@dotcom-reliability-kit/fetch-error-handler

Properly handle fetch errors and avoid a lot of boilerplate in your app. This module is part of FT.com Reliability Kit.

[!WARNING] This package is in beta and hasn't been tested extensively in production yet. Feel free to use, and any feedback is greatly appreciated.

• Usage
  • Wrap the fetch function
  • Handle errors with .then
  • Handle the response object
  • Errors thrown
    • Client errors
    • Server errors
    • DNS errors
    • Abort and timeout errors
    • Socket errors
    • Unknown errors
  • Creating your own handler
  • createFetchErrorHandler configuration options
    • options.upstreamSystemCode
• Contributing
• License

Usage

Install @dotcom-reliability-kit/fetch-error-handler as a dependency:

npm install --save @dotcom-reliability-kit/fetch-error-handler

Include in your code:

import { handleFetchErrors } from '@dotcom-reliability-kit/fetch-error-handler';
// or
const { handleFetchErrors } = require('@dotcom-reliability-kit/fetch-error-handler');

You can use this function with any fetch call to throw appropriate errors based on the HTTP status code that you get back.

There are several ways to use it, as long as it is awaited and is called with either a Response object or a Promise that resolves to a Response.

Some of the options below result in more errors being caught, you can weigh this up when implementing in your own code.

In all of the APIs below, if the response ok property is false, i.e. when the status code is 400 or greater, then errors will be thrown.

[!WARNING] If you're using node-fetch then it's important to read the body of the request because of a known memory leak. If an error is thrown then we automatically drain the response body stream but, if the request is successful, you'll need to do this yourself.

Wrap the fetch function

This is the recommended API as this will allow you to handle the most errors (even DNS and timeout errors) correctly:

const response = await handleFetchErrors(
    fetch('https://httpbin.org/status/500')
);

You must not await the fetch call itself if you want to handle DNS and timeout errors. This is safe to do and will not result in unhandled promise rejections – handleFetchErrors takes care of them all.

Handle errors with .then

This API allows you to handle most errors based on the HTTP response, but it will not allow you to handle errors which occur before a valid response is returned, e.g. DNS or timeout errors.

const response = await fetch('https://httpbin.org/status/500').then(handleFetchErrors);

Handle the response object

This API is for when you already have an HTTP response object, but it will not allow you to handle errors which occur before a valid response is returned, e.g. DNS or timeout errors.

const response = await fetch('https://httpbin.org/status/500');
await handleFetchErrors(response);

Errors thrown

We throw different errors depending on the status code we get back from the fetch call.

Client errors

If the URL you fetched responds with a status code in the range of 400–499 then this normally indicates that something is wrong with the current system. Maybe we're sending data in an invalid format or our API key is invalid. For this we throw a generic 500 error to indicate an issue with our system. It'll be an HTTPError. This error will have the following properties to help you debug:

error.statusCode // 500
error.code // FETCH_CLIENT_ERROR
error.data.upstreamUrl // The URL that was fetched
error.data.upstreamStatusCode // The status code that the URL responded with

Server errors

If the URL you fetched responds with a status code in the range of 500–599 then this indicates something is wrong with the upstream system. For this we can output an UpstreamServiceError and attribute it to the system we're fetching from. This error will have the following properties to help you debug:

error.statusCode // 502
error.code // FETCH_SERVER_ERROR
error.data.upstreamUrl // The URL that was fetched
error.data.upstreamStatusCode // The status code that the URL responded with

DNS errors

If the hostname of the URL you fetched cannot be resolved, a DNS error will be thrown, it'll be an OperationalError. This error will have the following properties to help you debug:

error.code // FETCH_DNS_LOOKUP_ERROR
error.cause // The underlying DNS error that was caught

[!IMPORTANT] This type of error will only be thrown if you use the "wrap the fetch function" API.

Abort and timeout errors

If the request times out or is aborted via AbortSignal, or the non-standard timeout option in node-fetch is used, then we throw an OperationalError. This error will have the following properties to help you debug:

error.code // FETCH_ABORT_ERROR or FETCH_TIMEOUT_ERROR
error.cause // The underlying abort or timeout error that was caught

[!IMPORTANT] This type of error will only be thrown if you use the "wrap the fetch function" API.

Socket errors

If the connection is closed early by the server then we throw an UpstreamServiceError. This error will have the following properties to help you debug:

error.code // FETCH_SOCKET_HANGUP_ERROR
error.cause // The underlying socket error that was caught

[!IMPORTANT] This type of error will only be thrown if you use the "wrap the fetch function" API.

Unknown errors

If the URL you fetched responds with an ok property of false and a status code outside of the 400–599 range, then it's unclear what's happened but we reject with an error anyway to make sure we're able to debug. We output an HTTPError:

error.statusCode // 500
error.code // FETCH_UNKNOWN_ERROR
error.data.upstreamUrl // The URL that was fetched
error.data.upstreamStatusCode // The status code that the URL responded with

Creating your own handler

You can customise the thrown errors by creating your own fetch handler and passing in options.

Include in your code:

import { createFetchErrorHandler } from '@dotcom-reliability-kit/fetch-error-handler';
// or
const { createFetchErrorHandler } = require('@dotcom-reliability-kit/fetch-error-handler');

Create and use your own handler (the handler supports all the same usage methods as outlined here):

const handleFetchErrors = createFetchErrorHandler({
    upstreamSystemCode: 'httpbin'
});
const response = await handleFetchErrors(fetch('https://httpbin.org/status/500'));

createFetchErrorHandler configuration options

Config options can be passed into the createFetchErrorHandler function to change the behaviour of the handler.

options.upstreamSystemCode

Attribute any fetch errors to a given Biz Ops system. This allows you to easily spot in your logs when an upstream system is the cause of an error. This must be a String and a valid system code.

const handleFetchErrors = createFetchErrorHandler({
    upstreamSystemCode: 'next-navigation-api'
});

When this is set, any errors thrown by handleFetchErrors will have a relatesToSystems property which includes the given system code.

Contributing

See the central contributing guide for Reliability Kit.

License

Licensed under the MIT license. Copyright © 2023, The Financial Times Ltd.