在後續一段時間裏, 我會寫一系列文章來講述如何實現一個RPC框架(我已經實現了一個示例框架, 代碼在我的github上)。 這是系列第四篇文章, 主要講述了客戶端和服務器之間的網絡通信問題。
模型定義
我們需要自己來定義RPC通信所傳遞的內容的模型, 也就是RPCRequest和RPCResponse。
@Data
@Builder
public class RPCRequest {
private String requestId;
private String interfaceName;
private String methodName;
private Class<?>[] parameterTypes;
private Object[] parameters;
}
@Data
public class RPCResponse {
private String requestId;
private Exception exception;
private Object result;
public boolean hasException() {
return exception != null;
}
}
這裏唯一需要說明一下的是requestId, 你可能會疑惑爲什麼我們需要這個東西。
原因是,發送請求的順序和收到返回的順序可能是不一致的, 因此我們需要有一個標識符來表明某一個返回所對應的請求是什麼。 具體怎麼利用這個字段, 本文後續會揭曉。
選擇NIO還是IO?
NIO和IO的選擇要視具體情況而定。對於我們的RPC框架來說, 一個服務可能與多個服務保持連接, 且每次通信只發送少量信息,那麼在這種情況下,NIO可能更適合一些。
我選擇使用Netty來簡化具體的實現, 自然地,我們就引入了Channel, Handler這些相關的概念。如果對Netty沒有任何瞭解, 建議先去簡單瞭解下相關內容再回過頭看這篇文章。
如何複用Channel
既然使用了NIO, 我們自然希望服務和服務之間是使用長連接進行通信, 而不是每個請求都重新創建一個channel。
那麼我們怎麼去複用channel呢? 既然我們已經通過前文的服務發現獲取到了service地址,並且與其建立了channel, 那麼我們自然就可以建立一個service地址與channel之間的映射關係, 每次拿到地址之後先判斷有沒有對應channel, 如果有的話就複用。這種映射關係我建立了ChannelManager去管理:
public class ChannelManager {
/**
* Singleton
*/
private static ChannelManager channelManager;
private ChannelManager(){}
public static ChannelManager getInstance() {
if (channelManager == null) {
synchronized (ChannelManager.class) {
if (channelManager == null) {
channelManager = new ChannelManager();
}
}
}
return channelManager;
}
// Service地址與channel之間的映射
private Map<InetSocketAddress, Channel> channels = new ConcurrentHashMap<>();
public Channel getChannel(InetSocketAddress inetSocketAddress) {
Channel channel = channels.get(inetSocketAddress);
if (null == channel) {
EventLoopGroup group = new NioEventLoopGroup();
try {
Bootstrap bootstrap = new Bootstrap();
bootstrap.group(group)
.channel(NioSocketChannel.class)
.handler(new RPCChannelInitializer())
.option(ChannelOption.SO_KEEPALIVE, true);
channel = bootstrap.connect(inetSocketAddress.getHostName(), inetSocketAddress.getPort()).sync()
.channel();
registerChannel(inetSocketAddress, channel);
channel.closeFuture().addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
removeChannel(inetSocketAddress);
}
});
} catch (Exception e) {
log.warn("Fail to get channel for address: {}", inetSocketAddress);
}
}
return channel;
}
private void registerChannel(InetSocketAddress inetSocketAddress, Channel channel) {
channels.put(inetSocketAddress, channel);
}
private void removeChannel(InetSocketAddress inetSocketAddress) {
channels.remove(inetSocketAddress);
}
}
有幾個地方需要解釋一下:
- 這裏用單例的目的是, 所有的proxybean都使用同一個ChannelManager。
- 創建Channel的過程很簡單,就是最普通的Netty客戶端創建channel的方法。
- 在channel被關閉(比如服務器端宕機了)後,需要從map中刪除對應的channel
- RPCChannelInitializer是整個過程的核心所在, 用於處理請求和返回的編解碼、 收到返回之後的回調等。 下文詳細說這個。
編解碼
上文的RPCChannelInitializer代碼如下:
private class RPCChannelInitializer extends ChannelInitializer<SocketChannel> {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline pipeline = ch.pipeline();
pipeline.addLast(new RPCEncoder(RPCRequest.class, new ProtobufSerializer()));
pipeline.addLast(new RPCDecoder(RPCResponse.class, new ProtobufSerializer()));
pipeline.addLast(new RPCResponseHandler()); //先不用管這個
}
}
這裏的Encoder和Decoder都很簡單, 繼承了Netty中的codec,做一些簡單的byte數組和Object對象之間的轉換工作:
@AllArgsConstructor
public class RPCDecoder extends ByteToMessageDecoder {
private Class<?> genericClass;
private Serializer serializer;
@Override
public void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
if (in.readableBytes() < 4) {
return;
}
in.markReaderIndex();
int dataLength = in.readInt();
if (in.readableBytes() < dataLength) {
in.resetReaderIndex();
return;
}
byte[] data = new byte[dataLength];
in.readBytes(data);
out.add(serializer.deserialize(data, genericClass));
}
}
@AllArgsConstructor
public class RPCEncoder extends MessageToByteEncoder {
private Class<?> genericClass;
private Serializer serializer;
@Override
public void encode(ChannelHandlerContext ctx, Object in, ByteBuf out) throws Exception {
if (genericClass.isInstance(in)) {
byte[] data = serializer.serialize(in);
out.writeInt(data.length);
out.writeBytes(data);
}
}
}
這裏我選擇使用Protobuf序列化協議來做這件事(具體的ProtobufSerializer的實現因爲篇幅原因就不貼在這裏了, 需要的話請看項目的github)。 總的來說, 這一塊還是很簡單很好理解的。
發送請求與處理返回內容
請求的發送很簡單, 直接用channel.writeAndFlush(request) 就行了。
問題是, 發送之後, 怎麼獲取這個請求的返回呢?這裏,我引入了RPCResponseFuture和ResponseFutureManager來解決這個問題。
RPCResponseFuture實現了Future接口,所包含的值就是RPCResponse, 每個RPCResponseFuture都與一個requestId相關聯, 除此之外, 還利用了CountDownLatch來做get方法的阻塞處理:
public class RPCResponseFuture implements Future<Object> {
private String requestId;
private RPCResponse response;
CountDownLatch latch = new CountDownLatch(1);
public RPCResponseFuture(String requestId) {
this.requestId = requestId;
}
public void done(RPCResponse response) {
this.response = response;
latch.countDown();
}
@Override
public RPCResponse get() throws InterruptedException, ExecutionException {
try {
latch.await();
} catch (InterruptedException e) {
log.error(e.getMessage());
}
return response;
}
// ....
}
既然每個請求都會產生一個ResponseFuture, 那麼自然要有一個Manager來管理這些future:
public class ResponseFutureManager {
/**
* Singleton
*/
private static ResponseFutureManager rpcFutureManager;
private ResponseFutureManager(){}
public static ResponseFutureManager getInstance() {
if (rpcFutureManager == null) {
synchronized (ChannelManager.class) {
if (rpcFutureManager == null) {
rpcFutureManager = new ResponseFutureManager();
}
}
}
return rpcFutureManager;
}
private ConcurrentHashMap<String, RPCResponseFuture> rpcFutureMap = new ConcurrentHashMap<>();
public void registerFuture(RPCResponseFuture rpcResponseFuture) {
rpcFutureMap.put(rpcResponseFuture.getRequestId(), rpcResponseFuture);
}
public void futureDone(RPCResponse response) {
rpcFutureMap.remove(response.getRequestId()).done(response);
}
}
ResponseFutureManager很好看懂, 就是提供了註冊future、完成future的接口。
現在我們再回過頭看RPCChannelInitializer中的RPCResponseHandler就很好理解了: 拿到返回值, 把對應的ResponseFuture標記成done就可以了!
/**
* 處理收到返回後的回調
*/
private class RPCResponseHandler extends SimpleChannelInboundHandler<RPCResponse> {
@Override
public void channelRead0(ChannelHandlerContext ctx, RPCResponse response) throws Exception {
log.debug("Get response: {}", response);
ResponseFutureManager.getInstance().futureDone(response);
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
log.warn("RPC request exception: {}", cause);
}
}
前文的FactoryBean的邏輯填充
到這裏,我們已經實現了客戶端的網絡通信, 現在只需要把它加到前文的FactoryBean的doInvoke方法就好了!
private Object doInvoke(Object proxy, Method method, Object[] args) throws Throwable {
String targetServiceName = type.getName();
// Create request
RPCRequest request = RPCRequest.builder()
.requestId(generateRequestId(targetServiceName))
.interfaceName(method.getDeclaringClass().getName())
.methodName(method.getName())
.parameters(args)
.parameterTypes(method.getParameterTypes()).build();
// Get service address
InetSocketAddress serviceAddress = getServiceAddress(targetServiceName);
// Get channel by service address
Channel channel = ChannelManager.getInstance().getChannel(serviceAddress);
if (null == channel) {
throw new RuntimeException("Cann't get channel for address" + serviceAddress);
}
// Send request
RPCResponse response = sendRequest(channel, request);
if (response == null) {
throw new RuntimeException("response is null");
}
if (response.hasException()) {
throw response.getException();
} else {
return response.getResult();
}
}
private String generateRequestId(String targetServiceName) {
return targetServiceName + "-" + UUID.randomUUID().toString();
}
private InetSocketAddress getServiceAddress(String targetServiceName) {
String serviceAddress = "";
if (serviceDiscovery != null) {
serviceAddress = serviceDiscovery.discover(targetServiceName);
log.debug("Get address: {} for service: {}", serviceAddress, targetServiceName);
}
if (StringUtils.isEmpty(serviceAddress)) {
throw new RuntimeException("server address is empty");
}
String[] array = StringUtils.split(serviceAddress, ":");
String host = array[0];
int port = Integer.parseInt(array[1]);
return new InetSocketAddress(host, port);
}
private RPCResponse sendRequest(Channel channel, RPCRequest request) {
log.debug("Send request, channel: {}, request: {}", channel, request);
CountDownLatch latch = new CountDownLatch(1);
RPCResponseFuture rpcResponseFuture = new RPCResponseFuture(request.getRequestId());
ResponseFutureManager.getInstance().registerFuture(rpcResponseFuture);
channel.writeAndFlush(request).addListener((ChannelFutureListener) future -> {
log.debug("Request sent.");
latch.countDown();
});
try {
latch.await();
} catch (InterruptedException e) {
log.error(e.getMessage());
}
try {
return rpcResponseFuture.get(1, TimeUnit.SECONDS);
} catch (Exception e) {
log.warn("Exception:", e);
return null;
}
}
就這樣, 一個簡單的RPC客戶端就實現了。 完整代碼請看我的github。