code: private void registerHandlerCallbackForClient(final MethodCall<Object> methodCall,
                                                    final Callback<Object> handler) {
  
          final HandlerKey handlerKey = new HandlerKey(methodCall.returnAddress(), methodCall.address(),
                  methodCall.id(), methodCall.timestamp());
  
          if (debug) {
              if (handlers.containsKey(handlerKey)) {
                  logger.debug("DUPLICATE HANDLERS {}", handlerKey);
              }
          }
  
          handlers.put(handlerKey, handler);
  
      }
docs: Register a callbackWithTimeout handler
  
  @param methodCall method call
  @param handler    call back handler to register