Promise Retry Design Patterns

There are many good solutions mentioned and now with async/await these problems can be solved without much effort.

If you don't mind a recursive approach then this is my solution.

function retry(fn, retries=3, err=null) {
  if (!retries) {
    return Promise.reject(err);
  }
  return fn().catch(err => {
      return retry(fn, (retries - 1), err);
    });
}

Something a bit different ...

Async retries can be achieved by building a .catch() chain, as opposed to the more usual .then() chain.

This approach is :

  • only possible with a specified maximum number of attempts. (The chain must be of finite length),
  • only advisable with a low maximum. (Promise chains consume memory roughly proportional to their length).

Otherwise, use a recursive solution.

First, a utility function to be used as a .catch() callback.

var t = 500;

function rejectDelay(reason) {
    return new Promise(function(resolve, reject) {
        setTimeout(reject.bind(null, reason), t); 
    });
}

Now you can build .catch chains very concisely :

1. Retry until the promise resolves, with delay

var max = 5;
var p = Promise.reject();

for(var i=0; i<max; i++) {
    p = p.catch(attempt).catch(rejectDelay);
}
p = p.then(processResult).catch(errorHandler);

DEMO: https://jsfiddle.net/duL0qjqe/

2. Retry until result meets some condition, without delay

var max = 5;
var p = Promise.reject();

for(var i=0; i<max; i++) {
    p = p.catch(attempt).then(test);
}
p = p.then(processResult).catch(errorHandler);

DEMO: https://jsfiddle.net/duL0qjqe/1/

3. Retry until result meets some condition, with delay

Having got your mind round (1) and (2), a combined test+delay is equally trivial.

var max = 5;
var p = Promise.reject();

for(var i=0; i<max; i++) {
    p = p.catch(attempt).then(test).catch(rejectDelay);
    // Don't be tempted to simplify this to `p.catch(attempt).then(test, rejectDelay)`. Test failures would not be caught.
}
p = p.then(processResult).catch(errorHandler);

test() can be synchronous or asynchronous.

It would also be trivial to add further tests. Simply sandwich a chain of thens between the two catches.

p = p.catch(attempt).then(test1).then(test2).then(test3).catch(rejectDelay);

DEMO: https://jsfiddle.net/duL0qjqe/3/

All versions are designed for attempt to be a promise-returning async function. It could also conceivably return a value, in which case the chain would follow its success path to the next/terminal .then().


2. Pattern that keeps on retrying until the condition meets on the result (with delay and maxRetries)

This is an nice way to do this with native promises in a recursive way:

const wait = ms => new Promise(r => setTimeout(r, ms));

const retryOperation = (operation, delay, retries) => new Promise((resolve, reject) => {
  return operation()
    .then(resolve)
    .catch((reason) => {
      if (retries > 0) {
        return wait(delay)
          .then(retryOperation.bind(null, operation, delay, retries - 1))
          .then(resolve)
          .catch(reject);
      }
      return reject(reason);
    });
});

This is how you call it, assuming that func sometimes succeeds and sometimes fails, always returning a string that we can log:

retryOperation(func, 1000, 5)
  .then(console.log)
  .catch(console.log);

Here we're calling retryOperation asking it to retry every second and with max retries = 5.

If you want something simpler without promises, RxJs would help with that: https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/retrywhen.md

Tags:

Javascript

Node.Js

Promise

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