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Retrying back-end service calls with back-off

26 Nov 2017 | javascript async promise generator

Recently I worked on a single-page ES6 application that calls many back-end services using the axios promise-based library.

Back off!

We wanted to implement the back-off pattern to retry service calls when they suffer transient failures. With back-off, the client does not retry immediately. Instead, it sleeps for a short delay before retrying again. If the same request continues to fail, the client sleeps for progressively longer delays before each retry.

There is the axios-retry plugin that adds retry capability to Axios but it works by counting the number of retries without providing any back-off delays.

A solution

We implemented retry with back-off by looping through an iterable of delay periods in milliseconds:

const retryAxios = async (delays, axiosFunc, ...axiosArgs) => {
  // Extract the iterator from the iterable.
  const iterator = delays[Symbol.iterator]();
  while (true) {
    try {
      // Always call the service at least once.
      return await axiosFunc(...axiosArgs);
    } catch (error) {
      const { done, value } = iterator.next();
      if (!done && isRetriable(error)) {
        await sleep(value);
      } else {
        // The error is not retriable or the iterable is exhausted.
        throw error;
      }
    }
  }
};

Notes:

  • delays is an ES6 Iterable. An array is a simple example.

  • A while loop is used so the service is called at least once. This requires the iterable to be unpacked manually so the done attribute of the object returned by the iterator’s next method can be checked after the service is called.

  • The sleep function is the usual promise-based mechanism to wait for a time period:

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

Transient failures

We consider network and server errors as transient, thus suitable to be retried:

// Network error rules from https://github.com/softonic/axios-retry.
const isNetworkError = error => (
  !error.response
  && error.code
  && Boolean(error.code)
  && error.code !== 'ECONNABORTED'
  && isRetryAllowed(error)
);
const isServerError = error => (
  error.response
  && error.response.status >= 500
  && error.response.status <= 599
);
const isRetriable = error => isNetworkError(error) || isServerError(error);

The isRetryAllowed function is from the is-retry-allowed module: it checks the error code against lists of values that can be retried and those that cannot.

Back-off implementations

A simple implementation of delays is an array, for example:

const delays = [50, 50, 100, 100, 200, 500, 1000, 1000, 1000, 1000];

This example provides ramp-up to repeated 1-second delays.

It is common to use exponentially-increasing delays between retries. This ExponentialDelays class returns an iterable of a specified number of delay times in milliseconds:

class ExponentialDelays {
  constructor(initialDelay, retryCount) {
    this.initialDelay = Math.max(1, initialDelay);
    this.retryCount = Math.max(0, retryCount);
  }
  // Iterator generator
  * iterator() {
    let delay = this.initialDelay;
    for (let retry = 0; retry < this.retryCount; retry += 1) {
      yield delay;
      delay *= 2;
    }
  }
  // Implement iterable protocol.
  [Symbol.iterator]() {
    return this.iterator();
  }
}

Some examples

  // GET with simple array of delays.
  const delays = [50, 50, 100, 100, 200, 200, 500, 1000, 1000, 1000];
  const { data } = await retryAxios(delays, axios.get, 'https://api.example.com/people/12345');

  // POST with exponential set of delays.
  const delays = new ExponentalDelays(10, 6);
  const { data } = await retryAxios(delays, axios.post, 'https://api.example.com/people', {
    firstName: 'Osbert',
    lastName: 'Sitwell',
  });

  // Using a custom axios instance as a function with 10 equally-spaced delays.
  const customAxios = axios.create({
   timeout: 30000,
   withCredentials: true,
   maxRedirects: 0,
  });
  const delays = new Array(10).fill(100);
  const { data } = await retryAxios(delays, customAxios, {
    method: 'put',
    url: 'http://api.example.com/people/12345',
    data: {
      firstName: 'Sacheverell',
      lastName: 'Sitwell',
    }
  });

A note on ESLint

The code for retryAxios shown here violates a number of default ESLint rules (no-await-in-loop, no-constant-condition and consistent-return). Comments to selectively disable those rules have been omitted here for clarity, but are present in the complete code in this Gist.

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