Throttle amount of promises open at a given time

Question

The following TypeScript performs each call to doSomething(action) one at a time. (Meaning the second item in the list does not get a call made until the first one is done).

async performActionsOneAtATime() {
    for (let action of listOfActions) {
        const actionResult = await doSomethingOnServer(action);
        console.log(`Action Done: ${actionResult}`);
    }
 }

This one will send all the requests to the server right away (without waiting for any responses):

async performActionsInParallel() {
    for (let action of listOfActions) {
        const actionResultPromise = doSomething(action);
        actionResultPromise.then((actionResult) => {
            console.log(`Action Done: ${actionResult}`);
        });
    }
}

But what I really need is a way to throttle them. Maybe have 10 or 20 calls open at a time. (One at at a time is too slow, but all 600 will overload the server.)

But I am having a hard time figuring this out.

Any suggestions on how I can throttle the number of calls to X open at a time?

(This question uses TypeScript, but I would be fine with an ES6 JavaScript answer.)

Answer 1

You can do this in one short function. (Returns values in order per Mulan's suggestion. Thanks!)

/**
 * Performs a list of callable actions (promise factories) so
 * that only a limited number of promises are pending at any
 * given time.
 *
 * @param listOfCallableActions An array of callable functions,
 *     which should return promises.
 * @param limit The maximum number of promises to have pending
 *     at once.
 * @returns A Promise that resolves to the full list of values
 *     when everything is done.
 */
function throttleActions(listOfCallableActions, limit) {
  // We'll need to store which is the next promise in the list.
  let i = 0;
  let resultArray = new Array(listOfCallableActions.length);

  // Now define what happens when any of the actions completes.
  // Javascript is (mostly) single-threaded, so only one
  // completion handler will call at a given time. Because we
  // return doNextAction, the Promise chain continues as long as
  // there's an action left in the list.
  function doNextAction() {
    if (i < listOfCallableActions.length) {
      // Save the current value of i, so we can put the result
      // in the right place
      let actionIndex = i++;
      let nextAction = listOfCallableActions[actionIndex];
      return Promise.resolve(nextAction()).then(result => {
        // Save results to the correct array index.
        resultArray[actionIndex] = result;
      }).then(doNextAction);
    }
  }

  // Now start up the original <limit> number of promises.
  // i advances in calls to doNextAction.
  let listOfPromises = [];
  while (i < limit && i < listOfCallableActions.length) {
    listOfPromises.push(doNextAction());
  }
  return Promise.all(listOfPromises).then(() => resultArray);
}

// Test harness:

function delay(name, ms) {
  return new Promise((resolve, reject) => setTimeout(() => {
    console.log(name);
    resolve(name);
  }, ms));
}

var ps = [];
for (let i = 0; i < 10; i++) {
  ps.push(() => {
    console.log(`begin ${i}`);
    return delay(`complete ${i}`, Math.random() * 3000);
  });
}

throttleActions(ps, 3).then(result => console.log(result));