHandlerAdapter() {
                         @Override
                         public void channelActive(ChannelHandlerContext ctx) {
                             Buffer message = allocator.allocate(256); // <2>
                             for (int i = 0; i < message.capacity(); i++) {
                                 message.writeByte((byte) i);
                             }
                             ctx.writeAndFlush(message); // <3>
                         }
                     });
                 }
             });

            // Start the client.
            ChannelFuture f = b.connect("127.0.0.1", 8007).sync();

            // Wait until the connection is closed.
            f.channel().closeFuture().sync();
        } finally {
            // Shut down the event loop to terminate all threads.
            group.shutdownGracefully();
        }
    }
}
----
<1> A life-cycled allocator is created to wrap the scope of our application.
<2> Buffers are allocated with one of the `allocate` methods.
<3> The buffer can then be sent down the pipeline, and will be written to the socket just like a `ByteBuf` would.

[NOTE]
--
The same is not the case for `BufferHolder`.
It is not treated the same as a `ByteBufHolder`.
--

On the server size, things are more complicated because Netty itself will be allocating the buffers, and the `ByteBufAllocator` API is only capable of returning `ByteBuf` instances.
The `ByteBufAllocatorAdaptor` will allocate `ByteBuf` instances that are backed by the new buffers.
The buffers can then we extracted from the `ByteBuf` instances with the `ByteBufAdaptor.extract` method.

We can tell a Netty server how to allocate buffers by setting the `ALLOCATOR` child-channel option:

[source,java]
----
ByteBufAllocatorAdaptor allocator = new ByteBufAllocatorAdaptor(); // <1>
ServerBootstrap server = new ServerBootstrap();
server.group(bossGroup, workerGroup)
      .channel(NioServerSocketChannel.class)
      .childOption(ChannelOption.ALLOCATOR, allocator) // <2>
      .handler(new EchoServerHandler());
----
<1> The `ByteBufAllocatorAdaptor` implements `ByteBufAllocator`, and directly allocates `ByteBuf` instances that are backed by buffers that use the new API.
<2> To make Netty use a given allocator when allocating buffers for receiving data, we set the allocator as a child option.

With the above, we just changed how the buffers are allocated, but we haven't changed the API we use for interacting with the buffers.
The buffers are still allocated at `ByteBuf` instances, and flow through the pipeline as such.
If we want to use the new buffer API in our server handlers, we have to extract the buffers from the `ByteBuf` instances that are passed down:

[source,java]
----
import io.netty.buffer.ByteBuf;
import io.netty.buffer.api.Buffer;
import io.netty.buffer.api.adaptor.ByteBufAdaptor;

@Sharable
public class EchoServerHandler implements ChannelHandler {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) { // <1>
        if (msg instanceof ByteBuf) { // <2>
            // For this example, we only echo back buffers that are using the new buffer API.
            Buffer buf = ByteBufAdaptor.extract((ByteBuf) msg); // <3>
            ctx.write(buf); // <4>
        }
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) {
        ctx.flush();
    }
}
----
<1> Netty pipelines are defined as transferring `Object` instances as messages.
<2> When we receive data directly from a socket, these messages will be `ByteBuf` instances with the received data.
<3> Since we set the allocator to create `ByteBuf` instances that are backed by buffers with the new API, we will be able to extract the backing `Buffer` instances.
<4> We can then operate on the extracted `Buffer` instances directly.
The `Buffer` and `ByteBuf` instances mirror each other exactly.
In this case, we just write them back to the client that sent the data to us.

The files in `src/test/java/io/netty/buffer/api/examples/echo` for the full source code to this example.