gRPC源码分析2-Server的建立

gRPC中，Server、Client共享的Class不是很多，所以我们可以单独的分别讲解Server和Client的源码。

通过第一篇，我们知道对于gRPC来说，建立Server是非常简单的，还记得怎么写的？还是以example里 HelloWorldServer 例子来看

server = ServerBuilder.forPort(port)
        .addService(new GreeterImpl())
        .build()
        .start();

你没有看错，就是这么几行搞定。

如果需要看懂gRPC的源码，首先有几点需要明白

Builder模式生成Entity
Provider（SPI）模式解耦，动态选择服务提供方
abstract class用于扩展

0. 流程图

1. Builder

ServerBuilder是一个抽象类，不同的服务提供方（Provider），将继承实现它。如何找到这些继承者呢？ServerProvider就是用来找到不同的provider的。

2. Provider

如上图，ServerProvider也是一个抽象类，实现者都有哪些呢？我们通过SPI模式找到他们。

通过搜索文件知道gRPC中 io.grpc.ServerProvider 的实现方只有：Netty

io.grpc.netty.NettyServerProvider，
这个类就是ServerProvider的实现者，它的builderForPort返回ServerBuilder

3. NettyServer

最后，我们来看下当链接建立时是如何创建handle的。

public void initChannel(Channel ch) throws Exception {
  eventLoopReferenceCounter.retain();
  ch.closeFuture().addListener(new ChannelFutureListener() {
    @Override
    public void operationComplete(ChannelFuture future) {
      eventLoopReferenceCounter.release();
    }
  });
  NettyServerTransport transport = new NettyServerTransport(ch, protocolNegotiator,
      maxStreamsPerConnection, flowControlWindow, maxMessageSize, maxHeaderListSize);
  ServerTransportListener transportListener;
  // This is to order callbacks on the listener, not to guard access to channel.
  synchronized (NettyServer.this) {
    if (channel != null && !channel.isOpen()) {
      // Server already shutdown.
      ch.close();
      return;
    }

    transportListener = listener.transportCreated(transport);
  }
  transport.start(transportListener);
}

看code可知，当一个链接建立时，会生成一个NettyServerTransport，所有的数据处理都将在这里实现。

4. NettyServerTransport

public void start(ServerTransportListener listener) {
  Preconditions.checkState(this.listener == null, "Handler already registered");
  this.listener = listener;

  // Create the Netty handler for the pipeline.
  final NettyServerHandler grpcHandler = createHandler(listener);
  HandlerSettings.setAutoWindow(grpcHandler);

  // Notify when the channel closes.
  channel.closeFuture().addListener(new ChannelFutureListener() {
    @Override
    public void operationComplete(ChannelFuture future) throws Exception {
      notifyTerminated(grpcHandler.connectionError());
    }
  });

  ChannelHandler negotiationHandler = protocolNegotiator.newHandler(grpcHandler);
  channel.pipeline().addLast(negotiationHandler);
}

我们看到当调用start方法是，最重要的就是createHandle，在这个方法里将看到如何绑定HTTP/2的处理器的。

5. NettyServerHandle

static NettyServerHandler newHandler(ServerTransportListener transportListener,
                                     int maxStreams,
                                     int flowControlWindow,
                                     int maxHeaderListSize,
                                     int maxMessageSize) {
  Preconditions.checkArgument(maxHeaderListSize > 0, "maxHeaderListSize must be positive");
  // 就是一个log
  Http2FrameLogger frameLogger = new Http2FrameLogger(LogLevel.DEBUG, NettyServerHandler.class);
  Http2HeadersDecoder headersDecoder = new GrpcHttp2ServerHeadersDecoder(maxHeaderListSize);
  // reader
  Http2FrameReader frameReader = new Http2InboundFrameLogger(
      new DefaultHttp2FrameReader(headersDecoder), frameLogger);
  // writer
  Http2FrameWriter frameWriter =
      new Http2OutboundFrameLogger(new DefaultHttp2FrameWriter(), frameLogger);
  return newHandler(frameReader, frameWriter, transportListener, maxStreams, flowControlWindow,
      maxMessageSize);
}

@VisibleForTesting
static NettyServerHandler newHandler(Http2FrameReader frameReader, Http2FrameWriter frameWriter,
                                     ServerTransportListener transportListener,
                                     int maxStreams,
                                     int flowControlWindow,
                                     int maxMessageSize) {
  Preconditions.checkArgument(maxStreams > 0, "maxStreams must be positive");
  Preconditions.checkArgument(flowControlWindow > 0, "flowControlWindow must be positive");
  Preconditions.checkArgument(maxMessageSize > 0, "maxMessageSize must be positive");
// 一个channel一个connection
  Http2Connection connection = new DefaultHttp2Connection(true);

  // Create the local flow controller configured to auto-refill the connection window.
  connection.local().flowController(
      new DefaultHttp2LocalFlowController(connection, DEFAULT_WINDOW_UPDATE_RATIO, true));


  Http2ConnectionEncoder encoder = new DefaultHttp2ConnectionEncoder(connection, frameWriter);
  Http2ConnectionDecoder decoder = new DefaultHttp2ConnectionDecoder(connection, encoder,
      frameReader);

  Http2Settings settings = new Http2Settings();
  settings.initialWindowSize(flowControlWindow);
  settings.maxConcurrentStreams(maxStreams);

  return new NettyServerHandler(transportListener, decoder, encoder, settings, maxMessageSize);
}