* special values [PAYLOAD_SHORT] or [PAYLOAD_LONG].
   */
  internal const val PAYLOAD_BYTE_MAX = 125L

  /** Maximum length of close message in bytes. */
  internal const val CLOSE_MESSAGE_MAX = PAYLOAD_BYTE_MAX - 2

  /**
   * Value for [B1_MASK_LENGTH] which indicates the next two bytes are the unsigned length.
   */
  internal const val PAYLOAD_SHORT = 126

The server handler function has this signature: `type SingleHandlerFn func(payload []byte) ([]byte, *RemoteErr)`.

Sample handler:
```go
    handler :=  func(payload []byte) ([]byte, *grid.RemoteErr) {
        // Do something with payload
        return []byte("response"), nil
    }

    err := manager.RegisterSingleHandler(grid.HandlerDiskInfo, handler)
```

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

  /** Send a ping with the supplied [payload]. */
  @Throws(IOException::class)
  fun writePing(payload: ByteString) {
    writeControlFrame(OPCODE_CONTROL_PING, payload)
  }

  /** Send a pong with the supplied [payload]. */
  @Throws(IOException::class)
  fun writePong(payload: ByteString) {
    writeControlFrame(OPCODE_CONTROL_PONG, payload)
  }

  /**

    val payload = Buffer()
    while (payload.completeSegmentByteCount() <= PAYLOAD_BYTE_MAX) {
      payload.writeByte('0'.code)
    }
    serverWriter.writeMessageFrame(OPCODE_BINARY, payload.snapshot())

    // Write directly to the unbuffered sink. This ensures it will become single frame.
    assertData("827e") // 'e' == 4-byte follow-up length.
    assertData(format("%04X", payload.completeSegmentByteCount()))

	errFatal(local.RegisterSingleHandler(handlerTest, func(payload []byte) ([]byte, *RemoteErr) {
		t.Log("1: server payload: ", len(payload), "bytes.")
		return append([]byte{}, payload...), nil
	}))
	// 2: Return as error
	errFatal(local.RegisterSingleHandler(handlerTest2, func(payload []byte) ([]byte, *RemoteErr) {
		t.Log("2: server payload: ", len(payload), "bytes.")
		err := RemoteErr(payload)
		return nil, &err
	}))

				fmt.Println("muxServer: Mux", m.ID, "send EOF", hErr)
			}
			msg.Flags |= FlagEOF
			if hErr != nil {
				msg.Flags |= FlagPayloadIsErr
				msg.Payload = []byte(*hErr)
			}
			msg.setZeroPayloadFlag()
			m.send(msg)
			return
		}
		msg.Payload = payload
		msg.setZeroPayloadFlag()
		m.send(msg)
	}
}

// handleRequests will handle the requests from the client and call the handler function.

}

// Post submit 'payload' to specified URL
func (c Config) Post(reqURL string, payload interface{}) (string, error) {
	if !c.Registered() {
		return "", errors.New("Deployment is not registered with SUBNET. Please register the deployment via 'mc license register ALIAS'")
	}
	body, err := json.Marshal(payload)
	if err != nil {
		return "", err
	}

  }

  fun assertTextMessage(payload: String?) {
    assertThat(nextEvent()).isEqualTo(Message(string = payload))
  }

  fun assertBinaryMessage(payload: ByteString?) {
    assertThat(nextEvent()).isEqualTo(Message(payload))
  }

  fun assertPing(payload: ByteString) {
    assertThat(nextEvent()).isEqualTo(Ping(payload))
  }

  fun assertPong(payload: ByteString) {

	var sub *subHandlerID
	if m.Payload == nil {
		m.Payload = GetByteBuffer()[:0]
	}
	h, err := m.UnmarshalMsg(b)
	if err != nil {
		return nil, nil, fmt.Errorf("read write: %v", err)
	}
	if len(m.Payload) == 0 && m.Flags&FlagPayloadIsZero == 0 {
		PutByteBuffer(m.Payload)
		m.Payload = nil
	}
	if m.Flags&FlagCRCxxh3 != 0 {
		const hashLen = 4