How to handle a long request with Express - node.js

Question

I'm working on a simple function I have for a specific GET request triggered in the browser. The objective of this request is to make multiple queries to a mongodb (mongoose) database and then perform some calculation and structure formating on the results to send it back to the browser.
The only problem is that everything takes too long and it results in an error in the browser:
net::ERR_EMPTY_RESPONSE
to give an example of part of the function I'm trying to build here it goes:
async function getPriceByMake(makes, id) {
return new Promise(async (resolve, reject) => {
let pMakes = {};
const makesArr = Object.keys(makes);
for (let i = 0; i < makesArr.length; i++) {
console.log('Getting the docs ... ' + Math.round(i/makesArr.length*100) + '%')
const currMake = makesArr[i];
pMakes[currMake] = {};
const modelsArr = Object.keys(makes[currMake]);
for (let j = 0; j < modelsArr.length; j++) {
const currModel = modelsArr[j];
await Listing.find({ catFrom: id, model: currModel }, 'year asking', (err, docs) => {
if (docs.length > 1) {
pMakes[currMake][currModel] = [docs];
} else {
pMakes[currMake][currModel] = {};
}
});
}
}
resolve(pMakes);
});
}
In this function, if I leave the async / await out, I get an empty {} on the other end. Which is obviously not the objective.
I've been searching the web a little and was able to find an article pointing to this scheme:
Browser:
Initiates request
displays progress
Show result
WebServer:
Submit event
Checks for completion
Return result
BackEndApp:
Picks up event
Runs task
Returns results
My question is the following:
How can I do that with NodeJS and Express?

Answer 1

In this code:
for (let j = 0; j < modelsArr.length; j++) {
const currModel = modelsArr[j];
await Listing.find({ catFrom: id, model: currModel }, 'year asking', (err, docs) => {
if (docs.length > 1) {
pMakes[currMake][currModel] = [docs];
} else {
pMakes[currMake][currModel] = {};
}
});
}
Your await isn't working because you're passing a callback to Listing.find(). When you do that, it does NOT return a promise and therefore the await does nothing useful. You get the empty response because the await doesn't work and thus you call resolve() before there's any actual data there.
Change the code to this:
for (let j = 0; j < modelsArr.length; j++) {
const currModel = modelsArr[j];
let docs = await Listing.find({ catFrom: id, model: currModel }, 'year asking');
if (docs.length > 1) {
pMakes[currMake][currModel] = [docs];
} else {
pMakes[currMake][currModel] = {};
}
}
And, then the await will work properly.
You also should remove the return new Promise() wrapper. You don't want that. Just make the function async and use await and it will already return a promise.
Here's your function with the unnecessary promise wrapper removed:
async function getPriceByMake(makes, id) {
let pMakes = {};
const makesArr = Object.keys(makes);
for (let i = 0; i < makesArr.length; i++) {
console.log('Getting the docs ... ' + Math.round(i/makesArr.length*100) + '%')
const currMake = makesArr[i];
pMakes[currMake] = {};
const modelsArr = Object.keys(makes[currMake]);
for (let j = 0; j < modelsArr.length; j++) {
const currModel = modelsArr[j];
let docs = await Listing.find({ catFrom: id, model: currModel }, 'year asking');
if (docs.length > 1) {
pMakes[currMake][currModel] = [docs];
} else {
pMakes[currMake][currModel] = {};
}
}
}
return pMakes;
}
Then, keep in mind that whatever code sends your actual response needs to use .then() or await when calling this async function in order to get the final result.
Your best bet to speed up this code would be to refactor either your queries or your database structure or both to not have to do N * M separate queries to get your final result. That's likely where your slowness is coming from. The biggest performance gains will probably come from reducing the number of queries you have to run here to far fewer.
Depending upon your database configuration and capabilities, it might speed things up to run the inner loop queries in parallel as shown here:
async function getPriceByMake(makes, id) {
let pMakes = {};
const makesArr = Object.keys(makes);
for (let i = 0; i < makesArr.length; i++) {
console.log('Getting the docs ... ' + Math.round(i/makesArr.length*100) + '%')
const currMake = makesArr[i];
pMakes[currMake] = {};
const modelsArr = Object.keys(makes[currMake]);
await Promise.all(modelsArr.map(async currModel => {
let docs = await Listing.find({ catFrom: id, model: currModel }, 'year asking');
if (docs.length > 1) {
pMakes[currMake][currModel] = [docs];
} else {
pMakes[currMake][currModel] = {};
}
}));
}
return pMakes;
}