Retries ​

Reties are a pretty common pattern for web application, particular operation can fail because of many reasons, for example because of connectivity problems on the client or short-term server unavailability. In this case, we can retry the request to get the data.

Farfetched provides a set of tools to make retries easy and flexible. Let's take a look at them.

Set-up retries ​

To retry a Query, or a Mutation we can use the retry operator. It takes an operation itself, number of retries and delay between them. After any error, it will retry the operation after the specified delay.

import { retry } from '@farfetched/core';

// let's retry our characterQuery
retry(characterQuery, {
  // up to 5 times
  times: 5,
  // with 500ms delay between attempts
  delay: 500,
});

This code works is pretty straightforward, after first failure of characterQuery, it calls it again after 500ms. If it fails again, it will call it again after 500ms and so on up yo 5 times. If it succeeded, it will stop retrying.

Filter retries ​

Sometimes, we want to retry only specific errors. For example, we want to retry only 500 errors, but not others. To do this, we can use the filter option of the retry operator.

import { isHttpErrorCode } from '@farfetched/core';

retry(characterQuery, {
  times: 5,
  delay: 500,
  // retry only 500 errors
  filter: isHttpErrorCode(500),
});

It is only the simplest way to use filter option. Actually, it could be used to disable retries by some external condition. For example, let's see how to allow retries only the particular page of the application.

retry(characterQuery, {
  times: 5,
  delay: 500,
  filter: characterRoute.$isOpened,
});

Also, we can combine the first and the second approaches to retry only 500 errors on the particular page.

retry(characterQuery, {
  times: 5,
  delay: 500,
  filter: {
    source: characterRoute.$isOpened,
    fn: ({ error, params }, isCharacterRouteOpened) =>
      isCharacterRouteOpened && isHttpErrorCode(500)({ error }),
  },
});

Dynamic delay ​

Not only filter option is sourced, you can formulate dynamic delay as well. For example, you can use linear or exponential back off to retry requests.

retry(characterQuery, {
  times: 5,
  // linear back off
  delay: ({ attempt }) => attempt * 50,
});

The most common cases for dynamic delay are linear or exponential back off. Farfetched provides a set of helpers to implement them.

import { linearDelay, exponentialDelay } from '@farfetched/core';

retry(characterQuery, {
  times: 5,
  delay: linearDelay(50),
});

retry(loginMutation, {
  times: 5,
  delay: exponentialDelay(50),
});

Alter parameters ​

Sometimes, we want to retry the same request with different parameters. For example, we want to retry the same request, but increased attempt parameter. To do this, we can use the mapParams option of the retry operator.

const characterQuery = createQuery({
  handler: async ({ id, attempt }) => {
    const response = await fetch(
      `https://rickandmortyapi.com/api/character/${id}?attempt=${attempt}`
    );

    return response.json();
  },
});

retry(characterQuery, {
  times: 5,
  delay: 500,
  mapParams: ({ params, error }, { attempt }) => ({
    ...params,
    attempt: attempt,
  }),
});

mapParams accepts a function that takes current parameters, occurred error and attempt number and returns new parameters. In this example, we just add attempt parameter to the current parameters.

API reference ​

You can find the full API reference for the retry operator in the API reference.