Hedge Hedge - 2 months ago 10
Javascript Question

Parallelize execution of Promise containing co wrapped generator

I'm creating seed data for my database. The seed function is co-wrapped generator (https://www.npmjs.com/package/co):

createSeedData () {
co(function * () {
[...]
}
}


At one point I'm trying to create 40 auctions with one bid each.
Up until now I had a for-loop like this:

for (let i = 0; i <= 40; i++) {
let auction = yield auction.create({
[...]
}) // Async creation of model and persisting it in db
yield bid.create({
auction: auction.id
[...]
})
}


Now I'd like to create the auction in parallel but I can't quite get it to work.

co
supports parallel execution when yield is used with arrays.

However I still need to create first an invoice, then a bid for it using yield.

I tried wrapping the above in a Promise and an inner co-wrapped generator function but it just never finishes execution.

How can I solve this problem? (If possible without using more packages)

Here is my attempt with the inner generator:

let promises = []
for (let i = 0; i <= 40; i++) {
promises.push(new Promise(
co(function *(resolve, reject) {
let auction = yield auction.create({
[...]
}) // Async creation of model and persisting it in db

yield bid.create({
auction: auction.id
[...]
})
)
resolve()
))
}
yield invoicePromises

Answer

There's no need to call new Promise (and it doesn't work because you never call resolve or reject). Just omit that part, co() already returns a promise:

let promises = [];
for (let i = 0; i <= 40; i++) {
  promises.push( co(function *() {
    let auction = yield auction.create({
      [...]
    });  // Async creation of model and persisting it in db
    yield bid.create({
      auction: auction.id
      [...]
    });
  ));
}
yield invoicePromises;

If you don't want to use an inner generator, you can do the chaining with plain promises as well:

let promises = [];
for (let i = 0; i <= 40; i++) {
  promises.push(
    auction.create({
      [...]
    }).then(auction =>
      bid.create({
        auction: auction.id
        [...]
      });
    )
  );
}
yield Promise.all(invoicePromises);