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Version: 1.12.0

Backend()

The Backend constructor lets you dynamically create new Fastly Backends for your Compute@Edge service.

Dynamically creating new Fastly Backends is disabled by default for Fastly Services. Please contact Fastly Support to request the feature be enabled on the Fastly Services which require Dynamic Backends.

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.

To enable Dynamic Backends the application will need to explicitly allow Dynamic Backends via:

import { allowDynamicBackends } from "fastly:experimental";
allowDynamicBackends(true);

Note: Can only be used when processing requests, not during build-time initialization.

Syntax

new Backend(backendConfiguration)

Note: Backend() can only be constructed with new. Attempting to call it without new throws a TypeError.

Parameters

  • backendConfiguration

    • : An Object which contains all the configuration options to apply to the newly created Backend.

      • name : string
        • The name of the backend.
        • The name has to be between 1 and 254 characters inclusive.
        • The name can be whatever you would like, as long as it does not match the name of any of the static service backends nor match any other dynamic backends built during a single execution of the application.
        • Throws a TypeError if the value is not valid. I.E. The value is null, undefined, an empty string or a string with more than 254 characters.
      • target : string
        • A hostname, IPv4, or IPv6 address for the backend as well as an optional port.
        • The target has to be at-least 1 character.
        • Throws a TypeError if the value is not valid. I.E. Is null, undefined, an empty string, not a valid IP address or host, or is the string ::
      • hostOverride : string optional
        • If set, will force the HTTP Host header on connections to this backend to be the supplied value.
        • Throws a TypeError if the value is an empty string.
      • 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
      • useSSL : boolean optional
        • Whether or not to require TLS for connections to this backend.
      • tlsMinVersion : 1 | 1.1 | 1.2 | 1.3 optional
        • Minimum allowed TLS version on SSL connections to this backend.
        • If the backend server is not able to negotiate a connection meeting this constraint, a 503 response will be presented instead.
        • Throws a RangeError if the value is not 1, 1.1, 1.2, or 1.3
      • tlsMaxVersion : 1 | 1.1 | 1.2 | 1.3 optional
        • Maximum allowed TLS version on SSL connections to this backend.
        • If the backend server is not able to negotiate a connection meeting this constraint, a 503 response will be presented instead.
        • Throws a RangeError if the value is not 1, 1.1, 1.2, or 1.3
      • certificateHostname : string optional
        • Define the hostname that the server certificate should declare.
        • Throws a TypeError if the value is an empty string.
      • caCertificate : string optional
        • The CA certificate to use when checking the validity of the backend.
        • Throws a TypeError if the value is an empty string.
      • ciphers : string optional
        • List of OpenSSL ciphers to support for connections to this origin.
        • If the backend server is not able to negotiate a connection meeting this constraint, a 503 response will be presented instead.
        • List of ciphers supported by Fastly.
        • Throws a TypeError if the value is an empty string.
      • sniHostname : string optional
        • The SNI hostname to use on connections to this backend.
        • Throws a TypeError if the value is an empty string.

Return value

A new Backend object.

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)));

In this example an explicit Dynamic Backend is created and supplied to the fetch request, the response is then returned to the client.

/// <reference types="@fastly/js-compute" />
import { allowDynamicBackends } from "fastly:experimental";
import { Backend } from "fastly:backend";
allowDynamicBackends(true);
async function app() {
// For any request, return the fastly homepage -- without defining a backend!
const backend = new Backend({
name: 'fastly',
target: 'fastly.com',
hostOverride: "www.fastly.com",
connectTimeout: 1000,
firstByteTimeout: 15000,
betweenBytesTimeout: 10000,
useSSL: true,
sslMinVersion: 1.3,
sslMaxVersion: 1.3,
});
return fetch('https://www.fastly.com/', {
backend // Here we are configuring this request to use the backend from above.
});
}
addEventListener("fetch", event => event.respondWith(app(event)));