NodeJS Worker Threads and AWS Lambda: Inconsistent Output - node.js

Question

I am trying to execute the following AWS lambda:
index.js:
const test = require("test");
exports.handler = async (event) => {
await test.main();
};
test.js:
const {Worker} = require("worker_threads");
const main = () => {
let num1 = 10;
let num2 = 20;
const worker = new Worker("./worker.js", {workerData: {num1, num2}});
worker.on("message", result => {
console.log(`${num1} + ${num2} is ${result}`);
});
worker.on("error", error => {
console.log(error);
});
worker.on("exit", exitCode => {
console.log(exitCode);
});
console.log("I am here!");
}
exports.main = main;
worker.js:
const {parentPort, workerData} = require("worker_threads");
const getSum = (num1, num2) => {
return num1 + num2;
}
parentPort.postMessage(getSum(workerData.num1, workerData.num2));
When I run the same program on my laptop it is working fine. I see the output of the worker thread consistently.
Output on my laptop:
❯ node index.js
I am here!
10 + 20 is 30
0
Output on the lambda:
START RequestId: c178d74b-da57-4765-9fa7-77d3fc83d645 Version: $LATEST
2021-08-31T14:33:37.353Z c178d74b-da57-4765-9fa7-77d3fc83d645 INFO I am here!
END RequestId: c178d74b-da57-4765-9fa7-77d3fc83d645
REPORT RequestId: c178d74b-da57-4765-9fa7-77d3fc83d645 Duration: 2.12 ms Billed Duration: 3 ms Memory Size: 10240 MB Max Memory Used: 108 MB
When I run the lambda, the output is very random. Sometimes I see the output of the worker thread and other times I don't.
Why is there a difference in execution of the program on AWS lambda and on my laptop?

Answer 1

You don't await for the worker async operation to complete in the test.js file. Try adding a promise that resolves when worker finishes. Like this:
const { Worker } = require("worker_threads");
const main = async () => {
let num1 = 10;
let num2 = 20;
const worker = new Worker("./worker.js", { workerData: { num1, num2 } });
worker.on("message", (result) => {
console.log(`${num1} + ${num2} is ${result}`);
});
worker.on("error", (error) => {
console.log(error);
});
console.log("I am here!");
// Awaiting for the worker to finish here
return new Promise((resolve) => {
worker.on("exit", (exitCode) => {
console.log(exitCode);
resolve();
});
});
};
exports.main = main;
Alternatively, you can set context.doNotWaitForEmptyEventLoop = false but it's not recommended as it's error-prone and hard to debug.