allowDynamicBackends

The allowDynamicBackends() function is used to control whether or not Dynamic Backends should be allowed within this Fastly Compute Service.

By default, Dynamic Backends are disabled within a JavaScript application as it can be a potential avenue for third-party JavaScript code to send requests, potentially including sensitive/secret data, off to destinations that the JavaScript project was not intending, which could be a security issue.

Note: This feature is in disabled by default for Fastly Services. Please contact Fastly Support to request the feature be enabled on the Fastly Services which require Dynamic Backends.

Syntax

allowDynamicBackends(enabledOrConfig)

Parameters

• enabledOrConfig : boolean
  • Whether or not to allow Dynamic Backends

or

• enabledOrConfig : object
  • connectTimeout : number optional
    • Maximum duration in milliseconds to wait for a connection to this backend to be established.
    • If exceeded, the connection is aborted and a 503 response will be presented instead.
    • Throws a RangeError if the value is negative or greater than or equal to 2^32
  • firstByteTimeout : number optional
    • Maximum duration in milliseconds to wait for the server response to begin after a TCP connection is established and the request has been sent.
    • If exceeded, the connection is aborted and a 503 response will be presented instead.
    • Throws a RangeError if the value is negative or greater than or equal to 2^32
  • betweenBytesTimeout : number optional
    • Maximum duration in milliseconds that Fastly will wait while receiving no data on a download from a backend.
    • If exceeded, the response received so far will be considered complete and the fetch will end.
    • Throws a RangeError if the value is negative or greater than or equal to 2^32

Return value

undefined.

Examples

In this example an implicit Dynamic Backend is created when making the fetch request to https://www.fastly.com/ and the response is then returned to the client.

/// <reference types="@fastly/js-compute" />
import { allowDynamicBackends } from "fastly:experimental";
allowDynamicBackends(true);
async function app() {
  // For any request, return the fastly homepage -- without defining a backend!
  return fetch('https://www.fastly.com/');
}
addEventListener("fetch", event => event.respondWith(app(event)));