Simple fetch limiter (Part 2)

#TypeScript #async #fetch #rate-limit #delay #event-loop

Leonardo Dicaprio Reaction GIF

It is very possible that you had such a reaction while reading my previous post, especially at the part where I mentioned that moving the delay to the bottom of the loop was a mistake. The move at the bottom was an attempt to address an issue that becomes obvious when using large values for the loop interval, e.g., 30 seconds.

const limiter = new HTTPLimiter({
  intervalMilliseconds: 30_000,
  requestsPerInterval: 2,
});

console.log("start");
for (let i = 10; i; i--) {
  limiter
    .fetch(`https://jsonplaceholder.typicode.com/todos/${i}`)
    .then((response) => response.json())
    .then((json) => console.log(json));
}

We’ll see start being printed in the console, and only after the 30 seconds delay (sleep), we will see the requests being fired, and the responses being printed as well. Let’s move it back to the bottom of the loop.

async #loop() {
  while (true) {
    const entities = this.#queue.splice(0, this.#options.requestsPerInterval);
    if (!entities.length) break;

    for (const entity of entities) {
      fetch(entity.input, entity.init)
        .then(entity.resolve)
        .catch(entity.reject);
    }

    await delay(this.#options.intervalMilliseconds);
  }
}

We are now back at the issue that all the items are being consumed immediately. Go back to the previous article for an explanation why that happens. In any case, the fix is easy. All we have to do is to allow “the producer” (the main thread) to produce all the requests it needs. For instance, allow our todos fetch loop to finish iterating over all the requests. We can easily achieve this by deferring the start of processing the items to the next event loop cycle using setTimeout.

fetch(input: FetchInput, init?: RequestInit): Promise<Response> {
  const promise = new Promise<Response>((resolve, reject) => {
    this.#queue.push({
      input,
      init,
      resolve,
      reject,
    });
  });

  if (this.#queue.length === 1) setTimeout(() => this.#loop(), 0);

  return promise;
}

Now everything works as expected, and the code remained simple.

/Elvis