Why aren't the Netty HTTP handlers sharable?

Question

Netty instantiates a set of request handler classes whenever a new connection is opened. This seems fine for something like a websocket where the connection will stay open for t

Answer 1

TL;DR:

If you get to the volume needed to make GC a problem with the default HTTP handlers it is time for scaling with a proxy server anyway.

---

After Norman's answer I ended up attempting a very bare bones sharable HTTP codec/aggregator POC to see if this was something to pursue or not.

My sharable decoder was a long ways from RFC 7230 but it gave me enough of the request for my current project.

I then used httperf and visualvm to get a concept of the GC load difference. For my efforts I only had a 10% decrease in the GC rate. In other words, it really doesn't make much of a difference.

The only real appreciated effect was that I had 5% less errors when running 1000 req/sec compared to using the packaged un-shared HTTP codec + aggregator versus my sharable one. And this only occurred when I was doing 1000 req/sec sustained for longer than 10 seconds.

In the end I'm not going to pursue it. The amount of time needed to make this into a fully HTTP compliant decoder for the tiny benefit that can be solved by using a proxy server is not worth the time at all.

For reference purposes here is the combined sharable decoder/aggregator that I tried:

import java.util.concurrent.ConcurrentHashMap;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelId;
import io.netty.channel.ChannelInboundHandlerAdapter;

@Sharable
public class SharableHttpDecoder extends ChannelInboundHandlerAdapter {

    private static final ConcurrentHashMap MAP = 
            new ConcurrentHashMap();

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) 
        throws Exception 
    {        
        if (msg instanceof ByteBuf) 
        {
            ByteBuf buf = (ByteBuf) msg;
            ChannelId channelId = ctx.channel().id();
            SharableHttpRequest request = MAP.get(channelId);

            if (request == null)
            {
                request = new SharableHttpRequest(buf);
                buf.release();
                if (request.isComplete()) 
                {
                    ctx.fireChannelRead(request);
                }
                else
                {
                    MAP.put(channelId, request);
                }
            }
            else
            {
                request.append(buf);
                buf.release();
                if (request.isComplete()) 
                {
                    ctx.fireChannelRead(request);
                }
            }
        }
        else
        {
            // TODO send 501
            System.out.println("WTF is this? " + msg.getClass().getName());
            ctx.fireChannelRead(msg);
        }
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) 
        throws Exception 
    {
        System.out.println("Unable to handle request on channel: " + 
            ctx.channel().id().asLongText());
        cause.printStackTrace(System.err);

        // TODO send 500
        ctx.fireExceptionCaught(cause);
        ctx.close();
    }

}

The resultant object created by the decoder for handling on the pipeline:

import java.util.Arrays;
import java.util.HashMap;
import io.netty.buffer.ByteBuf;

public class SharableHttpRequest
{

    private static final byte SPACE = 32;
    private static final byte COLON = 58;
    private static final byte CARRAIGE_RETURN = 13;

    private HashMap myHeaders;
    private Method myMethod;
    private String myPath;
    private byte[] myBody;
    private int myIndex = 0;

    public SharableHttpRequest(ByteBuf buf)
    {
        try
        {
            myHeaders = new HashMap();
            final StringBuilder builder = new StringBuilder(8);
            parseRequestLine(buf, builder);
            while (parseNextHeader(buf, builder));
            parseBody(buf);
        }
        catch (Exception e)
        {
            e.printStackTrace(System.err);
        }
    }

    public String getHeader(Header name)
    {
        return myHeaders.get(name);
    }

    public Method getMethod()
    {
        return myMethod;
    }

    public String getPath()
    {
        return myPath;
    }

    public byte[] getBody()
    {
        return myBody;
    }

    public boolean isComplete()
    {
        return myIndex >= myBody.length;
    }

    public void append(ByteBuf buf)
    {
        int length = buf.readableBytes();
        buf.getBytes(buf.readerIndex(), myBody, myIndex, length);
        myIndex += length;
    }

    private void parseRequestLine(ByteBuf buf, StringBuilder builder)
    {
        int idx = buf.readerIndex();
        int end = buf.writerIndex();
        for (; idx < end; ++idx)
        {
            byte next = buf.getByte(idx);

            // break on CR
            if (next == CARRAIGE_RETURN)
            {
                break;
            }

            // we need the method
            else if (myMethod == null)
            {
                if (next == SPACE)
                {
                    myMethod = Method.fromBuilder(builder);
                    builder.delete(0, builder.length());
                    builder.ensureCapacity(100);
                }
                else
                {
                    builder.append((char) next);
                }
            }

            // we need the path
            else if (myPath == null)
            {
                if (next == SPACE)
                {
                    myPath = builder.toString();
                    builder.delete(0, builder.length());
                }
                else
                {
                    builder.append((char) next);
                }
            }

            // don't need the version right now
        }
        idx += 2; // skip line endings
        buf.readerIndex(idx);
    }

    private boolean parseNextHeader(ByteBuf buf, StringBuilder builder)
    {
        Header header = null;
        int idx = buf.readerIndex();
        int end = buf.writerIndex();
        for (; idx < end; ++idx)
        {
            byte next = buf.getByte(idx);

            // break on CR
            if (next == CARRAIGE_RETURN)
            {
                if (header != Header.UNHANDLED)
                {
                    myHeaders.put(header,builder.toString());
                    builder.delete(0, builder.length());
                }
                break;
            }

            else if (header == null)
            {
                // we have the full header name
                if (next == COLON)
                {
                    header = Header.fromBuilder(builder);
                    builder.delete(0, builder.length());
                }

                // get header name as lower case for mapping purposes
                else
                {
                    builder.append(next > 64 && next < 91 ? 
                        (char) ( next | 32 ) : (char) next);
                }
            }

            // we don't care about some headers
            else if (header == Header.UNHANDLED)
            {
                continue;
            }

            // skip initial spaces
            else if (builder.length() == 0 && next == SPACE)
            {
                continue;
            }

            // get the header value
            else
            {
                builder.append((char) next);
            }
        }

        idx += 2; // skip line endings
        buf.readerIndex(idx);

        if (buf.getByte(idx) == CARRAIGE_RETURN)
        {
            idx += 2; // skip line endings
            buf.readerIndex(idx);
            return false;
        }
        else
        {
            return true;
        }
    }

    private void parseBody(ByteBuf buf)
    {
        int length = buf.readableBytes();
        if (length == 0)
        {
            myBody = new byte[0];
            myIndex = 1;
        }
        else
        {
            System.out.println("Content-Length: " + myHeaders.get(Header.CONTENT_LENGTH));
            if (myHeaders.get(Header.CONTENT_LENGTH) != null)
            {
                int totalLength = Integer.valueOf(myHeaders.get(Header.CONTENT_LENGTH));
                myBody = new byte[totalLength];
                buf.getBytes(buf.readerIndex(), myBody, myIndex, length);
                myIndex += length;
            }

            // TODO handle chunked
        }
    }




    public enum Method
    {
        GET(new char[]{71, 69, 84}), 
        POST(new char[]{80, 79, 83, 84}),
        UNHANDLED(new char[]{}); // could be expanded if needed

        private char[] chars;

        Method(char[] chars) 
        {
            this.chars = chars;
        }

        public static Method fromBuilder(StringBuilder builder) 
        {
            for (Method method : Method.values()) 
            {
                if (method.chars.length == builder.length()) 
                {
                    boolean match = true;
                    for (int i = 0; i < builder.length(); i++) 
                    {
                        if (method.chars[i] != builder.charAt(i)) 
                        {
                            match = false;
                            break;
                        }
                    }

                    if (match)
                    {
                        return method;
                    }
                }
            }
            return null;
        }
    }

    public enum Header
    {
        HOST(new char[]{104, 111, 115, 116}), 
        CONNECTION(new char[]{99, 111, 110, 110, 101, 99, 116, 105, 111, 110}),
        IF_MODIFIED_SINCE(new char[]{
            105, 102, 45, 109, 111, 100, 105, 102, 105, 101, 100, 45, 115, 
            105, 110, 99, 101}),
        COOKIE(new char[]{99, 111, 111, 107, 105, 101}),
        CONTENT_LENGTH(new char[]{
            99, 111, 110, 116, 101, 110, 116, 45, 108, 101, 110, 103, 116, 104}),
        UNHANDLED(new char[]{}); // could be expanded if needed

        private char[] chars;

        Header(char[] chars) 
        {
            this.chars = chars;
        }

        public static Header fromBuilder(StringBuilder builder) 
        {
            for (Header header : Header.values()) 
            {
                if (header.chars.length == builder.length()) 
                {                    
                    boolean match = true;
                    for (int i = 0; i < builder.length(); i++) 
                    {
                        if (header.chars[i] != builder.charAt(i)) 
                        {
                            match = false;
                            break;
                        }
                    }

                    if (match)
                    {
                        return header;
                    }
                }
            }
            return UNHANDLED;
        }
    }
}

A simple handler for the testing:

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.util.CharsetUtil;

@Sharable
public class SharableHttpHandler extends SimpleChannelInboundHandler
{    
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, SharableHttpRequest msg) 
        throws Exception
    {
        String message = "HTTP/1.1 200 OK\r\n" +
                "Content-type: text/html\r\n" + 
                "Content-length: 42\r\n\r\n" + 
                "Hello sharedworld";

        ByteBuf buffer = ctx.alloc().buffer(message.length());
        buffer.writeCharSequence(message, CharsetUtil.UTF_8);
        ChannelFuture flushPromise = ctx.channel().writeAndFlush(buffer);
        flushPromise.addListener(ChannelFutureListener.CLOSE);
        if (!flushPromise.isSuccess()) 
        {
            flushPromise.cause().printStackTrace(System.err);
        }
    }    
}

The full pipeline using these sharable handlers:

import tests.SharableHttpDecoder;
import tests.SharableHttpHandler;

import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.socket.SocketChannel;

public class ServerPipeline extends ChannelInitializer
{
    private final SharableHttpDecoder decoder = new SharableHttpDecoder();
    private final SharableHttpHandler handler = new SharableHttpHandler();

    @Override
    public void initChannel(SocketChannel channel)
    {
        ChannelPipeline pipeline = channel.pipeline();
        pipeline.addLast(decoder);
        pipeline.addLast(handler);

    }
}

---

The above was tested against this (more usual) unshared pipeline:

import static io.netty.handler.codec.http.HttpResponseStatus.OK;
import static io.netty.handler.codec.http.HttpVersion.HTTP_1_1;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.channel.socket.SocketChannel;
import io.netty.handler.codec.http.DefaultFullHttpResponse;
import io.netty.handler.codec.http.FullHttpRequest;
import io.netty.handler.codec.http.FullHttpResponse;
import io.netty.handler.codec.http.HttpHeaderNames;
import io.netty.handler.codec.http.HttpHeaderValues;
import io.netty.handler.codec.http.HttpObjectAggregator;
import io.netty.handler.codec.http.HttpServerCodec;
import io.netty.handler.codec.http.HttpUtil;
import io.netty.util.CharsetUtil;

public class ServerPipeline extends ChannelInitializer
{

    @Override
    public void initChannel(SocketChannel channel)
    {
        ChannelPipeline pipeline = channel.pipeline();
        pipeline.addLast(new HttpServerCodec());
        pipeline.addLast(new HttpObjectAggregator(65536));
        pipeline.addLast(new UnsharedHttpHandler());

    }

    class UnsharedHttpHandler extends SimpleChannelInboundHandler
    {

        @Override
        public void channelRead0(ChannelHandlerContext ctx, FullHttpRequest request) 
            throws Exception
        {
            String message = "Hello sharedworld";
            ByteBuf buffer = ctx.alloc().buffer(message.length());
            buffer.writeCharSequence(message.toString(), CharsetUtil.UTF_8);

            FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, OK, buffer);
            response.headers().set(HttpHeaderNames.CONTENT_TYPE, "text/html; charset=UTF-8");
            HttpUtil.setContentLength(response, response.content().readableBytes());
            response.headers().set(HttpHeaderNames.CONNECTION, HttpHeaderValues.CLOSE);
            ChannelFuture flushPromise = ctx.writeAndFlush(response);
            flushPromise.addListener(ChannelFutureListener.CLOSE);

        }
    }
}