nodejs multi threading async parallel - node.js

Question

Currently i have a dashboard that lists a bunch of records in a table. users can select 1 record and hit execute and i send a AJAX POST request to my routes middleware which executes 3 functions inside async.waterfall and returns a 200 response back to my client if everything works correctly. this async waterfall usually takes about 40-55 seconds to finish executing (fn_1,fn_2 and fn_3) and works perfectly fine.
router.post('/url', function(req, res, next) {
try {
async.waterfall([
fn_1,
fn_2,
fn_3
], function (err, body) {
res.writeHead(200, {'Content-Type': 'application/json'});
res.end(JSON.stringify({"error":err, "result":body}));
});
function fn_1(callback) {
callback(null, response);
}
function fn_1(result, callback) {
callback(err, result);
}
function fn_2(result, callback) {
callback(null, result);
}
}
catch (err){
console.log(err)
}
});
But, If i were to give provision for users to select MULTIPLE records and send that as an array back to my route middleware. how can i execute multiple async.waterfall methods for each item in the array in parallel
i can run a loop and execute the waterfall inside the loop but it again will wait for each item to complete and only then start the next iteration. this is not what i want.
is this doable in node / express . whats the easiest way to achieve this ? or are there modules/plugins that can help solve this case ?

Answer 1

Here is an abbreviated version of your code and how it could be changed to suit your needs. If none of your calls need data from any of the other calls, you can just run them in parallel with promises and use Promise.all to capture the result.
function fn_1(callback) {
// See function fn_2 for structure
}
function fn_1(result, callback) {
// See function fn_2 for structure
}
function fn_2(result, callback) {
return new Promise(resolve, reject => {
resolve(result)
})
.then(d => {
// Instead of callbacks, use a "then"
// block/statement.
//
// Do something with D here.
})
}
Promise.all([fn_1(), fn_2(), fn_3()])
.then(v => {
// Do somthing with v;
})
.catch(e => {
// Do something with e
})

Answer 2

I tend to advocate the use of native Promises over libraries like async, however, since your already using async...
You can use parallel and map each item in the array to a waterfall handler e.g.
async.parallel(
myArray.map(val => cb => async.waterfall(fn_1, fn_2, fn_3, cb)
, (err, results) => {
// return consolidated response
})
You would need to rework your waterfall handlers to not send a response but instead just propagate any errors.
It should also be noted that parallel is only useful if you are infact running I/O bound code, if the code is anything like your example then you won't really gain anything from using parallel over something like async.each