public void sendRequest(final DiscoveryNode node, final long requestId, final String action, final TransportRequest request, TransportRequestOptions options) throws IOException, TransportException {
        //參數(shù)說明：node發(fā)送的目的節(jié)點(diǎn)，requestId請(qǐng)求id，action action名稱，request請(qǐng)求，options包括以下幾種操作 RECOVERY,BULK,REG,STATE,PING;
　　　　　Channel targetChannel = nodeChannel(node, options);//獲取對(duì)應(yīng)節(jié)點(diǎn)的channel，channel在連接節(jié)點(diǎn)時(shí)初始化完成（請(qǐng)參考上一篇）
        if (compress) {
            options.withCompress(true);
        }
        byte status = 0;
　　　　　//設(shè)置status 包括以下幾種STATUS_REQRES = 1 << 0; STATUS_ERROR = 1 << 1; STATUS_COMPRESS = 1 << 2;
　　　　status = TransportStatus.setRequest(status); 
 　　　　ReleasableBytesStreamOutput bStream = new ReleasableBytesStreamOutput(bigArrays);//初始寫出流
        boolean addedReleaseListener = false;
        try {
            bStream.skip(NettyHeader.HEADER_SIZE);//留出message header的位置
            StreamOutput stream = bStream;
            // only compress if asked, and, the request is not bytes, since then only
            // the header part is compressed, and the "body" can't be extracted as compressed
            if (options.compress() && (!(request instanceof BytesTransportRequest))) {
                status = TransportStatus.setCompress(status);
                stream = CompressorFactory.defaultCompressor().streamOutput(stream);
            }
            stream = new HandlesStreamOutput(stream);
            // we pick the smallest of the 2, to support both backward and forward compatibility
            // note, this is the only place we need to do this, since from here on, we use the serialized version
            // as the version to use also when the node receiving this request will send the response with
            Version version = Version.smallest(this.version, node.version());
            stream.setVersion(version);
            stream.writeString(transportServiceAdapter.action(action, version));
            ReleasableBytesReference bytes;
            ChannelBuffer buffer;
            // it might be nice to somehow generalize this optimization, maybe a smart "paged" bytes output
            // that create paged channel buffers, but its tricky to know when to do it (where this option is
            // more explicit).
            if (request instanceof BytesTransportRequest) {
                BytesTransportRequest bRequest = (BytesTransportRequest) request;
                assert node.version().equals(bRequest.version());
                bRequest.writeThin(stream);
                stream.close();
                bytes = bStream.bytes();
                ChannelBuffer headerBuffer = bytes.toChannelBuffer();
                ChannelBuffer contentBuffer = bRequest.bytes().toChannelBuffer();
                buffer = ChannelBuffers.wrappedBuffer(NettyUtils.DEFAULT_GATHERING, headerBuffer, contentBuffer);
            } else {
                request.writeTo(stream);
                stream.close();
                bytes = bStream.bytes();
                buffer = bytes.toChannelBuffer();
            }
            NettyHeader.writeHeader(buffer, requestId, status, version);//寫信息頭
            ChannelFuture future = targetChannel.write(buffer);//寫buffer同時(shí)獲取future，發(fā)送信息發(fā)生在這里
            ReleaseChannelFutureListener listener = new ReleaseChannelFutureListener(bytes);
            future.addListener(listener);//添加listener
            addedReleaseListener = true;
            transportServiceAdapter.onRequestSent(node, requestId, action, request, options);
        } finally {
            if (!addedReleaseListener) {
                Releasables.close(bStream.bytes());
            }
        }
    }

public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
        Transports.assertTransportThread();
        Object m = e.getMessage();
        if (!(m instanceof ChannelBuffer)) {//非buffer之間返回
            ctx.sendUpstream(e);
            return;
        }
　　　　　//解析message頭
        ChannelBuffer buffer = (ChannelBuffer) m;
        int size = buffer.getInt(buffer.readerIndex() - 4);
        transportServiceAdapter.received(size + 6);
        // we have additional bytes to read, outside of the header
        boolean hasMessageBytesToRead = (size - (NettyHeader.HEADER_SIZE - 6)) != 0;
        int markedReaderIndex = buffer.readerIndex();
        int expectedIndexReader = markedReaderIndex + size;
        // netty always copies a buffer, either in NioWorker in its read handler, where it copies to a fresh
        // buffer, or in the cumlation buffer, which is cleaned each time
        StreamInput streamIn = ChannelBufferStreamInputFactory.create(buffer, size);
　　　　　　//讀取信息頭中的幾個(gè)重要元數(shù)據(jù)
        long requestId = buffer.readLong();
        byte status = buffer.readByte();
        Version version = Version.fromId(buffer.readInt());
        StreamInput wrappedStream;
　　　　　　…………
        if (TransportStatus.isRequest(status)) {//處理請(qǐng)求
            String action = handleRequest(ctx.getChannel(), wrappedStream, requestId, version);
            if (buffer.readerIndex() != expectedIndexReader) {
                if (buffer.readerIndex() < expectedIndexReader) {
                    logger.warn("Message not fully read (request) for [{}] and action [{}], resetting", requestId, action);
                } else {
                    logger.warn("Message read past expected size (request) for [{}] and action [{}], resetting", requestId, action);
                }
                buffer.readerIndex(expectedIndexReader);
            }
        } else {//處理響應(yīng)
            TransportResponseHandler handler = transportServiceAdapter.onResponseReceived(requestId);
            // ignore if its null, the adapter logs it
            if (handler != null) {
                if (TransportStatus.isError(status)) {
                    handlerResponseError(wrappedStream, handler);
                } else {
                    handleResponse(ctx.getChannel(), wrappedStream, handler);
                }
            } else {
                // if its null, skip those bytes
                buffer.readerIndex(markedReaderIndex + size);
            }
          …………
        wrappedStream.close();
    }

protected String handleRequest(Channel channel, StreamInput buffer, long requestId, Version version) throws IOException {
        final String action = buffer.readString();//讀出action的名字
        transportServiceAdapter.onRequestReceived(requestId, action);
        final NettyTransportChannel transportChannel = new NettyTransportChannel(transport, transportServiceAdapter, action, channel, requestId, version, profileName);
        try {
            final TransportRequestHandler handler = transportServiceAdapter.handler(action, version);//獲取處理該信息的handler
            if (handler == null) {
                throw new ActionNotFoundTransportException(action);
            }
            final TransportRequest request = handler.newInstance();
            request.remoteAddress(new InetSocketTransportAddress((InetSocketAddress) channel.getRemoteAddress()));
            request.readFrom(buffer);
            if (handler.executor() == ThreadPool.Names.SAME) {
                //noinspection unchecked
                handler.messageReceived(request, transportChannel);//使用該handler處理信息。
            } else {
                threadPool.executor(handler.executor()).execute(new RequestHandler(handler, request, transportChannel, action));
            }
        } catch (Throwable e) {
            try {
                transportChannel.sendResponse(e);
            } catch (IOException e1) {
                logger.warn("Failed to send error message back to client for action [" + action + "]", e);
                logger.warn("Actual Exception", e1);
            }
        }
        return action;
    }

protected void handleResponse(Channel channel, StreamInput buffer, final TransportResponseHandler handler) {
        final TransportResponse response = handler.newInstance();
        response.remoteAddress(new InetSocketTransportAddress((InetSocketAddress) channel.getRemoteAddress()));
        response.remoteAddress();
        try {
            response.readFrom(buffer);
        } catch (Throwable e) {
            handleException(handler, new TransportSerializationException("Failed to deserialize response of type [" + response.getClass().getName() + "]", e));
            return;
        }
        try {
            if (handler.executor() == ThreadPool.Names.SAME) {
                //noinspection unchecked
                handler.handleResponse(response);//轉(zhuǎn)發(fā)給對(duì)應(yīng)的handler
            } else {
                threadPool.executor(handler.executor()).execute(new ResponseHandler(handler, response));
            }
        } catch (Throwable e) {
            handleException(handler, new ResponseHandlerFailureTransportException(e));
        }
    }