进行缓存的 Ajax 调用时如何避免竞争条件

Question

Since many public APIs such as GitHub public API has a request limit, so it makes sense for us to implement some cache mechanism to avoid unnecessary request calls. However I discovered that this might incur race condition.

I coded up an example to demonstrate the situation https://codesandbox.io/s/race-condition-9kynm?file=/src/index.js

Here I first implement a cachedFetch ,

const cachedFetch = (url, options) => {
  // Use the URL as the cache key to sessionStorage
  let cacheKey = url;
  let cached = sessionStorage.getItem(cacheKey);
  if (cached !== null) {
    console.log("reading from cache....");
    let response = new Response(new Blob([cached]));
    return Promise.resolve(response);
  }

  return fetch(url, options).then(async response => {
    if (response.status === 200) {
      let ct = response.headers.get("Content-Type");
      if (ct && (ct.includes("application/json") || ct.includes("text"))) {
        response
          .clone()
          .text()
          .then(content => {
            sessionStorage.setItem(cacheKey, content);
          });
      }
    }
    return response;
  });
};

It uses sessionStorage to cache the results.

And I am making the requests to Github API. The idea is simple, there is a Input and a p tag, and the Input has a event listener to listen for input changes and uses the input value to get the github user's name and the p will render the name on the page.

The race condition might occur in the following situation:

1. User types jack in the input field, since this is the first time the user types jack so the result is not cached. The request will be made to fetch this jack user's Github profile
2. Then the user types david in the input field, since this is also the first time the user types david so the result is not cached. The request will be made to fetch this david user's Github profile
3. Finally the user types jack in the input field for the second time, since the result is already in the cache. The no request will be made and we can read the user profile from sessionStorage and render the result immediately.

Then you can image that, if the second request, ie request to fetch david 's profile takes too long, user will see david end up being the final result rendered on the page even if his/her last search was for jack . This is because jack 's result got overridden by the david 's result which takes much longer to get back.

In my example, I used this function to simulate the user typing

async function userTyping() {
  sessionStorage.clear();
  inputEl.value = "jack";
  inputEl.dispatchEvent(new Event("input"));

  await sleep(100);
  inputEl.value = "david";
  inputEl.dispatchEvent(new Event("input"));

  await sleep(100);
  inputEl.value = "jack";
  inputEl.dispatchEvent(new Event("input"));
}

the sleep function is defined as

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));

Right now what I can think of is using debounce to avoid the situation when the user is typing too fast. However it doesn't solve the problem in the fundamental level.

Also we can use some global variable to keep track of the latest input value, and use that to check if the result we are about to render is from the latest input value. Somehow I just don't think this is an elegant solution to this problem.

Any suggestions are appreciated.

Answer 1

You can save the current e.target.value in a variable inside the input handler. Then, once the cachedFetch response comes back, check if the same value is still in the input field. Only set the input field if the values match.

(If the values don't match, like if the input is a , then b , then a , and it takes the b request a longer time to finish, then b will be stored in the cache, but it won't be displayed to the user)

Also, make sure to only display the result to the user when an error does not occur:

inputEl.addEventListener("input", e => {
  const { value } = e.target;
  if (value === "") {
    return;
  }
  const url = endpoint + value;
  cachedFetch(url)
    .then(response => response.json())
    .then((result) => {
      if (e.target.value === value) {
        resultContainer.innerHTML = result.name;
      }
    })
    .catch(errorHandler);
});

Answer 2

You might be able to use the AbortController. It's experimental and not added to all browsers yet (missing in IE).

https://developer.mozilla.org/en-US/docs/Web/API/AbortController

Create an AbortController instance.

const controller = new AbortController();
const signal = controller.signal;

And connect it to your fetch.

return fetch(url, { ...options, signal }).then(async response => ...

And then cancel the request when you return something from the cache.

if (cached !== null) {
  controller.abort();
  ...
}