Nodejs and Express, use res.send() from a worker thread

Question

In Nodejs, using Express as a server, I offload a heavy computation onto a worker thread.

From the main application, I call the worker thread like this:

// file: main.js

const { Worker } = require("worker_threads");

function runService(fileName, workerData) {
    return new Promise((resolve, reject) => {
        const worker = new Worker(fileName, { workerData });
        worker.on("message", resolve);
        worker.on("error", reject);
        worker.on("exit", code => {
            if (code !== 0)
                reject(new Error(`Worker stopped with exit code ${code}`));
        });
    });
}

router.get("/some_url", async function(req, res) {
    const result = await runService(
        "./path/to/worker.js",
        { query: req.query, user: req.user } // using { req } causes an error
    );
});

The worker looks like this:

// file: worker.js

const { workerData, parentPort } = require('worker_threads');
const { query, user } = workerData;

async function run() {
    const result = await generateLotsOfData(query, user);

    parentPort.postMessage(result);

    // What I would like to do here (doesn't work): res.send(result);
}

The worker generates a huge amount of data and "postMessage" causes a server error.

Is there a way to send this data from the worker thread directly to the client, using res.send() or something alike?
(instead of using postMessage and then sending from the main thread)?

Answer 1

This sample of code works :

1. create worker
2. create event handler
3. sendMessage --> wrapperWorkerThreadBigComputing
4. sendBack message on our main thread
5. emit server response

Api routes :

app.get('/bigComputingWithWorker', (req, res) => {
     const worker = new Worker(`${__dirname}/src/wrapperWorkerThreadBigComputing.js`);
    res.set('Content-Type', 'text/html');
      worker.once("message", count => {
        res.status(200).send(`The final count :${count}`);
    });
    worker.once("error", err => {
        console.error(err);
        res.status(400).send(`error worker thread`);
    });
     worker.postMessage({coucou : 'john'});
});

wrapperWorkerThreadBigComputing

const { parentPort } = require('node:worker_threads');

console.log("* worker created");
parentPort.once('message', (message) => {
      let big = bigComputing();
      parentPort.postMessage(big);
})

function bigComputing() {
    let i = 0;
    console.log("* start big computing")
    for (i = 0; i < 300000000; i++) {
    }
    return i;
}