JS: handle multiple sequential failed async requests

Question

I'm using promises (with await). I have an async function that has to await an async request: an http request for example. The HTTP request can fail (timeout or other motivations) but I need to recall it until success or until a max number of attempts are done (let's say n attempts) and then continue the execution of the function. I wasn't able to find a clean and well organized way to do this. Below a pseudocode:

async function func(){
  //DO something before HTTP request
  try{
    let res = await http_request();
  } catch(e){
    //http request failed
    //WHAT TO DO HERE TO CALL AGAIN THE HTTP REQUEST until success??
    //or until max attempts == n?
  }
  //DO other stuff only after the http request succeeded
  return;
}

the idea would be to return at the end a promise which resolves if the http requests and the rest of the code succeeded or rejects if the http request attempts failed n times or other errors.

PS: the http request is an example but http_request() can be substituted with any other async function.

S.Davis · Accepted Answer · 2018-03-27 21:57:51Z

2

you could do a while loop that breaks once a successful request is made, and otherwise tries again. a counter can be used to limit the number of attemps.

async function func(){
  let counter = 0;
  while (counter < 100) {
    try{
      let res = await http_request();
      break;
    } catch(e){
      counter++
      continue;
    }
  }
  return;
}

answered Mar 27, 2018 at 21:57

S.Davis

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1 Comment

Michele Tuloski Furci Over a year ago

I was thinking to fancy stuff, but probably this is the cleanest and easiest solution :) I'll just need to handle different errors in the catch and continue the while only in case of some failures.

jfriend00 · Accepted Answer · 2018-03-27 21:58:03Z

You can just call your function again to retry and you can pass it a retry counter. You should also probably insert a short delay before retrying to avoid hammering a busy server.

function delay(t, v) {
   return new Promise(resolve => {
       setTimeout(resolve.bind(null, v), t);
   });
}

const kMaxAttempts = 10;
const kDelayBeforeRetry = 500;

async function func(cntr = 0){
  //DO something before HTTP request
  ++cntr;
  try{
    let res = await http_request();
    //DO other stuff only after the http request succeeded
    return finalValue;
  } catch(e){
    // test to see if max retries have been exceeded
    // also examine e to see if the error is retryable
    if (cntr > kMaxAttempts || e is not a retryable error) {
        throw e;
    }
    // retry after a short delay
    return delay(kDelayBeforeRetry, cntr).then(func);

  }
}

alex bennett · Accepted Answer · 2018-03-27 22:07:04Z

0

I would treat the request differently. You're trying to catch only an error. You also need to handle a failed response. Which ever request API you you are using, http_request() will most likely return a response and an error object. You can check for failed requests with something like response.status === 500. If the response has failed for whatever reason, you can then retry the request. I frequently see it done like this

http_request.GET(URL).then((response, error) => {
  if (error) // retry
  else if (response.status === 200 && response.body) // success
  else if (response.status === 500) // retry
})

As for the actual code, you can make a reusable request function that does the if / else error handling and takes a HTTP method and some options and data etc. and returns a promise of your data or error -- that is another common pattern.

answered Mar 27, 2018 at 22:07

alex bennett

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1 Comment

Michele Tuloski Furci Over a year ago

Yeah, of course I need to handle different responses and try again only in some cases.

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