客户端连接控制

<dubbo:reference interface="com.foo.BarService" connections="10" />

或

<dubbo:service interface="com.foo.BarService" connections="10" />

• 限制客户端服务使用连接不能超过 10 个。
• 如果 和 都配了 connections， 优先。
• 配置参数为connections。

public class DubboProtocol extends AbstractProtocol {

    private ExchangeClient[] getClients(URL url) {
        // whether to share connection
        boolean service_share_connect = false;
        // String CONNECTIONS_KEY = "connections";
        int connections = url.getParameter(Constants.CONNECTIONS_KEY, 0);
        // if not configured, connection is shared, otherwise, one connection for one service
        if (connections == 0) {
            service_share_connect = true;
            connections = 1;
        }
        // 如果设置了最大连接数，就不走共享连接，直接创建指定个数的连接。
        ExchangeClient[] clients = new ExchangeClient[connections];
        for (int i = 0; i < clients.length; i++) {
            if (service_share_connect) {
                clients[i] = getSharedClient(url);
            } else {
                clients[i] = initClient(url);
            }
        }
        return clients;
    }


    private ExchangeClient initClient(URL url) {

        // client type setting.
        String str = url.getParameter(Constants.CLIENT_KEY, url.getParameter(Constants.SERVER_KEY, Constants.DEFAULT_REMOTING_CLIENT));

        url = url.addParameter(Constants.CODEC_KEY, DubboCodec.NAME);
        // enable heartbeat by default
        url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));

        // BIO is not allowed since it has severe performance issue.
        if (str != null && str.length() > 0 && !ExtensionLoader.getExtensionLoader(Transporter.class).hasExtension(str)) {
            throw new RpcException("Unsupported client type: " + str + "," +
                    " supported client type is " + StringUtils.join(ExtensionLoader.getExtensionLoader(Transporter.class).getSupportedExtensions(), " "));
        }

        ExchangeClient client;
        try {
            // connection should be lazy
            if (url.getParameter(Constants.LAZY_CONNECT_KEY, false)) {
                client = new LazyConnectExchangeClient(url, requestHandler);
            } else {
                client = Exchangers.connect(url, requestHandler);
            }
        } catch (RemotingException e) {
            throw new RpcException("Fail to create remoting client for service(" + url + "): " + e.getMessage(), e);
        }
        return client;
    }
}

• 客户端的连接数控制是通过创建ExchangeClient的过程中进行控制。
• 如果设置了connections的变量，那么就只创建指定数量的的ExchangeClient对象。

服务端连接控制

<dubbo:provider protocol="dubbo" accepts="10" />

或

<dubbo:protocol name="dubbo" accepts="10" />

• 限制服务器端接受的连接不能超过 10 个。
• 配置参数为accepts。

public class NettyServer extends AbstractServer implements Server {

    private Map<String, Channel> channels; // <ip:port, channel>
    private ServerBootstrap bootstrap;
    private io.netty.channel.Channel channel;
    private EventLoopGroup bossGroup;
    private EventLoopGroup workerGroup;

    public NettyServer(URL url, ChannelHandler handler) throws RemotingException {
        super(url, ChannelHandlers.wrap(handler, ExecutorUtil.setThreadName(url, SERVER_THREAD_POOL_NAME)));
    }
}


public abstract class AbstractServer extends AbstractEndpoint implements Server {

    protected static final String SERVER_THREAD_POOL_NAME = "DubboServerHandler";
    private static final Logger logger = LoggerFactory.getLogger(AbstractServer.class);
    ExecutorService executor;
    private InetSocketAddress localAddress;
    private InetSocketAddress bindAddress;
    // 服务端连接控制
    private int accepts;
    private int idleTimeout = 600; //600 seconds

    public void connected(Channel ch) throws RemotingException {
        // If the server has entered the shutdown process, reject any new connection
        if (this.isClosing() || this.isClosed()) {
            logger.warn("Close new channel " + ch + ", cause: server is closing or has been closed. For example, receive a new connect request while in shutdown process.");
            ch.close();
            return;
        }

        Collection<Channel> channels = getChannels();
        // 超过服务端连接控制，直接关闭连接。
        if (accepts > 0 && channels.size() > accepts) {
            logger.error("Close channel " + ch + ", cause: The server " + ch.getLocalAddress() + " connections greater than max config " + accepts);
            ch.close();
            return;
        }
        super.connected(ch);
    }
}

• 服务端的连接控制数通过NettyServer的connected()方法在处理连接事件的时候进行判断。
• 如果连接数的数量超过了accepts的值那么就抛出异常并关闭连接数。

连接数说明

• 客户端没有配置connections，使用共享连接，以 ip+port 确定一个连接。
• 客户端配置了connections，客户端配置了两个引用（两个reference）且一个配置了connections=“2”，另一个没有配置connections，则没有配置connects的引用使用共享连接产生一个Client连接，配置connections=“2”使用独立连接，且连接数为2，则产生两个Client连接，所以一共产生三个Client连接。
  
  

参考

《Dubbo进阶二》——RPC协议之网络传输原理