How to throttle Promise.all() to 5 promises per second?

Question

I have a couple of items that I need to query a 3rd party API for and said API has a call limit of 5 calls per second. I need to somehow throttle my calls to the API to a maximum of 5 calls per second.

So far I've just used Promise.all() on an array of promises, where each promise sends a request to the API and resolves when the API responds with the HTTP status code 200 and rejects when it responds with some other status code. However, when I have more than 5 items in the array, I risk that the Promise.all() rejects.

How can I limit the Promise.all() call to 5 calls per second?

Answer 1

I hope this would help you.

And also to be said this would use Promise.all to resolve all requests and if you have a large list of queries, this would wait for all to resolve and may cause a lot waiting in your code to get all responses. And also if one of request rejects, Promise.all will reject.

I suggest if you don't need all results together it's better to use something else like lodash debounce or throttle or frameworks that handle this.

let items = [
    {name: 'item1'}, 
    {name: 'item2'}, 
    {name: 'item3'}, 
    {name: 'item4'}, 
    {name: 'item5'}, 
    {name: 'item6'}
];

// This is the api request that you send and return a promise
function apiCall(item) {
  return new Promise((resolve) => {
    setTimeout(() => resolve(item.name), 1000);
  })
}

new Promise((resolve) => {
  let results = [];

  function sendReq (itemsList, iterate, apiCall) {
    setTimeout(() => {
      // slice itemsList to send request according to the api limit
      let slicedArray = itemsList.slice(iterate * 5, (iterate * 5 + 5));
      result = slicedArray.map(item => apiCall(item));
      results = [...results, ...result];

      // This will resolve the promise when reaches to the last iteration
      if (iterate === Math.ceil(items.length / 5) - 1) {
          resolve(results);
      }
    }, (1000 * iterate)); // every 1000ms runs (api limit of one second)
  }

  // This will make iteration to split array (requests) to chunks of five items 
  for (i = 0; i < Math.ceil(items.length / 5); i++) {
    sendReq(items, i, apiCall);
  }
}).then(Promise.all.bind(Promise)).then(console.log);
// Use Promise.all to wait for all requests to resolve
// To use it this way binding is required

Answer 2

Using ES6 without libraries

export async function asyncForEach(array, callback) {
  for (let index = 0; index < array.length; index++) {
    await callback(array[index], index, array);
  }
}
export function split(arr, n) {
  var res = [];
  while (arr.length) {
    res.push(arr.splice(0, n));
  }
  return res;
}
export const delayMS = (t = 200) => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(t);
    }, t);
  });
};
export const throttledPromises = (
  asyncFunction,
  items = [],
  batchSize = 1,
  delay = 0
) => {
  return new Promise(async (resolve, reject) => {
    const output = [];
    const batches= split(items, batchSize);
    await asyncForEach(batches, async (batch) => {
      const promises = batch.map(asyncFunction).map(p => p.catch(reject));
      const results = await Promise.all(promises);
      output.push(...results);
      await delayMS(delay);
    });
    resolve(output);
  });
};

Answer 3

You can use the concurrency option in bluebird if you're not too worried about resolving promises sequentially.

The below would process 5 queries at a time only.

const Promise = require('bluebird');

const buildQueries = (count) => {
  let queries = [];

  for(let i = 0; i < count; i++) {
    queries.push({user: i});
  };

  return queries;
};

const apiCall = (item) => {
  return new Promise(async (resolve, reject) => {
    await Promise.delay(1000);
    resolve(item.user);
  });
};

const queries = buildQueries(20);

Promise.map(queries, async query => {
  console.log( await apiCall(query) );
}, {concurrency: 5});