Strangler (Netlify)

Library that allows to create a Netlify function that implements the strangler fig pattern. It allows to layer a Netlify website on top of on or more legacy systems, gradually replacing them.

Concept

Every path that is not part of the Netlify website, is handed to a list of legacy systems. If a system is able to handle that path, the response will be proxied back to the client. If not, the next system will be tried. If none of the systems can handle the path, a 404 will be returned.

Usage

Add the library to your project:

pnpm add @amazeelabs/strangler-netlify

Create a Netlify function that uses the library:

// file: netlify/functions/strangler.ts
import { createStrangler } from '@amazeelabs/strangler-netlify';
import fs from 'fs';

// Read the static 404 page from the file system.
// Thats the page content the function will return if none
// of the legacy systems can handle the request.
const notFoundPage = fs.readFileSync('./public/404.html');

export const handler = createStrangler(
  [
    {
      // Specify a URL to the legacy system.
      url: 'https://legacy.web.site',
      // Optional function that can check if the current url even
      // applies for the system. If not, the system will be skipped.
      applies: (url) => url.pathname.startsWith('/redirect/'),
      // Optional function that can modify the response from the legacy system.
      // If the function returns undefined, the response will be ignored and the
      // next system will be tried.
      process: (response) =>
        [301, 302].includes(response.status) ? response : undefined,
    },
  ],
  notFoundPage,
);

Add a catchall-redirect to the _redirects, that will pass the request to the strangler function:

# Pass all unhandled requests to the strangler function.
/* /.netlify/functions/strangler 200

Warning:
The redirect must be the last line in the file.

Optimizations

The redirect will not be executed for any files that are part of the Netlify build and for paths that match redirects or rewrites in the _redirects file. To reduce the number of invocation of the function, it is recommended to add manual rewrites for any known paths that should be handled by the legacy systems.

# Rewrite uploaded Drupal files to Drupal directly.
/sites/default/files/* https://legacy.web.site/sites/default/files/:splat 200

# Pass all unhandled requests to the strangler function.
/* /.netlify/functions/strangler 200