并行解决 Promise

Question

I am trying to resolve the array of promises together. Not sure how to do it. Let me share the pseudo code for it.

async function sendNotification(user, notificationInfo) {
    const options = {
        method: 'POST',
        url: 'http://xx.xx.xx:3000/notification/send',
        headers:
    { 'Content-Type': 'application/json' },
        body:
    { notificationInfo, user },
        json: true,
    };
    console.log('sent');

     return rp(options);
}

I have wrapped the sendNotification method in another method which returns the promise of rp(request-promise) module.

Next i am pushing this sendNotification method in array of promise, something like this

const notificationWorker = [];
for (const key3 in notificationObject) {
                          if(notificationObject[key3].users.length > 0) {
                            notificationWorker.push(sendNotification(notificationObject[key3].users, notificationObject[key3].payload));  // problem is notification are going as soon as i am pushing in notificationWorker array.
                          }
                }
    // task 1 - send all notifications
const result = await Promise.all(notificationWorker); // resolving all notification promises together
    // task 2 - update values in db , after sending all notifications
    const result2 = await Promise.all(updateWorker); // update some values in db

In above code, my problem is notifications are going as soon as i am pushing it in notificationWorker array. I want all notifications to go together, when i run await Promise.all(notificationWorker)

Not sure, how to achieve what i am trying?

Answer 1

I understood the question partially, but then i feel this is difference between nodejs working concurrently and we trying to achieve parallelism, isn't that so.

Nodejs just switching between the tasks by, and not actually parallely doing it.Child Process might help you in that case.

So for eg. if you go through a snippet

 function done(i){ try{ return new Promise((resolve,reject)=>{ console.log(i); resolve("resolved " + i + "th promise"); }) }catch(e){ return null; } } let promises = []; for(let i=0;i < 100000; i++){ promises.push(done(i)); }

So console starts even when you dont call Promise.all right? this was your question but infact Promise.all should not suffice your thing, should go by spwaning child processes to achieve parallelism to some extent.

The point i am trying to make it you are potraying the question to do something like first generate array of promises and start all of them once when Promise.all is called but in my opinion Promise.all also will be running concurrently not giving you what you want to achieve.

Something like this - https://htayyar.medium.com/multi-threading-in-javascript-with-paralleljs-10e1f7a1cf32 || How to create threads in nodejs

Though most of these cases show up when we need to do a cpu intensive task etc but here we can achieve something called map reduce to distribute you array of users in parts and start that array to loop and send notifications.

All of the solutions, i am presenting is to achieve some kind of parallelism but i dont think sending array of huge amount of users would ever be done easily (with less resources - instance config etc) at same instant