Elasticsearch源码解析之HTTP请求响应处理

因为ES已经存在多个版本，主要是每一个版本的启动流程都不一样，我这里不想单独去分析某一个版本如何进行启动的，解析ES如何去响应HTTP请求的，以及背后如何去实现。下面简单给大家分析下，HTTP服务器实现。

HTTP Server

Elasticsearch Netty注册服务器 Netty4HttpServerTransport

 protected void doStart() {
        boolean success = false;
        try {
            serverBootstrap = new ServerBootstrap();

            serverBootstrap.group(new NioEventLoopGroup(workerCount, daemonThreadFactory(settings,
                HTTP_SERVER_WORKER_THREAD_NAME_PREFIX)));

            // NettyAllocator will return the channel type designed to work with the configuredAllocator
            serverBootstrap.channel(NettyAllocator.getServerChannelType());

            // Set the allocators for both the server channel and the child channels created
            serverBootstrap.option(ChannelOption.ALLOCATOR, NettyAllocator.getAllocator());
            serverBootstrap.childOption(ChannelOption.ALLOCATOR, NettyAllocator.getAllocator());

            serverBootstrap.childHandler(configureServerChannelHandler());
            serverBootstrap.handler(new ServerChannelExceptionHandler(this));

            serverBootstrap.childOption(ChannelOption.TCP_NODELAY, SETTING_HTTP_TCP_NO_DELAY.get(settings));
            serverBootstrap.childOption(ChannelOption.SO_KEEPALIVE, SETTING_HTTP_TCP_KEEP_ALIVE.get(settings));
          // 省略部分代码

            final ByteSizeValue tcpSendBufferSize = SETTING_HTTP_TCP_SEND_BUFFER_SIZE.get(settings);
            if (tcpSendBufferSize.getBytes() > 0) {
                serverBootstrap.childOption(ChannelOption.SO_SNDBUF, Math.toIntExact(tcpSendBufferSize.getBytes()));
            }

            final ByteSizeValue tcpReceiveBufferSize = SETTING_HTTP_TCP_RECEIVE_BUFFER_SIZE.get(settings);
            if (tcpReceiveBufferSize.getBytes() > 0) {
                serverBootstrap.childOption(ChannelOption.SO_RCVBUF, Math.toIntExact(tcpReceiveBufferSize.getBytes()));
            }

            serverBootstrap.option(ChannelOption.RCVBUF_ALLOCATOR, recvByteBufAllocator);
            serverBootstrap.childOption(ChannelOption.RCVBUF_ALLOCATOR, recvByteBufAllocator);

            final boolean reuseAddress = SETTING_HTTP_TCP_REUSE_ADDRESS.get(settings);
            serverBootstrap.option(ChannelOption.SO_REUSEADDR, reuseAddress);
            serverBootstrap.childOption(ChannelOption.SO_REUSEADDR, reuseAddress);
            // 绑定端口和地址
            bindServer();
            success = true;
        } finally {
            if (success == false) {
                doStop(); // otherwise we leak threads since we never moved to started
            }
        }
    }

用过Netty知道上面代码什么意思，设置worker线程,TCP设置，设置管道handler。Netty的连接出来一般都是在childHandler()设置ChannelInitializer 实现类中添加，看下configureServerChannelHandler()主要初始化了HttpChannelHandler，在initChannel()能看到添加了那个处理器。

        protected void initChannel(Channel ch) throws Exception {
            Netty4HttpChannel nettyHttpChannel = new Netty4HttpChannel(ch);
            ch.attr(HTTP_CHANNEL_KEY).set(nettyHttpChannel);
            ch.pipeline().addLast("read_timeout", new ReadTimeoutHandler(transport.readTimeoutMillis, TimeUnit.MILLISECONDS));
            final HttpRequestDecoder decoder = new HttpRequestDecoder(
                handlingSettings.getMaxInitialLineLength(),
                handlingSettings.getMaxHeaderSize(),
                handlingSettings.getMaxChunkSize());
            decoder.setCumulator(ByteToMessageDecoder.COMPOSITE_CUMULATOR);
            ch.pipeline().addLast("decoder", decoder);
            ch.pipeline().addLast("decoder_compress", new HttpContentDecompressor());
            ch.pipeline().addLast("encoder", new HttpResponseEncoder());
            final HttpObjectAggregator aggregator = new HttpObjectAggregator(handlingSettings.getMaxContentLength());
            aggregator.setMaxCumulationBufferComponents(transport.maxCompositeBufferComponents);
            ch.pipeline().addLast("aggregator", aggregator);
            if (handlingSettings.isCompression()) {
                ch.pipeline().addLast("encoder_compress", new HttpContentCompressor(handlingSettings.getCompressionLevel()));
            }
            if (handlingSettings.isCorsEnabled()) {
                ch.pipeline().addLast("cors", new Netty4CorsHandler(transport.corsConfig));
            }
            ch.pipeline().addLast("pipelining", new Netty4HttpPipeliningHandler(logger, transport.pipeliningMaxEvents));
            ch.pipeline().addLast("handler", requestHandler);
            transport.serverAcceptedChannel(nettyHttpChannel);
        }

从上面代码知道处理请求的是： requestHandler，它的实现类： Netty4HttpRequestHandler

    protected void channelRead0(ChannelHandlerContext ctx, HttpPipelinedRequest<FullHttpRequest> msg) {
        Netty4HttpChannel channel = ctx.channel().attr(Netty4HttpServerTransport.HTTP_CHANNEL_KEY).get();
        FullHttpRequest request = msg.getRequest();
        boolean success = false;
        Netty4HttpRequest httpRequest = new Netty4HttpRequest(request, msg.getSequence());
        try {
            if (request.decoderResult().isFailure()) {
                Throwable cause = request.decoderResult().cause();
                if (cause instanceof Error) {
                    ExceptionsHelper.maybeDieOnAnotherThread(cause);
                    serverTransport.incomingRequestError(httpRequest, channel, new Exception(cause));
                } else {
                    serverTransport.incomingRequestError(httpRequest, channel, (Exception) cause);
                }
            } else {
                serverTransport.incomingRequest(httpRequest, channel);
            }
            success = true;
        } finally {
            if (success == false) {
                httpRequest.release();
            }

可以看出处理http请求的方法，委派了Netty4HttpServerTransport，也就是上面进行Netty server的类。这里的逻辑将httpRequest,channel 转换成Elasticsearch 模板对象，屏蔽掉底层api，再从线程池中获取ThreadContext进行任务执行。类似一个http分发器。详细代码就不展示出来，一连串的方法调用，看下图

TransportAction.doExecute是一个抽象方法，由NodeClient.transportAction返回的实现类去调用执行。每一个URL都会有对应的transportAction实现类，这个和我们平常MVC架构不一样。NodeClient内置了Map<ActionType, TransportAction> actions，里面包含所有HTTP请求处理方法，有300多个值对应不同场景的处理。
看下最简单的响应，当我请求ES:9200端口时，返回基础信息,由TransportMainAction如何响应的

public class TransportMainAction extends HandledTransportAction<MainRequest, MainResponse> {

    private final String nodeName;
    private final ClusterService clusterService;

    @Inject
    public TransportMainAction(Settings settings, TransportService transportService,
                               ActionFilters actionFilters, ClusterService clusterService) {
        super(MainAction.NAME, transportService, actionFilters, MainRequest::new);
        this.nodeName = Node.NODE_NAME_SETTING.get(settings);
        this.clusterService = clusterService;
    }

    @Override
    protected void doExecute(Task task, MainRequest request, ActionListener<MainResponse> listener) {
        ClusterState clusterState = clusterService.state();
        listener.onResponse(
            new MainResponse(nodeName, Version.CURRENT, clusterState.getClusterName(),
                    clusterState.metaData().clusterUUID(), Build.CURRENT));
    }
}

总结

分析这么多代码， ES处理HTTP请求链虽然是执行过程比较绕，但是实际代码还是比较简单，整体还是去分析TransportAction.doExecute如何响应请求的。