node js websockets used with api - node.js

Question

I have a node js API built with express. In one of my http endpoints, I would like to accept the request and send it over websockets to another server, and then return a reply. I'm using the ws library. The problem is that the websockets communication is not synchronous, so I don't know how to return an answer to the API client. Anyway, this is what I tried but it's not complete:
const express = require('express');
const app = express();
const WebSocket = require('ws');
const uuid = require('uuid');
const bodyParser = require('body-parser');
const ws = new WebSocket('ws://localhost:7465/');
app.use(bodyParser.json({ limit: '50mb' }));
app.use(bodyParser.urlencoded({ extended: false }));
ws.on('open', function open() {
console.log('connected');
});
ws.on('close', function close() {
console.log('disconnected');
});
ws.on('message', function incoming(data) {
console.log('Got data from server:');
console.log(data);
});
app.post('/my-http-endpoint', function (req, res) {
var payload = JSON.stringify({ body: req.body, requestID: uuid.v4() });
ws.send(Buffer.from(payload));
// How to send a reply?
})
app.listen(1337);

Answer 1

You will need to introduce message IDs. Generate them by counting up a number.
Add the following dictionary to your code:
let nextMessageId = 1;
const responseCallbacks = {};
When you send something (in the app.post callback in your case), do the following:
const messageId = obtainFreeMessageId(); // see below
ws.send(Buffer.from({ messageId, payload }));
responseCallbacks[messageId] = function(data) {
// make use of the response message data here
};
On message do the following (the response must contain messageId as well):
const messageId = message.messageId; // message has a message ID
const responseCallback = responseCallbacks[messageId];
if(responseCallback) {
responseCallback(message.data); // message contains your data
delete responseCallbacks[messageId];
}
Helper function for message ID generation which helps prevent possible overflow:
const MaximumMessageId = 0xFFFFFFFFFFFF; // 48-bit integer (64-bit is too much for javascript, 32-bit is a little bit too stingy)
function obtainFreeMessageId() {
const messageId = nextMessageId;
nextMessageId++;
if(nextMessageId > MaximumMessageId) {
nextMessageId = 1;
}
return messageId;
}
Consider setting a timeout which calls the response callback after a certain amount of time if you want to make sure the request comes to an end. Clear the timeout in the response callback.