SimpleServerRpcConnection register(SocketChannel channel) {
      SimpleServerRpcConnection connection =
        getConnection(channel, EnvironmentEdgeManager.currentTime());
      add(connection);
      if (LOG.isTraceEnabled()) {
        LOG.trace("Connection from " + connection + "; connections=" + size()
          + ", queued calls size (bytes)=" + callQueueSizeInBytes.sum() + ", general queued calls="
          + scheduler.getGeneralQueueLength() + ", priority queued calls="
          + scheduler.getPriorityQueueLength() + ", meta priority queued calls="
          + scheduler.getMetaPriorityQueueLength());
      }
      return connection;
    }
The number of open RPC conections
@return the number of open rpc connections
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/SimpleRpcServer.java