Limit Rate

mediumcodingJavaScriptFunctionsConcurrency~25 min

Asked at Stripe, Google, Amazon

Overview

Implement limitRate(fn, maxCalls, interval) that limits fn to at most maxCalls invocations per interval ms. Extra calls are queued.

Examples

const limited = limitRate(apiFetch, 3, 1000);
// At most 3 calls per second, rest queued

Solution

Reveal solution

function limitRate(fn, maxCalls, interval) {
  const queue = [];
  let calls = 0;
  let timer = null;
  function process() {
    while (calls < maxCalls && queue.length > 0) {
      calls++;
      const { args, resolve } = queue.shift();
      resolve(fn(...args));
    }
    if (!timer && calls > 0) {
      timer = setTimeout(() => { calls = 0; timer = null; process(); }, interval);
    }
  }
  return function(...args) {
    return new Promise(resolve => { queue.push({ args, resolve }); process(); });
  };
}

limit-rate.js

Limit Rate

mediumcodingJavaScriptFunctions

Overview

Implement limitRate(fn, maxCalls, interval) that limits fn to at most maxCalls invocations per interval ms. Extra calls are queued.

Examples

const limited = limitRate(apiFetch, 3, 1000);
// At most 3 calls per second, rest queued

Solution

Reveal solution

function limitRate(fn, maxCalls, interval) {
  const queue = [];
  let calls = 0;
  let timer = null;
  function process() {
    while (calls < maxCalls && queue.length > 0) {
      calls++;
      const { args, resolve } = queue.shift();
      resolve(fn(...args));
    }
    if (!timer && calls > 0) {
      timer = setTimeout(() => { calls = 0; timer = null; process(); }, interval);
    }
  }
  return function(...args) {
    return new Promise(resolve => { queue.push({ args, resolve }); process(); });
  };
}

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