Is this a correct way to parse incoming JSON over websocket, and respond depending on the type of message?

Question

So I am receiving JSON over a websocket from a chargepoint using OCPP 1.6 JSON. I am trying to parse the message and respond appropriately, depending on what the message is, using Node.js

Here is the message that I recieve:

[ 2,
  'bc7MRxWrWFnfQzepuhKSsevqXEqheQSqMcu3',
  'BootNotification',
  { chargePointVendor: 'AVT-Company',
    chargePointModel: 'AVT-Express',
    chargePointSerialNumber: 'avt.001.13.1',
    chargeBoxSerialNumber: 'avt.001.13.1.01',
    firmwareVersion: '0.9.87',
    iccid: '',
    imsi: '',
    meterType: 'AVT NQC-ACDC',
    meterSerialNumber: 'avt.001.13.1.01' } ]

In this case it is the 'BootNotification' message, to which I need to respond with an 'Accepted' message.

Here is my code:

const WebSocket = require('ws');

const wss = new WebSocket.Server({ port: 8080 });

wss.on('connection', function connection(ws) {
  ws.on('message', function incoming(message) {

    //Make incoming JSON into javascript object
    var msg = JSON.parse(message)

    // Print whole message to console
    console.log(msg)

    // Print only message type to console. For example BootNotification, Heartbeat etc...
   console.log("Message type: " + msg[2])

    // Send response depending on what the message type is
    if (msg[2] === "BootNotification") {
      //Send correct response

    } // Add all the message types

  });


});

With this I get the message type printed to the console as a string:

Message type: BootNotification

So my question is that is this the correct way to get the type of the message? I am new to this so I want to make sure.

The specification for OCPP 1.6 JSON is available here: OpenChargeAlliance website

Answer 1

I guess YES. JSON.parse is the built-in to, well, pares JSON strings. In case that goes wrong it throws an error, so you might try/catch this.

Since the response you get is an array, there is no other way as to access its items with a numeric index.

---

In such cases I personally prefer to have something like that:

const handlers = {
  'BootNotification': request => { 'msg': 'what a request' }
};

Than you can:

let respone = {'msg': 'Cannot handle this'}

if (handlers.hasOwnProperty(msg[2])) {
  response = handlers[msg[2]](msg);

}

But that is just the way I would go.

Answer 2

If you are asking about OCPP message structure (not parsing a JSON), I can provide you the details about OCPP 1.6 version.

In OCPP 1.6, the client (Charging Station) sends a CALL (similar to request in HTTP) to server (Charging Station Management Systems). All CALLs have a strict structure of 4 elements:

• MessageTypeId (integer)
• UniqueId (UUID, string)
• Action (string)
• Payload (JSON object containing the arguments relevant to the Action.)

or as in your example:

[
  2,
  'bc7MRxWrWFnfQzepuhKSsevqXEqheQSqMcu3',
  'BootNotification',
  { chargePointVendor: 'AVT-Company',
    chargePointModel: 'AVT-Express',
    chargePointSerialNumber: 'avt.001.13.1',
    chargeBoxSerialNumber: 'avt.001.13.1.01',
    firmwareVersion: '0.9.87',
    iccid: '',
    imsi: '',
    meterType: 'AVT NQC-ACDC',
    meterSerialNumber: 'avt.001.13.1.01' } 
]

So the Type of Action should be always at index of 2 (as you retrieve it when you parse received message). You can refer to @philipp anwser on way how to handle an errors when parsing.