javascript - Retry asyncawait try and catch block three times before responding with an error - Stack Overflow

admin管理员组

文章数量:1344942

Found similar issues, but none of which answered my question. I need to try to run an async function three times before throwing an error. Currently doing it with a while loop doing, which doesn't feel right to begin with. It is all happening in an express route. This is what my code currently looks like:

const MAX_NUMBER_OF_TRIES = 3
router.post('/', async function (req, res) {
  let tries = 0
  while (true) {
    try {
      await myAsyncFunction(body)
      res.json({ success: true, message: 'Done!' })
    } catch (err) {
      if (tries >= MAX_NUMBER_OF_TRIES) {
        res.json({ success: false, message: err && err.message })
      }
    }
    tries++
  }
}

One of the issues is that when I do this I always get the following error:

UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

This does not appear without the loop. So I suppose the question is, how do I limit these calls to three tries before responding with the error? Is this the correct path or is there another way to do it? And why am I getting issues with the headers?

Found similar issues, but none of which answered my question. I need to try to run an async function three times before throwing an error. Currently doing it with a while loop doing, which doesn't feel right to begin with. It is all happening in an express route. This is what my code currently looks like:

const MAX_NUMBER_OF_TRIES = 3
router.post('/', async function (req, res) {
  let tries = 0
  while (true) {
    try {
      await myAsyncFunction(body)
      res.json({ success: true, message: 'Done!' })
    } catch (err) {
      if (tries >= MAX_NUMBER_OF_TRIES) {
        res.json({ success: false, message: err && err.message })
      }
    }
    tries++
  }
}

One of the issues is that when I do this I always get the following error:

UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

This does not appear without the loop. So I suppose the question is, how do I limit these calls to three tries before responding with the error? Is this the correct path or is there another way to do it? And why am I getting issues with the headers?

• javascript
• node.js
• express
• async-await

Share Improve this question Follow asked Mar 18, 2019 at 14:47 theJulstheJuls 7,4901919 gold badges9696 silver badges182182 bronze badges 5

• 3 When does the loop end? Hint: never. – tkausl Commented Mar 18, 2019 at 14:50
• learnrxjs.io/operators/error_handling/retry.html May be the rxjs retry operator will do for you – Sergej Commented Mar 18, 2019 at 14:51
• to handle the error, you must call the catch function next ot your async function --> myAsyncFunction(body).catch(x => {}); – Sergej Commented Mar 18, 2019 at 14:52
• The error is about setting the headers, but your code is not related to the error. – Sergej Commented Mar 18, 2019 at 15:01
• @Sergej Not necessary to catch it like that, since myAsyncFunction is already awaited. Also, res.json sets content and headers, which is why it causes the error if it's called twice. – GolezTrol Commented Mar 18, 2019 at 15:03

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 5

Yes, that while loop looks sketchy. You never stop it. Just use a normal loop instead. Also you will need to break when you got a response.

const MAX_NUMBER_OF_TRIES = 3
router.post('/', async function (req, res) {
  var message;
  for (let tries = 0; tries < MAX_NUMBER_OF_TRIES; tries++) {
    try {
      await myAsyncFunction(body)
      res.json({ success: true, message: 'Done!' })
      return
//    ^^^^^^
    } catch (err) {
      message = err && err.message;
      console.log(`Try ${tries}: ${err}`)
    }
  }
  res.json({ success: false, message })
}

---

An alternative to this little weird control flow is to use recursion, which is well suited for retrying:

async function getResult(body, retries) {
  try {
    await myAsyncFunction(body)
    return {success: true, message: 'Done'}
  } catch(err) {
    if (retries > 0)
      return getResult(body, retries-1)
    else
      return {success: false, message: err && err.message}
  }
}

const MAX_NUMBER_OF_TRIES = 3
router.post('/', async function (req, res) {
  const RETRIES = MAX_NUMBER_OF_TRIES - 1
  res.json(await getResult(body, RETRIES))
})

You should return after res.json({ success: false, message: err && err.message }) like this:

...
res.json({ success: false, message: err && err.message })
return
...

Move the retry logic to the async function, something like the below This just an example of how it can be achieved, adjust it to your need.

router.post('/', async function(req, res) {
  try {
    var result = await asyncFunc(3 /* retries */);
    res.send(result);
  } catch(ex)
  {
    res.send(ex);
  }
})

in your async function, you can implement something like

async asyncFunc(retries)
{
  for(var i = 0; i < retries; i++)
  {
    var result = await doSomething();
    if(result)
    {
      // break the loop
      return result;
    }
  }

   // if we land here, so the number of retries was exceeded
   throw Error("...");
}

The async-retry package will do this for you; the amount of code actually ends up similar, but it's got a bit more capability and is less fiddly to get right.

For example:

const retry = require('async-retry')

try {
  await retry(async () => {
    await myAsyncFunction(body)
    res.json({ success: true, message: 'Done!' })
  }, {
    retries: 3,
  }) 
} catch (err) => 
{
   res.json({ success: false, message: err && err.message })
}

The outer catch handles the case where it retries three times and escapes the retry block. You can also set an onError function which has logic to be executed for each error (such as logging).

本文标签： javascriptRetry asyncawait try and catch block three times before responding with an errorStack Overflow